Unveiling the secrets of abiotic stress tolerance in plants through molecular and hormonal insights

Abstract

Phytohormones are signaling substances that control essential elements of growth, development, and reactions to environmental stress. Drought, salt, heat, cold, and floods are a few examples of abiotic factors that have a significant impact on plant development and survival. Complex sensing, signaling, and stress response systems are needed for adaptation and tolerance to such pressures. Abscisic acid (ABA) is a key phytohormone that regulates stress responses. It interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to direct resources toward reducing the impacts of abiotic stressors rather than fighting against pathogens. Under exposure to nanoparticles, the plant growth hormones also function as molecules that regulate stress and are known to be involved in a variety of signaling cascades. Reactive oxygen species (ROS) are detected in excess while under stress, and nanoparticles can control their formation. Understanding the way these many signaling pathways interact in plants will tremendously help breeders create food crops that can survive in deteriorating environmental circumstances brought on by climate change and that can sustain or even improve crop production. Recent studies have demonstrated that phytohormones, such as the traditional auxins, cytokinins, ethylene, and gibberellins, as well as more recent members like brassinosteroids, jasmonates, and strigolactones, may prove to be significant metabolic engineering targets for creating crop plants that are resistant to abiotic stress. In this review, we address recent developments in current understanding regarding the way various plant hormones regulate plant responses to abiotic stress and highlight instances of hormonal communication between plants during abiotic stress signaling. We also discuss new insights into plant gene and growth regulation mechanisms during stress, phytohormone engineering, nanotechnological crosstalk of phytohormones, and Plant Growth-Promoting Rhizobacteria’s Regulatory Powers (PGPR) via the involvement of phytohormones.

Introduction

Food, fuel, and fiber are mostly obtained from plants, which also significantly contribute to the biological variety and sustainability of our world. Plants have evolved complex systems to perceive and react to external pressures in order to maximize growth and production under changing environmental circumstances (Gupta et al. 2023). Abiotic stressors are one of them, and they may take many different forms. They are either linked to variations in weather, such as temperature, rainfall, and sun exposure, or to the quality of the soil where plants are grown (such as its water, nutrient, and pollutant levels). Climate change has been linked, in particular, to changes in temperature and water availability that cause drought and heat stress (Hamann et al. 2021).

Traditional breeding methods have few disadvantages because of the complex traits connected with stress tolerance; therefore, improvements are required to close the gap between the global food supply and demand. In this field, the development of fresh, efficient techniques is essential. Phytohormones are a practical and feasible method for raising climate-resilient crops, especially in horticultural plants. They play a crucial role in mediating growth, development, source/sink transitions, and nutrient allocation, helping plants adapt to changing environments. Phytohormones are essential endogenous chemicals for regulating physiological and molecular responses, which are crucial for plant survival as sessile creatures. They are an eco-friendly alternative to abiotic stress tolerance, especially in horticultural plants. After being transported, phytohormones might operate in plants at their site of production or in other parts of the plant. According to Altaf et al. (2023), phytohormones are chemicals that plants produce to control plant responses, growth, and development in response to environmental challenges like drought, high temperature, excessive rainfall, etc. Through the coordination of several signal transduction pathways, phytohormones play a significant role in the response to abiotic stressors (Jiang et al. 2023). They also participate in the control of many stimuli, both internal and external, which leads to important adjustments in plant growth (Ladeynova et al. 2023). ABA (abscisic acid) is one phytohormone that has been classified as a stress hormone. In addition to modulating abiotic and biotic stress responses, ABA performs important functions in plant development, including maintaining seed dormancy, stomatal closure, preventing germination, regulating growth, and fruit abscission (Habibpourmehraban et al. 2023). Biotechnologists may find that using phytohormone engineering is the best way to increase the nutritional value and economic viability of crops. For example, a class of plant hormones called gibberellins is in charge of growth and development. They play a crucial role in starting the germination of seeds. Low amounts of gibberellin promote cell elongation and can be utilized to speed up germination.

The issue of different stress conditions on plants has grown in recent years, which has prompted technological innovations. There is a strong view that nanotechnology is transforming the field and that it may be used to prevent global warming and climate change, according to Khan et al. (2012). “The understanding and control of matter at the nanoscale, at dimensions between approximately 1 and 100 nm, where unique phenomena enable novel applications”, is the definition of nanotechnology given by the US. National Nanotechnology Initiative. According to research based on observations and experiments, the effects of metallic nanoparticles on plants are determined by their composition, concentration, morphological characteristics, physical and chemical properties, and species. Nanoparticles can affect plant growth and development in both positive and negative ways (Polyakov et al. 2023). In this paper, we provide an overview of phytohormones, their functions in plant development, growth, and abiotic stress response, as well as how metabolic engineering might make agricultural plants more resistant to abiotic stress.

Plant hormones

Brassinosteroid

Polyhydroxylated steroids (PHs) are brassinosteroids (BRs). A variety of biological and cellular processes are influenced by the BRs, a group of naturally occurring plant steroid (Ahmad et al. 2023; Kaur et al. 2024). Research on the possible economic benefits of BRs in horticulture crops has been conducted since the 1980s. The BRs can be either free or conjugated, and around 69 and 5 conjugated and free BRs, respectively, have been discovered. Brassinolide (BL) has been identified as the most potent BR. The BR metabolism comprises many procedures to maintain the desired amounts of bioactive BRs, such as acylation, glycosylation, and sulphonation (Saini et al. 2015) in the cells. A key role of BRs in sustainable production is to protect plants from environmental challenges (Fig. 1) (Ahmad et al. 2023). In future economy, the combined effects of two properties of BRs (growth stimulation and stress resistance) would significantly improve crop yields. The signaling pathway in BR hormones involves binding to the BRI1 receptor on the cell surface, forming a complex with the BAK1 co-receptor. This triggers a phosphorylation cascade, inactivating BIN2 kinase, allowing transcription factors like BZR1 and BES1 to become active, regulating BR-responsive genes and promoting cell elongation and division.

Fig. 1.

Brassinosteroid signaling pathway in plants to combat abiotic stress response. The Brassinosteroid (BR) signaling pathway regulates plant growth, development, and stress responses. It begins when BR binds to the BRI1 receptor on the cell surface, forming a complex with the co-receptor BAK1. This triggers a phosphorylation cascade that inactivates the negative regulator BIN2. When BIN2 is inactivated, transcription factors like BZR1 and BES1 become active, move to the nucleus, and regulate BR-responsive genes. These genes control processes, such as cell elongation, division, and differentiation, while feedback mechanisms and cross-talk with other hormone pathways fine-tune the response

Gibberellins

Gibberellins are plant hormones important for a lot of things in the developmental processes of plants, such as seed germination and stem development, elongation, leaf extension, trichome development, pollen maturation, and the introduction of flowers (Achard and Genschik 2009). The pathogenic fungus Gibberella fujikuroi, the cause of the ‘foolish-seedling’ disease of rice that results in excessive elongation of afflicted plants, was where GA was originally discovered. More than 130 GAs have been discovered since their first discovery. They have been found in bacteria, fungi, and plants, but just a handful of biological activity may be seen in GAs (Yamaguchi 2008); many non-bioactive plants contain GAs, which serve as precursors to the bioactive forms, otherwise known as deactivated metabolites. The Gibberellin (GA) signaling pathway regulates plant growth, including seed germination, stem elongation, and flowering. Without GA, DELLA proteins inhibit growth by interacting with transcription factors. When GA is present, it binds to the GID1 receptor, forming a complex with DELLA proteins, allowing transcription factors to activate GA-responsive genes.

Jasmonic acid

Higher plants contain jasmonic acid, or JA, which is an endogenous growth regulator. JAs were first discovered to be a stress-related hormone, but they also have a role in the control of critical growth and developmental processes.

For instance, JAs can stimulate stomatal opening, prevent rubisco production, change nitrogen and phosphorus absorption, and influence the transport of organic materials like glucose. Nevertheless, in tomato plants, MYC gene is rapidly activated and a significant amount of JAs is synthesized in response to the exogenously appliedMeJA in order to protect the plant body against cold damage (Miao et al. 2018).

Chloroplasts are the site of the initial phase of JA biosynthesis, which results in the production of the intermediate cis-12-oxophytodienoic acid (OPDA). It has recently been demonstrated that OPDA is exported from chloroplasts via JASSY, a START protein localized to the GA3ox2 membrane of the chloroplast outer envelope (Guan et al. 2019). The peroxisomal membrane-localized CTS (also known as ABCD1) permits OPDA import into peroxisomes, where further JA biosynthesis takes place (Theodoulou et al. 2005). Recent studies employing grafting procedures reveal that OPDA and its derivatives—rather than the bioactive JA-Ile conjugate—travel through the phloem necessarily to start JA signaling in the root (Schulze et al. 2019). It is interesting to note that plant suspension cells demonstrated import-compatible transport activity, whereas in contrary to this, yeast experiments demonstrated export activity of this hormone. In addition, a number of NPF transporters facilitate JA and JA-Ile translocation in heterologous yeast and xenopus oocyte systems. Reduced JA transfer from injured to uninjured leaves is the result of the loss of the NPF2.10 function. Although these transporters can facilitate the transport of GA, ABA, and glucosinolates, the transport of JA by the NPFs is not specific. They are members of a vast protein family, and genetic redundancy is probably playing a substantial role in the physiological characterization of these transporters in plants.

Strigolactones

Strigolactones (SLs), a family of phytohormones and small signaling molecules generated from carotenoids, control a number of developmental processes and react to various environmental cues. By strategically altering plant growth, SLs also coordinate changes to the allocation of resources and help plants adapt to nutrient shortages. Instead of functioning alone, SL interacts with other plant phytohormones, including auxin, ethylene, cytokinin, and abscisic acid, to create complex signaling networks. Strigolactones (SLs) have been shown to regulate the growth of buds and lateral roots as well as to function as catalysts for the symbiotic relationship between plants and mycorrhizal fungi. Recently, it was discovered that the SL transporter ABCG59 regulates the mechanisms of mycorrhizal symbiosis in Medicago (Banasiak et al. 2020).

Additionally, SLs were discovered to control the auxin PIN-dependent feedback transport, which in turn regulates vascular tissue development and regeneration (Xie et al. 2020). The first identified SL transporter in plants was the ABCG-class protein PDR1 from petunias. In the root cortex and shoot axils, it is expressed as a plasma membrane-localized exporter1 (Sasse et al. 2015). Both the roots and the branches can produce SL. Grafting tests showed that SLs from the roots could restore a mutant shoot SL phenotype, proving that SL may be transferred across long distances. It has been demonstrated that lateral bud expansion is regulated by this long-distance transfer toward the shoot lateral axils (Shiratake et al. 2019).

Salicylic acid

An essential plant hormone called salicylic acid (SA) plays a crucial role in the plant’s defensive mechanism against pathogen invasion. Even after significant study over the past 30 years or more, we still don’t completely understand SA biosynthesis and its intricate involvement in plant defense. Multiple plant activities, including the plant’s immunological reaction to pathogens, are regulated by salicylic acid (SA). The importance of SA for systemic acquired resistance (SAR) has long been recognized. The physiologically inactive MeSA hormone is created as a result of SAR from the bioactive SA hormone. MeSA builds up in the phloem, travels to the distal tissues, and is then changed back into SA to cause SAR. The activity of the enzyme that catalyzes MeSA glucosylation, uridine diphosphate glycosyltransferase UGT71C3, further controls the process (Lin et al. 2019). The Salicylic Acid (SA) signaling pathway is crucial in plants for regulating defense responses against pathogens. When pathogen attacks are detected, SA levels increase, activating NPR1, a central regulator. This activates PR genes, enhancing the plant’s immune response. SA is also involved in systemic acquired resistance (SAR), providing long-lasting defense against various pathogens. It works with other signaling molecules like jasmonic acid and ethylene.

Abscisic acid

Abscisic acid (ABA) has a role in many different plant activities, including stomatal closure, root formation, drought tolerance, and growth. It has long been believed that ABA, which mediates stomatal closure, is produced in the root and transferred to the shoot. But it is now understood that ABA is also produced in the shoot. In stomatal apertures, ABA activity is recreated by phloem-specific ABA synthesis, demonstrating that ABA may travel to govern various responses (Kuromori et al. 2018).

The mechanism behind the molecular processes of ABA delivery has been deciphered which has become possible due to the discovery of many ABA transporters. Under natural conditions, both the forms of abscisic acid viz, protonated form (ABAH) and an anionic form (ABA) are found in plants. The diffusion of ABAH can passively pass across the plasma membrane although it mainly decreases (Karuppanapandian et al. 2017). In addition to ABA metabolism and (de)conjugation, plants are affected globally by the transit of ABA across the cytoplasm, which becomes more alkaline and rises under osmotic stress (Ikegami et al. 2009). They discovered that under water shortages, isotope-labeled ABA travels from leaves to roots and that ABA can only accumulate in leaves when leaves and roots are subjected to limited water independently. Other research has supported the idea that ABA is produced in leaves before being transferred to other organs (Zhang et al. 2018). As a result, an essential component of ABA activity in the overall plant’s systemic stress responses is the movement of ABA across cells, tissues, and organs. Although various potential salt, cold, and osmotic stress sensors have been discovered, it is still unclear how environmental sensing is connected to ABA accumulation (Jiang et al. 2019; Ma et al. 2015; Yuan et al. 2014).

Ethylene

The plant’s reaction to abiotic stressors that affect growth and development is regulated in part by the hormone ethylene (ET), which accumulates in the body at different concentrations (Khan et al. 2020). Additionally, higher ET production in plants under heat stress causes a reduction in grain yield, demonstrating that ET has a concentration-dependent role in plants. By activating various stress-related proteins that are essential in preserving the functional integrity and stability of the plant cells, exogenous administration of ET (such as ethephon) plays a significant role in thermotolerance. In the absence of ethylene, the pathway’s key repressor, CTR1, inhibits downstream signaling components, preventing ethylene responses. When ethylene is present, it binds to receptors, inactivating CTR1, allowing EIN2 protein accumulation and activating EIN3 and EIL1 transcription factors, triggering ethylene-responsive genes.

GABA

More than 70 years ago, GABA was initially discovered in potato (Solanum tuberosum) tubers (Gramazio et al. 2020; Ramos-Ruiz et al. 2019). It can also be created through the polyamine metabolic route. Plant development, particularly pollen tube elongation to enter the ovule, root growth, fruit ripening, and seed germination, is regulated by GABA, which also serves as a signal. It builds up as a result of how plants react to environmental challenges, disease invasions, and insect assaults. By enhancing photosynthesis, reducing the production of reactive oxygen species (ROS), activating antioxidant enzymes, and controlling stomatal opening in drought stress, a high concentration of GABA increases plant stress tolerance. Since then, a lot of research has been done on its physiological significance (Bown et al. 2020). However, it is challenging to locate instances of GABA sensors being employed in plants in the scientific literature. The GABA signaling pathway in plants plays a crucial role in stress responses, growth regulation, and metabolism. Produced by glutamate decarboxylase (GAD), it accumulates in plant tissues and interacts with ion channels like (ALMT) Aluminum-Activated Malate Transporters, influencing ion flux and plant responses. GABA also aids in the GABA shunt, balancing carbon, and nitrogen.

Melatonin

Melatonin, a common indoleamine present in both animals and plants, is thought to be a potential phytohormone that influences reactions to various biotic and abiotic stimuli. Melatonin and the auxin indole-3-acetic acid (IAA), both include tryptophan as a biosynthetic precursor, have comparable effects on plants. Melatonin is rapidly accumulated in plants under salt stress. Melatonin strengthens a plant’s ability to withstand salt stress in two ways: first, through direct pathways, such as the direct removal of reactive oxygen species, and second, through an indirect pathway that involves increasing antioxidant enzyme activity, photosynthetic efficiency, metabolite content, and transcription factor regulation linked to stress. The initial component of the MAPK cascade, MAPKKK, may need to be activated by a melatonin receptor. A membrane receptor kinase, such as a leucine-rich repeat (LRR) receptor kinase, would be such a melatonin receptor (Asai et al. 2002). Contrarily, it was shown that C and 2, a protein with seven transmembrane domains that is often assumed to be a G protein-coupled receptor (GPCR), functions as a melatonin receptor by interacting with melatonin to encourage stomatal closure through Ca²⁺ influx signaling and H₂O₂ generation. However, earlier studies (Chen et al. 2018; Li et al. 2017) do not support melatonin-induced H₂O₂ generation and stomatal closure.

The MET levels in transgenic plants are increased by genetic engineering using overexpression or the CRISPR/Cas system of MET biosynthetic genes, which also improves temperature stress tolerance.

Unraveling the science of plant gene regulation under abiotic stress

Many of the actions of phytohormones are mediated through modifications in gene expression. Increase in ABA levels associated with abiotic stress alters the mRNA levels of hundreds of genes, according to early genomic technologies (Goda et al. 2008). This revealed a crucial role for the control of genes in tolerance to abiotic stress, together with the finding that several frequent ABA-insensitive mutations were connected to transcriptional regulators (Finkelstein 2013).

ABA-mediated transcriptional control and hormone interplay

Early research identified the ABA-responsive element (ABRE), a conserved cis-acting regulatory element, in the promoters of genes triggered by drought (Marcotte et al. 1989). A family of four ABRE-binding proteins/ABRE-binding factors54 (AREBs/ABFs) and the closely related ABI5 are examples of basic leucine zipper-type transcription factors that identify ABREs (Fujita et al. 2013). The majority of the transcriptional responses to ABA in Arabidopsis during vegetative growth are controlled by the four largely redundant AREBs/ABFs although ABI5 is more significant during seed germination (Finkelstein et al. 2005). Numerous AREB/ABF targets are genes for other transcription factors, suggesting that ABA-dependent transcriptome remodeling is governed by a multilayer transcription factor hierarchy (Song et al. 2016). Importantly, the presence of nearby ABRE sites positively linked with the ABA-induced binding of several transcription factors, indicating that some transcription factors may work in concert with AREBs/ABFs. In reality, a number of NAC family transcription factors, including ANAC096 (which interacts with ABF2 to promote RD29A transcription), are necessary for ABA-dependent transcription processes.

In the absence of abiotic stress, ABA signaling is suppressed, which encourages healthy growth. For instance, ABI5 transcription increases in response to exposure to ABA or osmotic stress, while ABI5 mRNA levels are low in non-stress situations. The chromatin-remodeling ATPase BRAHMA68 from SWI2/SNF2 is needed for this suppression of ABI5. By encouraging nucleosome occupancy at the transcription start site of the ABI5 gene, BRAHMA prevents ABI5 from being transcribed when ABA is not present. It’s interesting to note that group A PP2C proteins remove the inhibitory phosphate that SnRK2.3, SnRK2.2, and SnRK2.6 phosphorylation of BRAHMA causes it to produce (Peirats-Llobet et al. 2016). In the absence of abiotic stress, several hormone conduits interact to control a variety of characteristics of plant life.

Post-transcriptional abiotic stress responses

Recent studies have revealed the roles of these systems in regulating ABA responses. Beyond transcriptional control, post-transcriptional activities expand the options for gene regulation. Abiotic stress alters alternative mRNA splicing, which controls ABA responses (Zhan et al. 2015; Carrasco-López et al. 2017). A group A PP2C gene called HAB1, for example, encodes a variety of splice isoforms, of which HAB1.2 preserves an intron and results in a non-functional protein and ABA hypersensitivity (Zhan et al. 2015). Recent research has revealed mRNA degradation as an additional mechanism influencing abiotic stress reactions. mRNA decapping mediates the breakdown of mRNA molecules (Schoenberg and Maquat 2012). Osmotic stress causes some transcripts to become unstable because the decapping activator VARICOSE (VCS) is phosphorylated by subclass I SnRK2-type protein kinases that are not reliant on ABA (Soma et al. 2017). Nicotinamide adenine dinucleotide (NAD⁺) can be used to alternatively modify the 5′ end of mRNAs. According to Zhang et al. (2019), the NAD⁺ cap, which is found on many transcripts in plants, is hypothesized to suppress gene expression by encouraging the destruction of tagged mRNAs.

Unraveling the science of plant growth under abiotic stress

Numerous aspects of plant growth and development rely on phytohormones. In response to abiotic stresses, plants orchestrate different growth and development strategies. New discoveries are beginning to shed light on these mechanisms. There are a variety of environmental pressures that plants are constantly subjected to that could potentially obstruct their growth and development. Plants have evolved sophisticated regulatory mechanisms that enable them to properly manage resources and alter their development patterns in response to these unfavorable environmental conditions. In order to create crop types that are stress-tolerant and to ensure food security in the face of climate change, it is essential to understand these mechanisms.

Transcription factors (TFs) play a crucial role in regulating the expression of genes that respond to stress, such as those involved in osmoprotection, antioxidant defense, and ion homeostasis. Under abiotic stress, plants adjust their metabolic pathways to better allocate resources and energy. The accumulation of osmotically adaptable solutes improves cellular water retention, while upregulating antioxidant enzymes can clean up reactive oxygen species (ROS). Changes in the metabolism of carbon and nitrogen can also influence growth rates and resource allocation.

Understanding the complex regulatory networks involved in plant growth under abiotic stress can lead to important implications for crop improvement methods. Breeding strategies can use pathways and genes that respond to stress to create types that can withstand it, while transgenic methods like overexpressing stress-related genes or adding stress-responsive promoters can make crop plants more tolerant. Modern methods like genome editing can precisely manipulate genes associated with stress for customized stress responses (Voesenek and Bailey, 2015).

Involvement of TOR in the response to abiotic stress

Rapamycin (TOR) is a conserved Ser/Thr protein kinase that regulates various developmental stages in plants, including embryogenesis, meristem activation, root and leaf growth, flowering, senescence, and lifespan. TOR (Target of Rapamycin) signaling integrates signals from nutrients, energy, hormones, and environmental cues, governing plant growth and development. Recent research has revealed TOR’s multifaceted contributions to plant responses to environmental stressors. TOR plays a vital role in sensing stress signals and providing predictions about key downstream effectors based on high-throughput proteomic analyses. This research has revealed its multifaceted contributions to plant responses to environmental stressors.

TOR, initially discovered in yeast through resistance to rapamycin, is a conserved Ser/Thr protein kinase present in all eukaryotes, including plants. In plants, its precise complex composition remains unclear, but it plays a vital role in regulating multiple aspects of growth, development, and stress responses by integrating signals related to nutrients, hormones, light, energy, and environmental cues. TOR governs various cellular processes, from cell division to metabolism and signaling networks.

In plants, glucose (Glc) derived from photosynthesis serves as an energy source and building block. TOR’s kinase activity is inhibited when Glc is depleted, triggering the activation of autophagy- and protein degradation-related genes. Glc reactivates TOR via glycolysis-mitochondria-electron transport chain, while Rho-like small GTPase ROP2 activates TOR in shoot apex, requiring Glc and auxin signaling. TOR regulates cell growth by crossing sugar and brassinosteroid signaling.

Sulfur is another essential nutrient for plants, and its assimilation involves various steps. Manipulating the allocation of sulfur flux from glutathione (GSH) biosynthesis to protein translation can affect plant growth via TOR regulation. Certain glucosinolates can also function as TOR inhibitors, influencing root meristem activation and elongation (Jobe et al. 2019; Malinovsky et al. 2017; Speiser et al. 2018). Branched-chain amino acids can activate TOR in Arabidopsis although plant orthologs of mammalian amino acid sensors have not been identified. Plants obtain organic nitrogen through nitrogen assimilation from nitrate or ammonium in the soil. TOR activity is inhibited during nitrogen starvation but quickly reactivated upon resupply of nitrate, ammonium, or amino acids (Couso et al. 2020; Saxton and Sabatini 2017).

It is also possible for organic nutrients to affect TOR activity, including phosphorus. A genome-wide analysis of transcriptional profiling reveals a positive feedback loop between Glc, sulfur, and nitrogen signaling, suggesting that TOR facilitates nutrient acquisition during aboveground and belowground plant growth (Couso et al. 2020; Fu et al. 2020; Rodriguez et al. 2019).

Recent studies have uncovered the complex roles that TOR plays in coordinating plant responses to various abiotic stresses. The nature and the length of the stress experienced will determine whether TOR acts as a positive or negative regulator (Li et al. 2019).

Temperature has a significant impact on how plants function and grow. According to research by Wang et al. (2017), after being exposed to cold treatment for as little as 10 min, Arabidopsis TOR activity encounters fast suppression but miraculously recovers within two hours. The inducible tor-es mutant’s elevated anthocyanin accumulation under normal temperature settings is also affected by cold treatment, suggesting that TOR may act as a negative regulator during cold acclimation. Such TOR inhibition may be related to the requirement for translation inhibition to increase cold tolerance (Wang et al. 2017). It’s interesting to note that a separate study by Dong et al. (2019) has offered a different viewpoint, arguing that TOR may possibly have a beneficial role in the plant’s reaction to cold stress. Reduced energy levels, decreased TOR activity, and a subsequent growth halt in Arabidopsis are caused by the loss of the AtTHADA gene, which encodes a protein that is an ortholog of the cold response regulator HsTHADA in humans. Furthermore, the Atthada mutant and TOR-RNAi (35-7) lines also have increased sensitivity to cold environments (Dong et al. 2019). These divergent results highlight the complex and dynamic nature of the TOR-mediated response to cold stress and may be explained by variations in the silencing efficacy of distinct TOR-RNAi lines or variations in growing conditions.

TOR is integrally involved in boosting tolerance to hot temperatures in addition to its role in reducing cold stress. As a result of the application of exogenous glucose (Glc), TOR overexpression, and enhanced production of the transcription factor E2Fa, heat shock genes are expressed more strongly, and seedling survival rates in the process of recovering from heat stress are improved. On the other hand, reduced seedling survival results from the downregulation of TOR, E2Fa, or treatment with the TOR inhibitors AZD-8055 or Torin1 (Sharma et al. 2019). By attaching to the promoter of AtHLP1, Glc-activated TOR works with E2Fa to activate the gene. Then, AtHLP1 binds directly to the promoters of several heat shock genes, causing histone acetylation and an increase in H3K4me3 to activate and maintain thermos-memory and subsequently improve thermotolerance (Sharma et al. 2019). In particular, the proHLP1: After 24 h of heat stress recovery, GUS expression is noticeably increased in the proliferation zone of the shoot apex, especially when Glc is present. This highlights the coordination between cell proliferation in the shoot apex and internal as well as external cues for maintaining growth and survival, orchestrated by Glc-TOR energy signaling.

TOR develops in plants as a positive regulator in response to osmotic and drought stressors. Particularly in Arabidopsis, overexpression of TOR causes primary roots to be noticeably longer in comparison to control lines subjected to high potassium chloride concentrations (Deprost et al. 2007). In addition, rice (Oryza indica) that has the Arabidopsis TOR gene expressed ectopically shows improved water use efficiency, growth, and yield when water is scarce. It’s significant that these transgenic rice lines show resistance to ABA treatment when seeds germinate (Bakshi et al. 2017). Collectively, these results suggest that constitutive TOR expression may be able to lessen the negative impacts of drought or osmotic stress on plant growth.

TOR appears to have a negative regulatory function in plants exposed to oxidative stress or DNA/RNA damage, with Maf1, a well-known RNA polymerase III repressor, controlling its repressive activity through phosphorylation and dephosphorylation. It is believed that these stressors impede TOR activity to boost Maf1 dephosphorylation, which in turn activates its repressive function. Oxidative stress and TOR silencing are both linked to the dephosphorylation of Maf1.

Over the past decade, our understanding of plant TOR signaling has improved, revealing that it functions as a central regulator that detects and communicates various inputs, including nutrients, energy, hormones, metabolism, and environmental stresses, regulating physiological, molecular, and developmental responses for growth and adaptation. However, many questions remain unanswered and new ones emerge. It is crucial to understand how TOR signaling suppresses various primary target gene pathways involved in stress responses and immune functions. The phosphorylation of Thr-449 in TOR substrate, ribosomal S6 kinase1, serves as a conserved marker for assessing endogenous TOR activity. The development of markers specific to TOR kinase activity that can be visualized with fluorescence in different tissues, such as sink and source tissues, will enable the quantitative measurement of TOR activity and specific signaling outcomes. This advancement will enhance our ability to accurately interpret varying or even opposing phenotypes that arise when TOR signaling is disrupted under different environmental conditions.

Gibberellic acid and ABA: the dynamic duo behind germination

The pivotal determinant in the decision to germinate is the equilibrium between two competing hormone signaling pathways, namely gibberellic acid (GA) and abscisic acid (ABA). During the seed maturation process, a complex network of transcription factors, including ABI3, ABI4, and ABI5, which are regulated by ABA, activates the genes necessary for seed desiccation and the production of ABA while simultaneously suppressing the genes responsible for GA synthesis. Environmental cues, such as cold temperatures and exposure to light, induce seed dormancy release by tilting the hormonal balance toward GA. DELLAs act as inhibitors of gibberellic acid (GA) responses, and GA signaling, in part, deactivates DELLAs by inducing their proteasomal degradation (Fig. 2) (Achard et al. 2006; Sun et al. 2011). ABI3 and ABI5 are among the TFs that interact with DELLAs. Together, these protein complexes facilitate the transcription of SOMNUS, a crucial factor promoting dormancy. SOMNUS, in turn, activates genes involved in abscisic acid (ABA) biosynthesis and suppresses genes related to GA biosynthesis (Lim et al. 2013). Intriguingly, the interaction between DELLA and ABI5 is counteracted by the basic helix–loop–helix (bHLH) TF Inducer of CBF Expression1 (ICE1) (Hu et al. 2019). ICE1 binding hinders ABI5’s DNA-binding activity, and GA treatment may stimulate this interaction due to DELLA degradation. Cold temperatures may promote germination through cold exposure. ABI5 expression is crucial for environmental signals during germination. The light-responsive component HY5 triggers ABI5 transcription (Chen et al. 2008), while the DELLA protein RGL2 enhances ABA signaling. GA production may reduce ABI5 expression through degradation of RGL2 (Borghi et al. 2016).

Fig. 2.

GID 1-mediated gibberellin signaling transduction pathway in plants. The Gibberellin (GA) signaling pathway controls key aspects of plant growth, such as seed germination, stem elongation, and flowering. In the absence of GA, DELLA proteins act as growth repressors by inhibiting transcription factors. When GA is present, it binds to the GID1 receptor, forming a complex that interacts with DELLA proteins. This interaction leads to the degradation of DELLA proteins, releasing the transcription factors to activate GA-responsive genes that promote growth and development. The pathway is finely tuned through interactions with other hormonal signals to coordinate plant growth

Auxin, ABA, and their roles in root growth under stress conditions

ABA regulates root tissue patterning during water stress by inducing the expression of microRNAs miR165 and 166, crucial for vascular development (Bloch et al. 2019; Ramachandran et al. 2018). It also activates Vascular-Related NAC Domain (VND) transcription factors within xylem cells, further promoting xylem differentiation and promoting root tissue patterning (Ramachandran et al. 2018). Research on two interrelated root-branching strategies dependent on water availability, namely hydro-patterning and xero-branching, has unveiled the involvement of auxin and ABA signaling (Dinneny et al. 2019). Lateral roots originate from primary root pericycle cells and are controlled by an auxin-regulated transcriptional network (De Smet et al. 2007; Moreno-Risueno et al. 2010). Hydro-patterning, a process where water content differences in primary roots favor lateral root initiation, is linked to auxin biosynthesis and signaling on the water-contacting side of the root (Bao et al. 2014). Recent research has highlighted the essential role of the auxin response factor ARF7 in hydro-patterning (Orosa-Puente et al. 2018). Another phenomenon, termed xero-branching, entails the repression of lateral root formation along the entire root circumference when roots encounter large air spaces in the soil. ABA signaling has been implicated in this xero-branching response (Orman-Ligeza et al. 2018). Barley plants accumulate ABA after a water deficit, mimicking xero-branching. Short-term ABA treatment in maize and barley roots leads to lateral root repression and disrupts auxin signaling, suggesting lateral root repression (Orman-Ligeza et al. 2018).

Gibberellin, ABA, and ethylene: their roles in flowering regulation amidst abiotic stress

The regulation of flowering time in plants is orchestrated by a fundamental genetic network that integrates signals from internal, environmental, and seasonal cues (Andrés and Coupland 2012). Here, we delve into the intricate interplay between hormone signaling and the core regulators of flowering to elucidate how abiotic stress influences the timing of flowering. In the face of prolonged drought, many plant species exhibit an accelerated transition to the flowering phase, a phenomenon known as drought escape (Riboni et al. 2013). The study focuses on drought-induced flowering, where evidence suggests a positive role for ABA signaling. Mutations in ABA biosynthesis genes lead to delayed flowering under long-day conditions, while an ABA-hypersensitive pp2c triple mutant exhibits accelerated flowering. Drought stress exacerbates this delay, indicating a critical role for ABA in promoting drought escape (Riboni et al. 2013, 2016). Salt stress, on the other hand, exerts an ethylene-dependent delay on flowering time in Arabidopsis (Achard et al. 2007). Additionally, salt stress exerts a repressive effect on flowering by inducing the degradation of GIGANTEA (Kim et al. 2013).

Ethylene and gibberellin: orchestrating responses to flooding

In agriculture, flooding poses a significant threat, often resulting in substantial crop losses, while in natural ecosystems, certain plant species encounter it as a formidable environmental challenge (Voesenek and Bailey 2015). To navigate these inundated conditions, plants employ a repertoire of developmental and physiological strategies, with variations observed among different species. Here, we delve into recent advances pertaining to hormone signaling during submergence, with a particular focus on the hormonal regulation of a flood-escape mechanism in rice. For a comprehensive exploration of the broader topic of flooding responses, we direct readers to recent reviews (Hartman et al. 2021; Lee and Bailey, 2021).

The submergence of plant tissues presents a daunting challenge, severely restricting cellular access to essential gasses like O₂ and CO₂, leading to profound metabolic disruptions. Furthermore, underwater environments foster the accumulation of ethylene within plant tissues (Voesenek and Bailey 2015). Flooding triggers hypoxia in Arabidopsis, affecting gene expression under low oxygen conditions. ERF-VIIs, five transcription factors, control these genes’ transcription (Gasch et al. 2016). Both hypoxia and elevated ethylene levels bolster the stability of ERF-VIIs, consequently promoting the transcription of target genes (Gibbs et al. 2011; Licausi et al. 2011; Hartman et al. 2019; Lin et al. 2019). ERF-VII transcription factors bind to conserved cis-regulatory elements, and in response to flooding, the chromatin accessibility at these regulatory sites increases (Gasch et al. 2016; Reynoso et al. 2019).

Certain flood-adapted plant species employ an escape strategy characterized by the elongation of underwater shoots and leaves to reach the surface (Voesenek and Bailey 2015). Deepwater rice, a flooding-tolerant variety of rice, is influenced by ethylene and gibberellin (GA) signaling. Fascinatingly, recent research has proposed that compacted soil conditions trigger ethylene accumulation in roots, subsequently inhibiting further growth. This phenomenon may serve as an adaptive mechanism enabling plants to avoid regions characterized by poor soil aeration (Pandey et al. 2021). These findings hint at the possibility that elevated ethylene concentrations could be a common and early signal employed by plants to respond to air deficiency stress and modulate their growth accordingly.

From the onset of seed germination throughout their entire life cycle, plants encounter a multitude of stressors. These stressors, which include salinity, heat, drought, and nutrient deficiencies, can lead to reduced crop quality and yield, contributing to crop losses (Andreotti 2020). Abiotic stresses don’t just affect yield, they also influence product quality by inducing changes in plant morphology, physiology, and biochemistry (Rao et al. 2016). Recent shifts in climate patterns have added to the challenges faced by horticultural crops (Fig. 3). Climate change is widely recognized as a looming threat to the agricultural sector (Francini and Sebastiani 2019; Gao et al. 2022; Shahid et al. 2021).

Fig. 3.

Impact of abiotic stress on various physiological and morphological plant processes

Plants exhibit a diverse array of responses to these stresses, including reductions in photosynthetic efficiency, leaf water content, membrane integrity, and levels of photosynthetic pigments, as well as impacts on plant growth and overall yield (Ullah et al. 2018). Furthermore, it’s worth noting that a significant portion of agricultural land is affected by one or more stress factors. Consequently, the horticultural industry is actively exploring new agronomic approaches to combat the challenges posed by these environmental stressors while upholding both sustainability and product quality. To this end, horticultural plants employ various plant hormones as protective mechanisms against abiotic stressors.

Engineering phytohormones to produce crop plants capable of sustaining abiotic stress

Engineering techniques

In several economically significant crop species, genetic engineering has created new opportunities for abiotic stress tolerance introduction. Genetic engineering techniques are now among the most effective ways to increase agricultural output in difficult situations as a result of recent achievements. However, efficient plant transformation techniques are crucial for the functional expression and stable integration of foreign genes in the plant genome (Rahman et al. 2022). Rapid advancements in transformation technology since the first reports on tobacco have led to the genetic modification of numerous plant species (Reddy et al. 2022).

There are two primary methods for transferring genes to plants: direct gene transfer and vector-mediated gene transfer. Direct gene transfer involves inserting DNA into cells using chemical, physical, or electrical methods, or vector-mediated gene transfer using a biological vector like Agrobacterium. Examples of direct or non-biological gene transfer include particle bombardment, protoplast fusion, electro-transfection, polyfection, liposomes, injection-based methods, and desiccation-based transformation. Biolistics, or particle bombardment, is the most commonly used direct gene transfer technique. Agrobacterium tumefaciens has expanded its host range to the point where it is now the technique of choice for transferring genes to all of the major crop plants. This soil bacterium is naturally able to transfer (or T-DNA) a specific portion of its plasmid DNA into the genome of its plant host. Although direct gene transfer techniques are effective for both stable transient and transformation expression, they still have some drawbacks, including unwanted genetic rearrangements, a low frequency of stable transformation caused by the high copy number of genes, and a lengthy time frame needed to regenerate entire transgenic plants. It’s interesting to note that it is possible to successfully substitute bacterial genes in Agrobacterium with the desired gene(s) and that this change has no impact on the frequency or process of transformation.

The effectiveness of Agrobacterium-mediated approaches depends on a number of variables for transformation and subsequent regeneration. The explant chosen, the hormonal makeup of the medium used, the conditions of culture prior to and during inoculation, the length of co-cultivation, dietary supplements, the virulence of the strain of Agrobacterium, and the concentration and makeup of the bacteriostatic drug used are some of these factors. The duration of the selection and the level of the selection marker are equally crucial. Other crucial elements include the plant cultivar and varied tissue culture conditions, such as a strong plant regeneration system (Yookongkaew et al. 2007). However, over the past two to three decades, amazing advancements in plant genetic engineering have been made through intense efforts. Agrobacterium-mediated genetic transformation of a wide variety of crop plants, including monocots, which were earlier thought to lie beyond the Agrobacterium host range, has been made possible by the development of optimized methods. Recently, strategies for in-plant transformation have been developed with success. However, Broothaerts et al. (2005) have identified three non-Agrobacterium species: Sinorhizobium meliloti, Rhizobium sp. NGR234, and Mesorhizobium loti as being proficient in effective genetic transformation of various plant species. The difficult Agrobacterium patent environment and the need for a superior plant biotechnology platform served as the driving forces for this study. Virus-based vectors have just been added to this list of biological vectors for plant genetic modification.

Metabolic engineering of phytohormones for crop plant abiotic stress tolerance

Abiotic stress tolerance in agricultural plants can be improved by modifying hormone metabolism and signaling mechanisms as phytohormones are crucial growth and development regulators for plants. To maintain hormonal balance, it is essential to overexpress essential enzymes in the ABA biosynthesis pathway (Waadt et al. 2022). Studies have shown that overexpression of genes associated with ABA synthesis or catabolic pathways can boost drought tolerance but lower growth due to pleiotropic implications. Zhang et al. (2013) overexpressed CRK45, a stress-inducible kinase involved in ABA signaling, to address these adverse developmental defects (Jewell et al. 2010).

Recently, transgenic poplars were created by overexpressing the Arabidopsis YUCCA6 gene, which is regulated by the stress-inducible SWPA2 promoter and involved in the tryptophan-dependent IAA biosynthesis pathway. These transgenic lines displayed enhanced drought resilience but underwent stricter regulation of ABA levels and signaling. IPT was also expressed to avoid pleiotropic effects (Ke et al. 2015).

OsGA2ox1, a gene that encodes GA2-oxidase, was overexpressed by Sakamoto et al. (2003) to alter GA (Gibberellic acid) levels. However, transgenic rice did not set grain when the actin promoter constitutively produced the gene. Under the direction of the GA biosynthesis gene OsGA3ox2 (D18), OsGA2ox1 was expressed ectopically at the location of bioactive GA synthesis in shoots, resulting in a semi-dwarf phenotype with typical flowering and grain development.

As biosynthetic routes and convergence places for crosstalk remain unknown, there is still a forum for improvement in this field and the discovery of new genes coding phytohormone metabolic processes to be targeted to engineer abiotic stress tolerance in agricultural plants. Recent research has opened up numerous possibilities for genetically modifying phytohormone targets to give important crop species resistance to abiotic stress.

Crosstalk between plant hormones and nanoparticles: a nanotechnological strategy for mitigating plant stress

Plant physiologic performance is linked to hormone pool modulation, which regulates growth under abiotic and biotic stress. Understanding plant hormone signaling and metabolism is crucial for understanding regulatory networks under environmental stress (Xiao et al. 2021). Plants develop a defensible system under stressful conditions, making it essential to characterize these processes. According to Yang et al. (2017), plant hormone content and activity are regarded as key indicators of plant toxicity. Presently, researchers are beginning to grasp the complex interplay between phytohormone signaling and nanoparticles. Plant hormones and nanoparticle exposure interact in synergistic or antagonistic ways that are critical for determining how plants react to stress. According to Vinkovi'c et al. (2017), plant hormones (as adaptable regulators of plant development and growth) constitute an untouched arena in investigations of the interactions between plants and nanoparticles. Plant hormones auxin and cytokinin play a major role in controlling the effects of metallic nanoparticle exposure on plant development, which may be either favorable or adverse depending on concentration (Kaur et al. 2023; Tripathi et al. 2021). In response to exposure to metal oxide-based nanoparticles, Vankova et al. (2017) correlated the hormonal profile with the physiological condition of Arabidopsis. They observed the plant’s reaction to ZnO NP exposure at various concentrations of auxin, cytokinins, ABA, SA (salicylic acid), and JA (jasmonic acid). In Arabidopsis, it was discovered that the levels of cytokinin and their active precursor, cytokinin phosphatase, had been upregulated to the relatively low ZnO NP (nanoparticle) concentration. In order to control the growth of Brassica napus roots, Xie et al. (2020) hypothesized that a nanomaterial (graphene oxide) would interact with many plant growth hormones. In the past few decades, numerous studies have shed light on the regulatory function that nanoparticles play in plants under stressed conditions. Research on nanomaterials’ potential to improve environmental constraints is limited. More research is needed to understand how nanomaterials affect the expression of genes related to phytohormones essential for plant growth. Recent studies have shown that reactive oxygen species (ROS) play a crucial role in controlling plant development and defense reactions (Kawaguchi et al. 2023). The manufacture of numerous phytohormones, including ethylene, ABA, brassinosteroids, JA, and SA, is facilitated by ROS at their optimal concentration. However, in unfavorable circumstances, plant hormones have been discovered to be in charge of accelerating the plant’s defense system and suppressing ROS (Xia et al. 2015).

Abiotic stress and pathogens have been linked to severe toxicity and yield loss in commercial cash crops like rice, wheat, and groundnuts, among others (Abhinandan et al. 2018). However, studies have also suggested that stress may be mitigated with a nanotechnological approach (Kashyap et al. 2020). Studies have shown that treatment with nanoparticles increases the activity of the plant’s antioxidant defense system and hormonal signaling, which reduces the negative effects of stress on the plant. For example, treatment with nanoparticles improved morphological parameters in Solanum lycopersicum by activating the plant’s antioxidant defense mechanism and overexpressing JA genes, which lessened the negative effects of salt stress (Hernandez-Hernandez et al. 2018). SiO₂ NPs were discovered to increase the defensive activity against pathogenic bacteria in A. thaliana and to be a secure, economical, and long-lasting substitute that causes the plant to develop systemic acquired resistance (El-Shetehy et al. 2021).

miRNAs (microRNAs) have been shown to play a regulatory role in some instances of nanoparticle exposure (Balaban Hanoglu et al. 2023). miRNAs are known to play an important role in post-transcriptional gene regulation and the regulation of plant responses to a variety of environmental challenges, including abiotic stresses like nanoparticles (Khraiwesh et al. 2012). Aluminum oxide nanoparticles were used by Burklew et al. (2012) to observe the modification in miRNA expression in tobacco. MiR167 and miR159 were discovered to be significantly overexpressed (11.4 and 5.9-fold, respectively). MiR159 and miR167 are two genes that regulate plant stress. MiR159 suppresses the expression of genes MYB33 and MYB101, controlling the accumulation of ABA phytohormone, and miR167 targets transcription factors Auxin Response Factor 8 and 6 (Reyes and Chua 2007). It has also been advised to use transcription factors to control the effects of nanoparticle exposure on stress. For instance, Linh et al. (2020) examined the high expression of WRKY27, which controls ABA production and the signaling pathway under drought stress, in the leaves of Glycine max L. This method of maintaining endogenous phytohormone levels under stressful conditions by overexpressing biosynthetic and transcription factors, signaling-related genes, and miRNAs with nanoparticles is widely known.

Unveiling plant growth: promoting rhizobacteria’s regulatory powers to combat abiotic stresses in plants via the involvement of phytohormones

Climate change has negatively impacted agricultural output and arable land due to increased global temperatures and excessive use of chemical inputs (Mohanty et al. 2021; Shah et al. 2021). Plant biomass and soil fertility have decreased due to environmental challenges like drought, salinity, high temperatures, and nutrient shortages (Delitte et al. 2021; Oleska et al. 2020; Srivastava et al. 2021). Rapid industrialization and population expansion have led to the buildup of hazardous metals in soil, posing a threat to crop output. Most cultivated plants lack adaptation to these conditions, leading to significant crop output losses. To address these issues, environmentally friendly strategies are needed to improve resource utilization efficiency and reduce unsustainable agrochemical inputs. Plant Growth-Promoting Rhizobacteria (PGPR) has been isolated and characterized since the 1980s, and the use of beneficial microbes in agriculture is crucial for sustainability in the context of climate change and ecosystem deterioration (Kloepper et al. 1980; Leontidou et al. 2020; Mohanty et al. 2021; Jiao et al. 2021). Understanding the ecological and evolutionary relationships between plants and their microbiomes is essential for developing novel solutions. Combining PGPR with cutting-edge technologies in plant–microbiome interactions can increase plant resilience.

Intense interactions between plant roots and diverse soil microbial groups, which include bacteria, archaea, fungus, and viruses, occur in the rhizosphere (Paez-Espino et al. 2016; Pérez-Jaramillo et al. 2018; Genitsaris et al. 2020). Bacteria are known to be the most common taxonomic group among them (Mohanty et al. 2021; Grover et al. 2021; Ullah et al. 2021). Due to the complex and highly dynamic interactions between plants and microbes, grasping the idea of the soil metagenome—which is crucial for understanding the dynamics of microbial genes in the ensuing soil metaphenome—presents major obstacles (Jansson and Hofmockel, 2018). Although there are many mechanisms attributed to the role of PGPRs in conferring abiotic stress resistance, discussing all of them is out of the scope of the current review, and focus is made on the role of phytohormones only.

Most PGPR strains can synthesize phytohormones, along with related metabolites and signaling molecules that resemble hormones produced by plants (Jiao et al. 2021; Shah et al. 2021). These include hormones including abscisic acid (ABA), cytokinins, gibberellic acid, and indole acetic acid (IAA) (Ullah et al. 2021). Among the almost 80% of soil bacteria, including several Pseudomonas, Bacillus, Burkholderia, and Rhizobium species, IAA is the hormone that is generated the most frequently (dos Santos et al. 2020; Khan et al. 2020). Numerous studies emphasize the value of IAA-producing PGPR in enhancing plant development and agricultural yield in a variety of difficult environments, including salinity brought on by heavy pesticide usage or water scarcity (Mellidou et al. 2021).

Root elongation, root meristem activity, the creation of lateral roots, and nodule development are all influenced by cytokinins, which regulate many aspects of plant growth and development. Arthrobacter, Azotobacter, Bacillus, Pseudomonas, and Pantoea are just a few of the bacterial species that have been shown to generate cytokinin, which helps the host create nutrients and respond to infections (Shah et al. 2021). Plant growth and yield can also be increased by PGPR that releases gibberellins, such as Azospirillum, Shingomonas, Bacillus amyloliquafaciens, and Bacillus pumilus (Dos Santos et al. 2020). Numerous plant species, including maize, rice, and soybean, have shown the positive impacts of these PGPR, which are fueled by the production of phytohormones (Kang et al. 2019).

Some PGPR strains have the ACC deaminase enzyme, which can scavenge plant ACC, limiting its availability for ethylene synthesis and regulating excessive levels. These strains have been shown to boost plant growth in various crops, including rice, rapeseed, wheat, beans, and tomatoes, when exposed to salt stress, drought, or flooding (Nautiyal et al. 2013; Singh and Jha 2017; Danish et al. 2020; Chandra et al. 2020). They also promote root elongation. However, a mutation in the ACC deaminase gene hampered rice plant development under salt stress, affecting nitrogenase activity (Han et al. 2015). Additionally, some PGPR strains have the ability to produce abscisic acid (ABA), which counteracts the effects of ethylene in regulating plant growth and responses to abiotic stress, especially through stomatal conductance modulation (Müller 2021). Despite the fact that plant and bacterial species have different mechanisms for ABA-mediated stress tolerance, several PGPR have been identified as ABA makers or stimulators, especially in water-stressed environments (Salmon et al. 2014). Furthermore, in situations where there is an excess of salt, ABA-induced reactive oxygen species (ROS) buildup mediated by PGPR can cause stomatal closure, preventing water loss and assisting plants in surviving salt stress (Grover et al. 2021). In contrast, some PGPR may use ABA as a source of carbon and energy, potentially reducing the buildup of ABA in plant tissues. This has been shown in wheat that has had psychrophilic Bacillus species inoculation, which increases resistance to cold stress by regulating the expression of ABA-related genes (Zubair et al. 2019). According to their modes of action and the particular environmental signals they come across, PGPR can exert its effects through various routes as seen by the variety of reactions to ABA. Certain PGPR strains can also make polyamines, which can improve photosynthesis, stomatal conductance, and root architecture in addition to hormone production.

Conclusion and future perspective

In order to increase agricultural yields in the face of abiotic stressors, applications and translational research can be used to better understand the mechanisms, genes, and pathways driving these qualities. The model organism Arabidopsis thaliana has been the subject of numerous insights that are discussed in this paper. Therefore, it is crucial to look into whether the same or different processes are operating in crop plants. It has also become clear that different cell types have different hormone signaling pathways and effects. Therefore, altering characteristics at the cellular or tissue level requires a thorough investigation of the hormone signaling pathways inside those particular cell types.

Single-cell sequencing, protein complex identification techniques, hormone reporters, and other cutting-edge technologies are only a few of the cutting-edge tools that will make it possible to examine the various species- and cell-type-specific signal transduction pathways involved in abiotic stress. Using genetic tools like CRISPR-Cas9 gene editing and genomics-accelerated breeding, it is possible to hasten the emergence of characteristics that confer resistance to abiotic stress. Furthermore, the genomics revolution and automated phenotyping have improved our ability to identify the genes and pathways that strong wild relatives use most frequently. This information can guide focused breeding initiatives intended to introduce better traits into cultivated crops. Furthermore, knowledge-guided de novo domestication of crops provides a possible avenue for incorporating advantageous hormone signaling features, increasing the yields of climate change-resistant wild types. The development of abiotic stress-resistant crops will continue to depend on the understanding of how plant hormones interact in various responses to abiotic stress that is still being made.

Scientific communities and businesses should capitalize on the increasing public apprehension regarding the use of agrochemicals. They can achieve this by advocating the advantages of alternative technologies, such as harnessing the phyto-microbiome in the form of “plant probiotics.” Numerous studies have explored beneficial microbes for enhancing plant growth, particularly in challenging environmental conditions. However, there is a pressing need for more comprehensive and large-scale research efforts to identify and characterize potential novel microbial biostimulants. These studies should aim to uncover their functions in host plants when acting as consortia, addressing crucial knowledge gaps before progressing to their commercialization. Furthermore, the integration of cutting-edge omics approaches with computational biology can help overcome the technical challenges associated with understanding the unpredictable behavior of plant–microbiome interactions in the face of constantly changing environmental conditions. This integration will drive innovations in the application of microbiomes in the agriculture sector.

Author contributions

SG: Writing: Original draft preparation and Visualization, Supervision; RK: Original draft preparation and Visualization; AU: Writing—Original draft preparation; AC: Original draft preparation and Visualization, Supervision; VT: Critical review.

Funding

This review did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

No data were used for the research described in the article.

Declarations

Conflict of interest

The authors claim they have no known financial conflict of interest.