Priming Salt Resilience in Melon ( Cucumis melo ) via Pseudomonas oryzihabitans ‐Mediated Metabolic Reprogramming

ABSTRACT

Melon ( Cucumis melo L.) is a globally significant crop, yet its productivity is increasingly threatened by rising soil salinity in many cultivation areas. In this context, plant growth‐promoting rhizobacteria (PGPR) have emerged as a promising strategy to enhance plant growth and improve tolerance to salinity stress. This study investigated the effect of inoculating melon seedlings with a previously characterized strain AXSa06 (identified as Pseudomonas oryzihabitans ) under salt stress (250 mM NaCl), focusing on plant growth, photosynthesis, antioxidant responses, metabolomic profiles, nutrient balance, abscisic acid (ABA) levels, and expression of ABA signaling pathway genes. Inoculation improved the growth and photosynthetic performance of melon plants under salt stress, while reducing Na⁺ accumulation and facilitating better nutrient homeostasis. The elevated levels of malondialdehyde (MDA) and catalase (CAT), along with the increased ABA content upon inoculation, pointed to a priming effect that prepared plants to manage oxidative stress, as evidenced by the reduced antioxidant activity upon stress. Expression of CmPYL9 and several CmPP2AC isoforms was upregulated only in inoculated stressed plants, suggesting improved ABA sensitivity and a fine‐tuned transcriptional control of signaling. AXSa06‐inoculated stressed plants also displayed notable metabolic reprogramming, primarily affecting carbohydrate metabolism, following shifts in TCA cycle metabolites already evident under control conditions. Taken together, these findings suggest that AXSa06 could serve as a promising biostimulant agent improving melon tolerance to salt stress by modulating physiological and biochemical responses. This study offers valuable insights into PGPR‐driven stress priming and its role in developing strategies for sustainable crop production under saline conditions.

1. Introduction

Salinity stress is a major abiotic factor hindering plant growth and productivity, posing a significant challenge to agriculture and crop production worldwide, especially in arid and semi‐arid regions (Munns and Gilliham 2015; Arzani and Ashraf 2016). High soil salinity triggers various morphological, physiological, biochemical, and metabolic alterations in plants, with the severity of these effects depending on factors such as plant species, genotype, age, and the intensity and duration of stress. High salinity reduces plant germination, vigor, and crop yield, often leading to substantial productivity losses (Paul and Lade 2014). Excessive Na⁺ and Cl⁻ accumulation disturbs ion balance and promotes the generation of reactive oxygen species (ROS), which damages proteins, lipids, and chlorophyll, reducing photosynthetic activity and destabilizing cellular membranes (Apel and Hirt 2004; Grover et al. 2011). Oxidative damage, along with osmotic stress, hormonal imbalances, elevated ethylene levels, and impaired stomatal regulation, further suppresses growth and accelerates senescence (Glick 2014; Hashem et al. 2015).

Efforts to improve plant tolerance to salinity have employed several strategies, including the development of resistant varieties through breeding and genetic engineering, as well as the application of various agronomic practices. Among them, the use of plant growth‐promoting rhizobacteria (PGPR) and seed biopriming has emerged as a promising and sustainable approach. These beneficial microorganisms colonize the rhizosphere, forming symbiotic relationships with plant roots and enhancing plant resilience to abiotic stresses, such as salinity (Banaei‐Asl et al. 2016; Genitsaris et al. 2020; Ha‐Tran et al. 2021).

PGPR‐mediated salt tolerance involves several mechanisms. They enhance the ability of plants to absorb water and essential nutrients while facilitating the accumulation of osmolytes (e.g., proline, glutamate, glycine betaine, soluble sugars, choline, O‐sulphate, and polyols), which help regulate osmotic balance and maintain cell turgor pressure (Yasin et al. 2018; Jogawat 2019). Moreover, PGPR promotes the activation of antioxidant enzymes and increases the levels of nonenzymatic antioxidants in plant tissues, thereby safeguarding against oxidative damage caused by salinity stress (Gill and Tuteja 2010; Neshat et al. 2022). Furthermore, PGPR can exert their protective effects by synthesizing Na⁺‐binding exopolysaccharides (EPS), improving ion homeostasis, and decreasing ethylene levels through 1‐aminocyclopropane‐1‐carboxylate (ACC) deaminase activity. PGPR can also synthesize and modulate other key hormones like auxins, cytokinins, and abscisic acid (ABA) (Ha‐Tran et al. 2021; Gao et al. 2022). Notably, ABA plays a critical role in regulating plant responses to salinity by modulating water loss, stomatal closure, and activating stress‐related gene expression, making it a key hormone in managing salt stress in plants (Hsu et al. 2021). At the molecular level, ABA perception is mediated by the PYL (PYR/PYL/RCAR) family of receptors, which bind ABA and inhibit clade A protein phosphatases 2C (PP2Cs). This interaction allows the activation of downstream SnRK2 kinases and ABA‐responsive gene expression involved in plant stress responses (Ma et al. 2009; Fujii et al. 2009; Cutler et al. 2010). Changes in the expression of PYL and PP2C genes have been associated with altered ABA sensitivity and stress adaptation. It has been reported that PGPR significantly increased ABA levels in plants under these conditions, which may play a crucial role in enhancing salt tolerance and promoting resilience to saline conditions (Arkhipova et al. 2020; Hidri et al. 2024). Although the mechanistic studies focusing exclusively on Pseudomonas oryzihabitans remain limited, recent evidence demonstrates that this species can modulate root system architecture through long‐distance, nonvolatile signaling involving redox‐related mechanisms and nutrient transport pathways in Arabidopsis thaliana , supporting its capacity to reprogram host physiology, even in the absence of direct root colonization (Cantabella et al. 2022).

Melon ( Cucumis melo ) is a globally significant fruit crop, with worldwide production reaching 29 million tons in 2023 (Food and Agriculture Organization of the United Nations (FAO) 2025), although its cultivation is often challenged by high soil salinity (Tedeschi et al. 2011). Salinity stress in melon can lead to reduced growth, lower yield, leaf wilting, and fruit deformities such as reduced fruit size and flesh (pulp) content, thereby compromising overall fruit quality (Franco et al. 1993; Mavrogianopoulos et al. 1999; Colla et al. 2006; Akrami and Arzani 2019). Recent studies highlight the potential of PGPR in enhancing melon tolerance to salinity, as inoculation can stimulate root and shoot growth, increase leaf area, enhance photosynthetic efficiency, improve nutrient uptake, and induce stress‐resilient metabolic pathways. As a result, PGPR helps maintain plant vigor, leading to improved growth, yield, and fruit quality (Seido et al. 2019; Gopalakrishnan et al. 2022). By harnessing the beneficial interactions between melon plants and PGPR, melon crop resilience to soil salinity can be increased, leading to improved yield and quality even in challenging growing conditions.

Our study investigates the impact of PGPR inoculation on melon cultivated under salt conditions. In particular, we examined plant growth, nutrient homeostasis, primary metabolism, antioxidant defenses, ABA content, and ABA‐related gene expression in melon plants colonized by the saline‐origin strain P. oryzihabitans AXSa06. By examining these factors, we aim to uncover the mechanisms by which these beneficial microbes enhance melon resilience against salt. This research provides valuable insights into how PGPR help maintain metabolic balance and reinforce antioxidant defenses in melon plants exposed to saline environments.

2. Materials and Methods

2.1. Plant Material, Bacterial Strain, and Preparation of Bacterial Suspension

Seeds of Cucumis melo L. (melon), Greek traditional cultivar “St. Vasileios”, maintained at the Institute of Plant Breeding & Genetic Resources (ELGO), were used to explore plant responses to salt stress. To our knowledge, ‘St. Vasileios’ cultivar salinity tolerance has not been systematically benchmarked against commercial cultivars under standardized NaCl regimes. The PGPR strain used for the inoculation in this study was the AXSa06, isolated from the rhizosphere of halophyte Sarcocornia sp. plants at Axios Delta National Park‐an adverse, highly saline ecosystem‐and identified as Pseudomonas oryzihabitants using a Whole Genome Sequencing approach (Leontidou et al. 2020). Earlier publications include detailed information about its genome, along with its PGP traits and its potential for alleviating salinity stress in tomato plants (Leontidou et al. 2020; Mellidou et al. 2021; Papadopoulou et al. 2022). For seedling inoculation, fresh overnight bacterial cultures were prepared in Nutrient Glycerol (NG) broth (containing per liter: 3.3 g Bacto Peptone, 2.7 g nutrient broth. 2.0 g yeast extract, and 25 mL glycerol) at 28°C with continuous shaking (150 rpm). Bacterial cells were harvested by centrifugation (2200 g, 15 min, 4°C), washed with sterile distilled water, and bacterial pellets were collected and resuspended in water to obtain an inoculum corresponding to approximately 10⁸ colony‐forming units (CFU mL⁻¹), based on optical density measurements at 600 nm (OD₆₀₀)‐CFU correlations, commonly used for bacterial strains.

2.2. Growth Conditions, Inoculation and Salt Stress Treatment

Melon seeds were grown in plastic pots with internal dimensions 8 × 8 × 10 cm (L × W × H) (one plant per pot) containing a peat:perlite mixture (3:1 v/v) under noncontrolled greenhouse conditions. After 10 days, each seedling was inoculated with 4 mL of bacterial suspension through root drenching, while non‐inoculated plants received the same volume of water. Prior to stress application, all plants were supplemented with a plant nutrient solution (PNS) containing essential macro‐ and micronutrients, prepared according to a Hoagland‐type formulation, to ensure baseline nutritional support. Three weeks after sowing, uniformly sized and healthy seedlings were divided into the following groups (15 plants per group): (1) non‐inoculated control plants, (2) inoculated control plants, (3) non‐inoculated salt‐stressed plants, and (4) AXSa06‐inoculated salt‐stressed plants. Irrigation was withheld for 3 days before salinity application in order to standardize substrate moisture content across pots and to minimize uncontrolled leaching during the subsequent salt treatment. Before salinity application, the substrate exhibited low baseline electrical conductivity (EC < 1 dS m⁻¹) as measured using a TEROS 12 soil sensor (METER Group Inc.). Salt stress was applied by irrigating each pot with 100 mL of 250 mM NaCl solution on three occasions over a 7‐day treatment period (stressed plants), while control plants received the same volume of tap water. Substrate salinity was regularly monitored by measuring bulk substrate EC after irrigation, aiming for values between 3.0–3.5 dS m⁻¹. At the end of experimentation (after 7 days of treatment), physiological and growth parameters were evaluated, while leaf tissues from 10 plants per treatment were collected (all fully expanded leaves) following the same procedure across treatments, immediately frozen in liquid nitrogen, and subsequently stored at −80°C until further analysis.

2.3. Growth Parameters, Photosynthetic Efficiency and Leaf Gas Exchange Measurements

At the end of experimentation, the number of leaves was determined by direct counting, plant height and midrib length were measured using a ruler, while shoot diameter was determined with a digital caliper. Above‐ground fresh biomass was determined by weighing the harvested shoots using an analytical balance for 10 plants per treatment. Below‐ground fresh biomass was recorded for five plants following careful removal of the substrate and gentle washing of roots. Chlorophyll content index (CCI, CCM‐200, Opti‐Sciences) and photochemical quantum yield of photosystem II (PSII) (Fv/Fm) (QY, potable fluorometer, FluorPen FP 100, Photon Systems Instruments) were determined at the top fully expanded leaf with 10 replications per treatment. Net photosynthesis (Anet; μmol CO₂ m⁻² s⁻¹), transpiration rate (E; mmol m⁻² s⁻¹) and stomatal conductance (Gs; mol H₂O m⁻² s⁻¹) were determined using a portable gas‐exchange photosynthesis system (LCPROT‐001/BW Advanced Portable Photosynthesis System, ADC Bioscientific Ltd.) in five plants per treatment.

2.4. MDA Content

Lipid peroxidation, quantified as malondialdehyde (MDA) content, was assessed by the thiobarbituric acid (TBA) method described by Heath and Packer (1968). Briefly, 200 mg of frozen melon leaf tissue powder was homogenized in 600 μL of 0.1% trichloroacetic acid (TCA), followed by centrifugation at 15,000 g at 4°C for 20 min. Subsequently, 0.5 mL of the supernatant was mixed with 1.5 mL of 20% TCA containing 0.5% TBA. The mixture was then heated in a water bath at 95°C for 25 min, and the reaction was terminated in an ice bath. After centrifugation at 15,000 g at 4°C for 5 min, the absorbance of the supernatant was measured at 532 and 600 nm. MDA concentration was calculated using its extinction coefficient of 155 μM⁻¹ cm⁻¹. Four biological replicates were analyzed per treatment.

2.5. Enzyme Activity

For enzyme activity analysis, frozen powder of the same leaf samples was used for total protein extraction. Briefly, 500 mg of leaf tissues were purified using an ion‐exclusion Sephadex G‐25 column (PD 10, GE Healthcare), following established protocols (Mellidou et al. 2012). Bovine serum albumin (Sigma) served as standard for protein content quantification of the extracts according to the Bradford method (Bradford 1976). Subsequently, the activities of catalase (EC 1.11.1.6, CAT), and superoxide dismutase (SOD; EC 1.15.1.1) were determined using optimized “in‐house” protocols (Mellidou et al. 2012). Four biological replicates were analyzed per treatment.

2.6. ABA Content

ABA concentration was determined using the Phytodetek Immunoassay Kit for ABA (Agdia) according to the manufacturer's instructions. For the extraction process, 200 mg of frozen leaf tissue powder was homogenized with 500 μL of 90% methanol containing 200 mg L⁻¹ diethyldithiocarbamate trihydrate in Eppendorf tubes. The mixture was incubated overnight at 4°C to allow thorough extraction. After incubation, the samples were centrifuged at 8000 g for 10 min at 4°C. The supernatant was transferred into precooled 1.5 mL Eppendorf tubes and evaporated to dryness using nitrogen flow. The dried residues were then reconstituted in 500 μL of methanol‐Tris buffer and stored at −20°C until further analysis using the ABA ELISA assay. ABA concentrations were quantified by measuring absorbance at 405 nm, and results were expressed as ng g⁻¹ of fresh weight. Three biological replicates were analyzed per treatment.

2.7. Quantitative Real‐Time PCR (qRT‐PCR)

Total RNA was isolated from frozen melon leaf tissues (200 mg; same pool as the above assays) using an RNeasy Plant Mini Kit (QIAGEN, Qiagen Inc.), following the manufacturer's instructions, with three biological replicates per treatment. RNA concentration and purity were assessed using Nanodrop 2000 (NanoDrop Technologies) and 1% agarose gel electrophoresis prior to downstream analyses. Six genes related to ABA signaling, including CmPYL9 (MELO3C006033.2) and five CmPP2AC isoforms (MELO3C010694.2, MELO3C011469.2, MELO3C009297.2, MELO3C018362.2, MELO3C011713.2), were analyzed using qRT‐PCR. All sequences were retrieved from the Melonomics database (https://www.melonomics.net/; Organism: Cucumis melo L., cultivar DHL92, genome version v3.6.1). RNA samples underwent first‐strand complementary DNA (cDNA) synthesis using the QuantiTect Rev. Transcription Kit for cDNA synthesis (QIAGEN, Qiagen Inc.) according to the manufacturer's instructions. Gene expression analysis was conducted using the QuantiNova SYBR Green PCR Kit (QIAGEN) in the QuantStudio 5 Real‐Time PCR system (Applied Biosystems), following the manufacturer's instructions. Expression data were normalized against the melon β‐actin gene. The qPCR program protocol, adapted from Xanthopoulou et al. (2021) with minor adjustments, included an initial denaturation step at 95°C for 60 s, followed by a forty‐cycle program involving denaturation at 95°C for 15 s and extension at 60°C for 20 s. A melt curve analysis was constructed to confirm the specificity of PCR products, with the cycle threshold (Ct) recorded at 0.2 ΔRn. Data analysis was performed using the ΔΔCt method (Livak and Schmittgen 2001). Primer sequences for all analyzed genes are listed in Table S1.

2.8. Nutrient Content Determination

The aboveground tissue from six plants (two plants pooled per replicate) was dried at 70°C until constant weight was achieved, weighed, and ground to determine the nutrient content, as previously described (Papadopoulou et al. 2022). Subsequently, samples underwent ashing at 500°C for a minimum of 4 h (Mills and Jones 1996). The resulting ash was dissolved in 2 M HCl, filtered, and the filtrate was analyzed for phosphorus (P) using the molybdenum blue‐ascorbic acid method (Kuo 1996). Potassium (K⁺) and sodium (Na⁺) levels were determined via flame photometry, while calcium (Ca²⁺), magnesium (Mg²⁺), copper (Cu), zinc (Zn), iron (Fe), and manganese (Mn) were quantified using atomic absorption spectrometry, following standard analytical procedures.

2.9. Untargeted Primary Metabolic Profiling in Leaf Tissue by GC–MS

Leaf polar metabolite analysis was conducted following established protocols (Ainalidou et al. 2016). Briefly, frozen powder (500 mg) was transferred into 2 mL screw cap tubes containing 1400 μL of precooled methanol (−20°C). Adonitol (100 μL of 0.2 mg mL⁻¹) was added as an internal quantitative standard, followed by vortexing and incubation at 70°C for 10 min. After centrifugation (11,000 g, 4°C, 10 min), supernatants were transferred to glass vials and 750 μL of precooled chloroform (−20°C) and 1500 μL of dH₂O (4°C) were added. After another centrifugation (2200 g, 4°C, 15 min), 150 μL from the upper polar phase was transferred into 1.5 mL glass vials and placed in a vacuum desiccator with anhydrous CaCl₂ for overnight drying. For derivatization, dried residues were dissolved in 40 μL of 20 mg mL⁻¹ methoxyamine hydrochloride in pyridine and gently shaken at 37°C for 120 min. Afterwards, 70 μL of N‐Methyl‐N‐(trimethylsilyl)‐trifluoroacetamide (MSTFA) was added, and samples were incubated at 37°C for 30 min and subsequently stored at −20°C until further GC–MS analysis. GC–MS analysis was performed using a Thermo Trace Ultra GC equipped with ISQ MS and a TriPlus RSH autosampler. The system was programmed as previously described (Papadopoulou et al. 2023), and sample volumes of 1 μL were injected. Peak area integration and chromatogram visualization were performed using the Xcalibur processing program. The NIST11 database was utilized for peak identification, and mass spectra were cross‐referenced with authentic standards available in the GOLM metabolome database. Quantification of the detected metabolites was assessed based on the relative response compared to adonitol, serving as the internal standard. Results were expressed as relative abundance, with four biological replicates analyzed for each treatment.

2.10. Statistical Analysis

Data analysis was conducted using IBM SPSS Statistics (version 25; SPSS Inc.) and GraphPad Prism (version 10; GraphPad Software). Experiments followed a factorial design, and data for growth, photosynthetic parameters, antioxidant levels, nutrient elements, ABA content, and gene expression were analyzed using two‐way analysis of variance (ANOVA) with bacterial inoculation and salinity as fixed factors. To address specific biological questions, pairwise comparisons were conducted using two‐tailed Student's t‐tests to evaluate (1) the effect of bacterial inoculation within each salinity condition and (2) the effect of salinity within each inoculation status. When a significant interaction between bacterial inoculation and salinity was detected, pairwise comparisons were used to interpret the interaction. When the interaction was not significant, pairwise comparisons were reported to facilitate interpretation of condition‐specific responses. Prior to statistical analysis, data were tested for compliance with parametric assumptions. Normality within groups was assessed using the Shapiro–Wilk test, and homogeneity of variances across groups was evaluated using Levene's test. Significant statistical differences (p ≤ 0.05) were visualized in bar plots and denoted with asterisks according to conventional thresholds. Metabolite data were analyzed separately, as metabolomic profiling was treated as an exploratory comparison across all treatments. For individual metabolites, one‐way analysis of variance (ANOVA) followed by Duncan's Multiple Range Test (p ≤ 0.05) was performed to manage multiple comparisons and to identify statistically distinct groups across treatments. Principal Component Analysis (PCA) was conducted to visualize overall treatment‐related differences in metabolite accumulation patterns. Additionally, targeted comparisons using Student's t‐tests were performed to selected key metabolites of particular biological interest to provide a focused analysis on relevant changes.

3. Results

3.1. Enhanced Growth and Photosynthetic Performance Upon AXSa06 Inoculation

Two‐way ANOVA revealed significant effects of salinity and bacterial inoculation on growth‐related traits, with significant inoculation × salinity interactions detected for several parameters (Table S2). Consistent with these results, the evaluation of individual growth parameters revealed notable effects of inoculation on the growth performance of stressed melon plants. Specifically, inoculation under stress conditions resulted in a 19% increase in plant height compared to controls (Figure 1A), while the number of leaves per plant reduced significantly (40%) under stress only in non‐inoculated plants (Figure 1B). Additionally, shoot diameter and midrib length exhibited more pronounced decreases in the absence of inoculum under salinity, while inoculated plants showed a mitigated decline (Figure 1C–E). For above‐ground fresh biomass, two‐way ANOVA indicated a significant main effect of salinity and inoculation, whereas the inoculation × salinity interaction was not significant (Table S2). Accordingly, salinity was associated with a marked reduction in shoot biomass, with inoculated plants exhibiting higher biomass values under stress compared to non‐inoculated plants (Figure 1E). Below‐ground biomass was slightly enhanced by AXSA06 inoculation under control conditions (27%). Under salt stress, root biomass values were reduced relative to this elevated baseline (−25%). However, two‐way ANOVA indicated that salinity was the primary factor affecting root biomass, while neither inoculation nor the interaction was significant (Figure 1F; Table S2). Thus, the apparent reduction reflects the strong prior stimulation under non‐stress conditions rather than a negative effect of inoculation. Overall, inoculation with AXSa06 promoted plant growth under salt stress conditions (Figure 1G).

FIGURE 1.

Changes in (A) plant height (cm), (B) number of leaves, (C) shoot diameter (mm), (D) midrib length (cm), (E) above‐ground fresh biomass (g) and (F) below‐ground fresh biomass (g) in non‐inoculated and AXSa06‐inoculated melon plants grown under control and salt stress conditions. (G) Representative images of non‐inoculated and AXSa06‐inoculated melon plants under control and salt stress conditions. Data are shown as mean values ± SD (n = 10 for A–E; n = 5 for F). Asterisks indicate significant differences between treatments, as determined by a two‐tailed t‐test (*p < 0.05; **p < 0.005; ***p < 0.001; ****p < 0.0001; ns, nonsignificant).

Regarding photosynthetic traits, two‐way ANOVA revealed significant main effects of salinity on CCI, Anet, and Gs, with a significant inoculation × salinity interaction detected exclusively for QY. In addition, significant main effects of inoculation were observed for QY and Anet (Table S2). In particular, CCI increased under salinity in both inoculated (24.8%) and non‐inoculated plants (15.8%), indicating a general salinity‐driven response. Yet the rise was significantly greater in the presence of AXSa06 (Figure 2A). In contrast, QY increased only in inoculated stressed plants, reflecting a condition‐dependent effect of AXSa06 inoculation (Figure 2B). Additionally, the net photosynthetic rate (Anet) decreased solely in non‐inoculated stressed plants, suggesting a mitigating effect of PGPR on photosynthetic decline (Figure 2C). Stomatal conductance (Gs) showed significant reductions in response to salinity, regardless of inoculation, while the transpiration rate (E) also tended to decrease in both stressed groups, although this effect was not statistically significant (Figure 2D,E).

FIGURE 2.

Changes in (A) chlorophyll content index (CCI), (B) quantum yield (QY), (C) net photosynthesis (Anet), (D) stomatal conductance (Gs), and (E) transpiration rate in non‐inoculated and AXSa06‐inoculated melon plants grown under control and salt stress conditions. Data represent mean values ± SD (n = 10 for A, B; n = 5 for C–E). Asterisks indicate significant differences between treatments, as determined by a two‐tailed t‐test (*p < 0.05; **p < 0.005; ****p < 0.0001; ns, nonsignificant).

3.2. Biochemical Responses and Nutrient Dynamics in Melon Plants Upon AXSa06 Inoculation at Exposure to Salt Stress

Two‐way ANOVA revealed significant effects of salinity on antioxidant‐related parameters and ABA. Significant inoculation × salinity interactions were detected for CAT, SOD, and MDA, while a significant main effect of inoculation was additionally observed for MDA and SOD (Table S2). Under control conditions, CAT activity increased significantly by 49% upon inoculation, while salinity stress induced a further 31.2% increase. Nevertheless, in the absence of inoculation, the increase was more pronounced, reaching 167% (Figure 3A). Similarly, SOD activity increased under stress conditions regardless of inoculation. However, the increase was more pronounced in non‐inoculated plants (85%) compared to inoculated plants (19%) (Figure 3B). Additionally, MDA content rose significantly by 191% under control conditions in the presence of AXSa06, but did not show any further increase under stress. Conversely, MDA levels increased markedly by 195% only in non‐inoculated plants subjected to stress (Figure 3C). Under salt stress, ABA levels increased significantly in both treatments, indicating a salinity‐driven response. Although inoculated plants displayed higher ABA levels under stress, approximately fourfold compared to non‐stressed conditions, two‐way ANOVA indicated that salinity was the primary factor affecting ABA accumulation (Figure 3D).

FIGURE 3.

Changes in (A) catalase activity (CAT), (B) superoxide dismutase activity (SOD), (C) malondialdehyde (MDA) content, and (D) ABA content in non‐inoculated and AXSa06‐inoculated melon plants grown under control and salt stress conditions. Data are shown in mean ± SD (n = 4 for A–C; n = 3 for D). Asterisks indicate significant differences between treatments, as determined by a two‐tailed t‐test (*p < 0.05; **p < 0.005; ***p < 0.001; ****p < 0.0001; ns, nonsignificant).

For leaf mineral nutrients, two‐way ANOVA revealed significant inoculation × salinity interactions for P, K⁺, and Na⁺. In addition, significant main effects of inoculation were observed for Na⁺, Zn, and Fe, while salinity also exerted significant main effects on Na⁺ and K⁺ (Table S2). Specifically, P levels exhibited a condition‐dependent response to salinity, with salinity increasing P accumulation in non‐inoculated plants but not in inoculated plants (Figure 4A). Potassium levels decreased under salinity, with a significant interaction indicating that the magnitude of this reduction differed between inoculated and non‐inoculated plants, with inoculated plants showing a smaller reduction (Figure 4B). Na⁺ levels increased significantly under salt stress in both inoculated and non‐inoculated plants, although the increase was less prominent in inoculated plants (Figure 4E). Concentrations of Ca²⁺, Mg²⁺, Cu, Mn, and B remained relatively stable across treatments, showing no significant variations (Figure 4).

FIGURE 4.

Changes in nutrient concentrations in non‐inoculated and AXSa06‐inoculated melon plants grown under control and salt stress conditions. Data are shown in mean values ± SD (n = 3). Asterisks indicate significant differences between treatments, as determined by two‐tailed t‐test (*p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; ns, nonsignificant).

3.3. Expression of ABA‐Related Genes in Response to AXSa06 Inoculation and Salt Stress

To explore the molecular basis of the observed ABA accumulation, we quantified the transcript levels of genes associated with ABA perception and signaling, including CmPYL9 (MELO3C006033.2) and several CmPP2AC isoforms (MELO3C010694.2, MELO3C011469.2, MELO3C009297.2, MELO3C018362.2, MELO3C011713.2). Across these genes, two‐way ANOVA revealed significant main effects of both salinity and bacterial inoculation, as well as significant inoculation × salinity interactions for all of them (Table S2). In non‐inoculated plants, CmPYL9 transcript levels were consistently detectable but remained low under both control and salt stress conditions (Figure 5A), indicating limited induction of ABA receptor genes. Similarly, the expression of CmPP2AC isoforms showed only minor fluctuations in non‐inoculated plants (Figure 5B–F), suggesting that the inhibitory branch of ABA signaling was not strongly modulated. In contrast, in AXSa06‐inoculated plants, CmPYL9 expression was already elevated under control conditions and exhibited a further pronounced increase under salinity, reflecting a strong salinity‐dependent induction (Figure 5A). In addition, all analyzed CmPP2AC isoforms displayed a marked salinity‐induced upregulation specifically in inoculated plants, consistent with the significant inoculation × salinity interactions detected by ANOVA (Figure 5B–F). These results highlight the substantial impact of AXSa06 inoculation on ABA signaling‐related transcriptional responses under salt stress. The numerical values underlying the relative expression data shown in Figure 5 are provided in Table S3.

FIGURE 5.

Relative expression of ABA‐related genes in non‐inoculated and AXSa06‐inoculated melon plants grown under control and salt stress conditions. (A) CmPYL9 and (B–F) CmPP2AC isoforms. Data are shown as mean values ± SD (n = 3). Asterisks indicate significant differences between treatments, as determined by two‐tailed t‐test (*p < 0.05; **p < 0.01; ****p < 0.0001; ns, nonsignificant). Expression data were normalized against the melon Actin gene.

3.4. Primary Metabolite Responses to AXSa06 Inoculation and Exposure to Salt Stress

A total of 40 metabolites were identified in melon leaf tissues (Figure 6A). The PCA was employed to visualize the relationships between treatments based on metabolite accumulation. The four treatment groups were distinctly separated in the PCA plot, indicating significant differences at the primary metabolite level. PCA revealed that PC1 accounted for 49.1% of the total variance and primarily separated samples according to salinity treatment, with control samples positioned on the negative side and salt‐stressed samples on the positive side of PC1. PC2 explained an additional 19.7% of the variance and mainly reflected the effect of AXSa06 inoculation, distinguishing inoculated from non‐inoculated plants (Figure 6B). This indicated that both the presence of AXSa06 and salt stress had a clear impact on metabolite profiles. Specifically, inoculation alone under control conditions led to differential accumulation of 13 metabolites, with nine increasing and four decreasing. Regarding stress treatments, 17 metabolites were differentially accumulated in non‐inoculated plants, with eight being increased and nine being decreased compared to their respective controls. Accordingly, in AXSa06‐inoculated plants, 16 metabolites showed differential accumulation, with nine being increased and seven being decreased in response to stress, compared to their respective controls (Table S4).

FIGURE 6.

(A) Heatmap illustration of the differential accumulation of primary metabolites in non‐inoculated and AXSa06‐inoculated melon plants grown under control (0 mM) and salt stress (250 mM NaCl) conditions. Data are presented as Z‐score‐normalized values. (B) Principal Component Analysis (PCA) plot displaying the overall metabolite profiles for all treatment groups.

The results indicated that inoculation with the AXSa06 strain led to significant metabolic adjustment in melon plants, both under control and salt stress conditions (Figure 6). Key metabolites, including galactose, arabitol, oxalate, succinate, glycerate, erythro‐pentonate, threonine, alanine, valine, and γ‐aminobutyrate (GABA), were elevated following AXSa06 inoculation, while serine and citrate decreased, suggesting a shift in metabolic pathways even before exposure to stress. Salt stress notably influenced sugar levels; in non‐inoculated plants, sucrose, 2‐deoxy‐inosose, and 2‐deoxy‐D‐ribose concentrations increased, while fructose and glucose decreased. By contrast, in AXSa06‐inoculated plants under salt stress, sucrose, 2‐deoxy‐D‐ribose, and turanose levels increased, whereas fructose, glucose, and galactose concentrations decreased. Additionally, myo‐inositol levels increased in all plants under stress, while arabitol was elevated only in non‐inoculated plants. For organic acids, stress induced distinct effects. In particular, non‐inoculated plants showed increased levels of glutaconate, gulonate, and 2‐keto‐l‐gluconate, accompanied by decreased concentrations of glycerate, gluconate, fumarate, citrate, and malate. In AXSa06‐inoculated plants, however, citrate and threonate increased, while oxalate, succinate, glycerate, malate, and gluconate decreased. The amino acid profile revealed reductions in alanine, serine, aspartate, and GABA in non‐inoculated plants under stress, while cystathionine and serine levels increased in inoculated plants. Putrescine levels significantly decreased under stress only in non‐inoculated plants.

The heatmap provided a global overview of treatment‐dependent metabolic changes, indicating that AXSa06 inoculation predominantly affected amino acid and organic acid metabolism, whereas carbohydrate‐related metabolites were mainly influenced by salinity, particularly in inoculated plants (Figure 6A). To further refine these patterns, pairwise t‐test analysis was performed, confirming significant differences (p < 0.05) for several metabolites of interest. Figure 7 presents these metabolites, highlighting distinct metabolic signatures associated with priming under non‐stress conditions and with stress‐responsive metabolic adjustments under salinity. Together, these results indicate a coordinated metabolic reconfiguration that is consistent with enhanced stress resilience in AXSa06‐inoculated melon plants.

FIGURE 7.

Schematic representation of the metabolites most strongly affected by AXSa06 inoculation under control (0 mΜ) and salinity (250 mM) conditions in melon leaves. Metabolites are grouped according to their involvement in amino acid the TCA cycle metabolism, as well as carbohydrate metabolism. (A) Amino acid and TCA‐related metabolites that are differentially accumulated mainly upon AXSa06 inoculation under control conditions. Metabolites shown in red indicate significant increases compared to their respective non‐inoculated control plants, while those in blue indicate significant decreases. (B) Carbohydrate‐related metabolites that are differentially accumulated mainly upon AXSa06 inoculation under salinity stress conditions. Metabolites shown in red indicate significant increases compared to their respective AXSa06‐inoculated control plants, while those in blue indicate significant decreases. Data are presented as mean ± SD (n = 4). Asterisks indicate significant differences between treatments, as determined by t‐test (*p < 0.05; **p < 0.005; ***p < 0.001; ****p < 0.0001; ns, nonsignificant).

4. Discussion

The present study demonstrated that inoculation with the PGPR strain AXSa06 significantly enhanced melon growth and stress tolerance under salt stress conditions. The selected strain was originally isolated from the rhizosphere of plants in the National Park of Delta Axios (Northern Greece), a Mediterranean coastal wetland characterized by alkaline (pH > 8.0) and highly saline‐sodic soils (Leontidou et al. 2020). As a halophytic ecosystem, this habitat harbors native plants and associated microbes naturally adapted to high salt concentrations. Consistent with its origin, AXSa06 has been shown to tolerate considerable salinity, demonstrating growth at NaCl concentrations up to 3.25 M (Papadopoulou et al. 2022), thus contributing to enhanced salt tolerance in tomato upon salt stress (Mellidou et al. 2021). Our findings further support previous reports highlighting the capacity of rhizobacteria to mitigate abiotic stresses in melon through various mechanisms, including enhanced nutrient uptake, modulation of antioxidant responses, and metabolic adjustments (Gopalakrishnan et al. 2022; Murgese et al. 2020; Unnikrishnan et al. 2023).

4.1. AXSa06 Mitigates Growth Inhibition and Maintains Photosynthetic Performance Upon Salt Stress

High salinity disrupts water uptake and nutrient balance, leading to stunted shoot growth, smaller leaves, and reduced overall biomass production in plants (Hasanuzzaman and Fujita 2022). In melon plants, AXSa06 inoculation significantly alleviated salt‐induced growth inhibition, resulting in less pronounced reductions in plant height, midrib length, shoot diameter, leaf number, and above‐ground biomass compared to non‐inoculated stressed plants. Although the relative reduction in root biomass under salinity appeared more pronounced in AXSa06‐inoculated plants, this pattern reflects the higher root biomass attained under control conditions rather than the increased sensitivity to salt stress. Under salinity, root growth was strongly constrained in both inoculated and non‐inoculated plants, indicating that salt‐induced inhibition of root development represents a dominant physiological limitation that cannot be fully alleviated by inoculation. Overall, AXSa06 significantly reduced the severity of stress‐related growth inhibition across multiple shoot‐related traits compared to non‐inoculated plants. These findings are consistent with the well‐documented role of PGPR in mitigating growth suppression under salt stress (Bhat et al. 2020; Ha‐Tran et al. 2021; Mellidou et al. 2021; Shultana et al. 2022).

Salinity stress often disrupts photosynthetic performance, leading to declines in gas exchange parameters such as Anet, Gs, and E (Parihar et al. 2015). In the present study, salinity caused a significant reduction in Anet in non‐inoculated plants, whereas AXSa06‐inoculated plants maintained significantly higher Anet levels under salt stress. Stomatal conductance (Gs) was significantly reduced by salinity in both inoculated and non‐inoculated plants. However, AXSa06‐inoculated plants exhibited higher Gs values under stress compared to non‐inoculated plants. In contrast, transpiration rate (E) was not significantly affected by either salinity or inoculation, indicating that AXSa06 did not markedly alter transpirational water loss under the conditions tested. The maintenance of higher Anet in inoculated plants, despite comparable reductions in E, suggests that AXSa06 may support photosynthetic performance primarily through non‐stomatal mechanisms, potentially involving biochemical or mesophyll‐related processes, rather than through changes in transpiration. Similar non‐stomatal limitations under salinity have been reported previously (Liu et al. 2019). CCI increased under salinity in both inoculated and non‐inoculated plants, with the rise being more pronounced in the presence of AXSa06. However, CCI is an optical index that reflects chlorophyll concentration per unit leaf area and is strongly influenced by leaf structural traits rather than representing a direct measure of total chlorophyll content per plant (Richardson et al. 2002). Notably, the greater increase in CCI observed in AXSa06‐inoculated plants coincided with the maintenance of QY and Αnet under stress, suggesting functional preservation of the photosynthetic apparatus rather than a purely optical artifact. These results are aligned with reports showing that PGPR inoculation can enhance photosynthetic performance in Cucumis species exposed to salinity stress (Li et al. 2020; Saberi‐Riseh et al. 2020; Zhou et al. 2022).

4.2. AXSa06 Inoculation Reduces Oxidative Damage and Enhances ABA‐Mediated Stress Adaptation

One of the key outcomes of the study was the differential responses of antioxidant enzymes and MDA under various treatments. In non‐inoculated plants subjected to salt stress, elevated CAT and SOD activity, along with significantly higher MDA levels, reflected heightened oxidative stress. In combination with concurrent declines in photosynthetic performance and growth‐related parameters, this oxidative imbalance indicates cellular dysfunction and membrane damage (Morales and Munné‐Bosch 2019). In contrast, AXSa06‐inoculated plants displayed less pronounced antioxidant enzyme activity and stable MDA levels under salinity stress, suggesting that the strain effectively limited oxidative damage. This mitigation of oxidative stress likely contributed to the improved growth and photosynthetic performance observed in inoculated plants under stress conditions. Although MDA is typically regarded as an oxidative stress marker, moderate increases in lipid peroxidation products have recently been proposed to act as signaling cues involved in the activation of antioxidant defenses and stress acclimation, rather than reflecting irreversible damage (Schmid‐Siegert et al. 2012; Morales and Munné‐Bosch 2019; Munoz et al. 2018). Notably, AXSa06 increased MDA levels under control conditions, which may reflect a mild oxidative signal contributing to the priming of plant defense mechanisms before stress exposure. This interpretation is consistent with previous findings in inoculated tomato plants treated with the same strain (Mellidou et al. 2021), where early activation of defense‐related responses was associated with enhanced stress tolerance. Nevertheless, transient oxidative signaling linked to bacterial colonization or metabolic adjustments during early plant–microbe interactions may also contribute to the observed priming effect. Additionally, CAT activity was enhanced in inoculated plants, in accordance with earlier studies (Bhattacharyya et al. 2020; Mellidou et al. 2021; Papadopoulou et al. 2022), supporting the notion that AXSa06 induces a primed antioxidant state.

PGPR are known to modulate hormone signaling pathways in plants (Jha and Saraf 2012; Jalmi and Sinha 2022). Among these, ABA plays a critical role in abiotic stress responses by regulating stomatal closure, osmoprotection, and the activation of stress‐responsive genes (Fujii et al. 2009). In our study, inoculated plants accumulated higher ABA levels under stress, indicating a stronger hormonal adjustment. This response may have supported enhanced stomatal regulation, limiting water loss while sustaining higher Gs and maintaining photosynthetic activity compared with non‐inoculated stressed plants. In parallel, AXSa06 inoculation also influenced sugar metabolism, with elevated turanose and sucrose levels under stress, consistent with the role of ABA in regulating carbohydrate dynamics during abiotic stress (Kempa et al. 2008; Mathan et al. 2021). At the transcriptional level, AXSa06 strongly and consistently induced CmPYL9, a core ABA receptor involved in signal perception (Weiner et al. 2010), along with several CmPP2C isoforms. The CmPYL9 upregulation may have provided more receptor sites to perceive ABA signals, while the concomitant induction of CmPP2Cs indicated a dynamic feedback mechanism. Although CmPP2Cs are generally negative regulators of ABA signaling (Cutler et al. 2010), their induction alongside CmPYL9 may reflect a fine‐tuning strategy to prevent excessive signaling, maintaining a balanced ABA response. Previous studies demonstrated that PYL–PP2C interactions can stabilize receptor function under stress (Ma et al. 2009; Santiago et al. 2009), which supports our finding that AXSa06 inoculation primes a more efficient and tightly regulated ABA signaling cascade. Overall, this dual modulation, elevated ABA accumulation along with a reconfigured receptor–regulator network, highlighted a likely mechanism by which AXSa06 enhances the resilience of melon plants to salinity. Further research is needed to clarify the molecular mechanisms by which AXSa06 shapes ABA signaling to improve stress tolerance in melon.

4.3. AXSa06 Inoculation Reduces Na⁺ Accumulation and Maintains Nutrient Homeostasis Under Salinity

The variation in nutrient concentrations across treatments highlights the distinct effects of salinity and AXSa06 inoculation on ion homeostasis. Under salt stress, Na⁺ levels increased in both inoculated and non‐inoculated plants. Ηowever, the rise was notably lower in inoculated plants, pointing to the role of AXSa06 in mitigating Na⁺ accumulation. Previous research has shown that salt‐tolerant PGPR strains, likewise the AXSa06 strain, produce extracellular polysaccharides (EPS) (Papadopoulou et al. 2022) capable of binding Na⁺ in the rhizosphere, thereby reducing its uptake by plants (Shultana et al. 2022). This mitigation not only alleviates osmotic stress but also helps maintain nutrient balance, supporting plant growth and stress resilience. Salinity also disrupted the uptake of essential nutrients, particularly reducing K⁺ and Fe levels in non‐inoculated plants, likely due to ionic imbalance and membrane dysfunction, a well‐documented response under salt stress (Munns and Tester 2008; Lopez‐Zaplana et al. 2022). In contrast, AXSa06‐inoculated plants exhibited a more balanced nutrient profile, with moderated changes in K⁺ and Fe, indicating improved nutrient retention under stress. Additionally, salinity increased P and Zn concentrations, especially in non‐inoculated plants. Elevated P under stress has previously been reported in melon (Uygur and Yetisir 2009), and may support increased energy demands associated with stress adaptation (Kaya et al. 2001), while higher Zn levels may contribute to antioxidant enzyme function (Marreiro et al. 2017). Interestingly, the presence of the AXSa06 strain effectively regulated nutrient levels in the plants, preventing both their over‐ and under‐accumulation, indicating its crucial role in maintaining nutrient homeostasis under stress conditions.

4.4. AXSa06 Inoculation Primes Amino Acid and TCA Metabolism, and Reprograms Carbohydrate Pathways Under Salt Stress

Metabolic profiling of AXSa06‐inoculated melon plants revealed distinct adjustments under both control and stress conditions, particularly in amino acid, TCA cycle, and carbohydrate‐related metabolites, indicating a complex reprogramming process that supports stress tolerance. Even in the absence of stress, inoculation triggered a reconfiguration of primary metabolism, with notable increases in amino acids and TCA intermediates. Elevated levels of valine, alanine, and GABA, together with metabolites such as oxalate and succinate, suggest that AXSa06 primes the plant metabolic system prior to stress exposure. Elevated succinate, together with increased oxalate, reflects enhanced TCA activity and associated pathways. Oxalate supports detoxification and calcium regulation, while succinate, as a central TCA intermediate, contributes to energy generation needed for sustaining cellular defenses under stress (Li et al. 2022; Karabourniotis et al. 2020).

Valine, a branched‐chain amino acid, supports protein stability and serves as a precursor for various osmoprotective metabolites (Ganie 2021; Ingrisano et al. 2023). Alanine links amino acid and carbon metabolism through its conversion to pyruvate, facilitating rapid metabolic adjustments under stress (Kim et al. 2007; Ganie 2021). The accumulation of GABA further underscores its priming role in stress tolerance, where it functions as a signaling molecule, an osmoprotectant, and a stabilizer of cellular pH. Furthermore, through the GABA shunt, it also connects succinate metabolism directly to the TCA cycle, providing an alternative pathway for succinate production (Ansari et al. 2021). Consistent with this, inoculated plants displayed increased succinate and oxalate levels, pointing to enhanced TCA activity and associated metabolic pathways that help sustain energy balance and cellular homeostasis under salinity stress. Regarding non‐inoculated stressed plants, significant increases in sugars in response to salinity stress, such as sucrose, 2‐deoxy‐D‐ribose and 2‐deoxy‐inosose, together with the sugar alcohols myo‐inositol and arabitol, suggest their role as osmoprotectants and signaling molecules that help maintain cell turgor and modulate stress responses (Singh et al. 2015; Jeandet et al. 2022). In contrast, the reduction in key stress‐responsive compounds, like alanine, serine, aspartate, GABA, and putrescine (Batista‐Silva et al. 2019), along with decreases in TCA cycle intermediates (malate, citrate, succinate), reflects metabolic imbalance and impaired stress adaptation. These patterns highlight the contrasting metabolic regulation in non‐inoculated versus inoculated plants.

On the other hand, AXSa06‐inoculated stressed plants displayed pronounced metabolic shifts, with primary changes in carbohydrate metabolism and secondary adjustments in TCA cycle‐related metabolites, which were already modulated under control conditions. Elevated levels of sucrose, 2‐deoxy‐d‐ribose, turanose, and arabitol suggest an increased role of these carbohydrates in osmoprotection and signaling. Notably, turanose has been shown to enhance plant stress tolerance by activating diverse sugar metabolism‐related signaling pathways and by regulating source‐sink dynamics (Khalil et al. 2021). Similarly, Dixit et al. (2023) reported enhanced accumulation of turanose in PGPR‐inoculated tomato plants exposed to salinity stress, following inoculation with Bacillus velezensis , highlighting the involvement of sugar metabolism in PGPR‐induced stress adaptation. Stress also increased serine and citrate levels, which were reduced upon inoculation under control conditions. This indicates a dynamic adjustment in which inoculation primes these metabolites, boosting their availability when needed for stress resilience. Putrescine levels decreased under salinity in both treatments, although the decrease was less pronounced in AXSa06‐inoculated plants. Putrescine, a key polyamine, supports cell growth, differentiation, and stress adaptation by functioning as an osmoprotectant and stabilizing cellular structures (Gill and Tuteja 2010).

Taken together, our results support a model in which AXSa06 acts as a multifaceted priming agent that integrates early signaling events with transcriptional, metabolic, and physiological reprogramming, thereby enhancing melon tolerance to salinity stress. AXSa06 colonization under non‐stress conditions likely initiates low‐intensity oxidative and hormonal signaling, as reflected by elevated basal MDA levels, increased CAT activity, and detectable CmPYL9 expression, establishing a primed physiological state rather than causing cellular damage.

Upon exposure to salinity, this primed state enables a more efficient ABA‐mediated response, characterized by increased ABA accumulation and strong induction of CmPYL9 alongside dynamic regulation of CmPP2AC isoforms, suggesting optimized stress perception coupled with controlled feedback regulation. In parallel, AXSa06 limits Na⁺ accumulation and moderates salinity‐induced nutrient imbalance, supporting cellular stability. At the metabolic level, AXSa06 appears to promote a two‐phase reprogramming pattern. Under control conditions, inoculation seems to reshape amino acid and TCA‐related metabolism, thereby reinforcing central carbon fluxes and redox buffering capacity, while under salinity, it facilitates a strategic shift toward carbohydrate accumulation, supporting osmoprotection, energy supply, and signaling functions. Simultaneously, the attenuated decline of stress‐sensitive metabolites in inoculated plants indicates improved metabolic stability rather than simple stress avoidance. These interconnected responses converge at the physiological level, where AXSa06‐inoculated plants maintain higher photosynthetic performance, balanced antioxidant activity, and moderated growth inhibition under salinity. Collectively, our findings indicate that AXSa06 does not act through a single protective pathway, but rather through an integrated priming mechanism that links early signaling events to transcriptional regulation, metabolic plasticity, and whole‐plant physiological resilience.

5. Conclusions

Collectively, our findings indicate that AXSa06 does not act through a single protective pathway, but rather through an integrated priming mechanism that links early signaling events to transcriptional regulation, metabolic plasticity, and whole‐plant physiological resilience. Overall, AXSa06‐mediated stress tolerance involves: (1) antioxidant preconditioning, (2) improved nutrient homeostasis, (3) reconfiguration of ABA signaling for stress sensing and response, and (4) coordinated metabolic plasticity that optimizes energy balance and osmotic adjustment. This integrated framework not only deepens our understanding of PGPR‐mediated stress tolerance but also underscores the potential of AXSa06 as a biostimulant for sustainable crop production under saline conditions. Future research should further dissect the molecular basis of this ABA signaling reconfiguration and evaluate the performance of AXSa06 across different crop species and developmental stages.

Author Contributions

Anastasia Papadopoulou: data curation, formal analysis, investigation, methodology, visualization, writing – original draft, writing – review and editing. Ifigeneia Gkourmi: investigation. Theodora Matsi: methodology, resources. Aliki Xanthopoulou: methodology, formal analysis, writing – review and editing. Kalliopi Kadoglidou: methodology, formal analysis, writing – review and editing. Aikaterini Triantafyllou: investigation. Katerina Karamanoli: methodology, supervision, writing – review and editing. Ifigeneia Mellidou: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, supervision, writing – review, and editing.

Funding

This project was funded by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Innovation (GSRI), under Grant Agreement No. 705 (2018–2022), awarded to IM.

Supporting information

Table S1: Details of primers sets used for qRT‐PCR analysis.

Table S2: Two‐way ANOVA of growth, physiological, biochemical and gene expression traits in melon plants in response to bacterial inoculation and salinity.

Table S3: Relative expression levels of ABA‐related genesin Cucumis melo L. leaves under control (0 mM NaCl) and salt stress (250 mM NaCl), in non‐inoculated and AXSa06‐inoculated plants.

Table S4: Relative concentrations of primary metabolites detected in Cucumis melo L. leaves under control (0 mM NaCl) and salt stress (250 mM NaCl), in non‐inoculated and AXSa06‐inoculated plants.

Papadopoulou, A. , Gkourmi I., Matsi T., et al. 2026. “Priming Salt Resilience in Melon ( Cucumis melo ) via Pseudomonas oryzihabitans ‐Mediated Metabolic Reprogramming.” Physiologia Plantarum 178, no. 2: e70866. 10.1111/ppl.70866.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.