Table 1.
Mechanisms of drought stress tolerance through fungal symbiosis in plants.
| Stress tolerance at Biochemical Level | ||
|---|---|---|
| Specific Mechanisms | Representative examples | References |
|
Antioxidant mechanisms
(Generation of diverse antioxidants through a symbiotic relationship thereby mitigating drought stress) |
Ascorbate eliminates H2O2 through the action of ascorbate peroxidases thereby aiding drought tolerance in plants during symbiotic association. | Foyer and Noctor, 2011 ; Bárzana et al., 2015 ). |
| Enhanced activities of catalase (CAT), glutathione reductase (GR), guaiacol peroxidase (G-POD), and ascorbate peroxidase (APX) in citrus plants that had been inoculated with AMF have been reported. | Wu et al., 2006a; Wu et al., 2006b, Wu et al., 2006c; Wu et al., 2007b, Wu et al., 2013. | |
| Phytohormone mediated mechanisms (Enhanced ability of mycorrhizal plants to tolerate water-stressed conditions are associated with alterations in hormonal regulation) | Enhanced production of Abscisic acid (ABA), the abiotic stress hormone in AMF host plants thereby improving the plant’s ability to withstand drought conditions. | Calvo-Polanco et al., 2013 ; Hong et al., 2013 ; Martín-Rodríguez et al., 2016 ; Bahadur et al., 2019. |
| Rise in the endogenous concentrations of Jasmonic Acid (JA), its precursor 12-oxophytodienoic acid, as well as derivatives such as 11-hydroxy jasmonic acid and 12-hydroxy jasmonic acid in Digitaria eriantha inoculated with AMF after exposure to drought and salinity stress. | López-Ráez, 2016. | |
| Strigolactones (SLs) are phytohormones derived from carotenoids and are secreted by plants. SLs have been found to mitigate the negative impacts of drought stress through the regulation of plant physiological processes during Arbuscular mycorrhizal fungus (AMF) symbiosis. | Ruiz-Lozano et al., 2016. | |
| Proline Mediated Mechanisms (Proline produced by plants serves as an osmoprotectant in response to drought stress. It maintains the cellular osmotic balance in plants thereby alleviating the negative effects of drought stress) | AMF colonization of plant roots leads to proline accumulation under water-limited conditions. Accumulation of proline in plants was observed to be associated with the drought resistance induced by AMF symbiosis with proline serving as an osmoprotectant. | Ruiz-Lozano and Azcón, 1995 ; Azcon et al., 1996 ; Goicoechea et al., 1998 ; Yooyongwech et al., 2013 ; Rapparini and Peñuelas, 2014 ; Goicoechea et al., 1998. |
| Stress tolerance through Water Absorption and Transport | ||
| Regulation through Aquaporins (AMF symbiosis regulates various aquaporins within the host plant during drought stress) | Tomato plants infected with AMF showed an increase in the ability of water transport through the roots of AMF. This can be attributed to the overexpression of
LeNIP3;1, which encodes for NOD26-like intrinsic proteins (NIP).
AMF colonization induced the expression of certain plant genes encoding AQPs, such as RpPIP2;1 in Robinia pseudoacacia. This induction serves as a mechanism to enhance the flow of water to particular plant tissues during periods of drought. |
Chitarra et al., 2016. He et al., 2016 ; Bahadur et al., 2019. |
| Regulation through Osmotic adjustment (Osmotic adjustment aids plants in maintaining a water potential gradient, facilitating the movement of water from the soil into the roots) | Inoculation of AMF can enhance the drought stress tolerance of citrus plants by improving osmotic adjustment.
The growth performance and osmotic adjustment in Macadamia tetraphylla L. were improved by forming a symbiotic relationship with AMF through the buildup of soluble sugar, proline, and free amino acids in response to drought stress. |
Kubikova et al., 2001
;
Wu and Xia, 2006b
;
Wu et al., 2007a, Wu et al., 2013
;
Abbaspour et al., 2012. Yooyongwech et al., 2013 |
| Regulation through Stomatal Aperture (The role of stomatal architecture in host plants has been extensively regulated during AMF symbiosis under drought conditions) | Mycorrhizal symbiosis impacts the stomatal density in plants inoculated with R.intraradices in water-stressed conditions. High stomatal density enhances a plant’s ability to absorb CO2. | Chitarra et al., 2016 |
| Stress Tolerance through Morphological Modifications | ||
| Regulation through root system architecture (Root System Architecture (RSA), organization of roots within the soil that plays a significant role in a plant’s ability to withstand under adverse soil conditions) | Drought stress restricts the effectiveness of RSA in trifoliate orange seedlings. Inoculation with G. mosseae resulted in higher active and total absorption regions of the root structures thereby mitigating drought stress. | Wu and Xia, 2006a |
| Regulation through extraradical hyphae (Extraradical hyphae, with a diameter of 2-5 μm, penetrate through soil pores, typically inaccessible to root hairs) | Movement of water through mycorrhizal extraradical hyphae results in the apoplastic water flow within plant roots. | Bárzana et al., 2012 |
| Stress Tolerance through through Photosynthesis | ||
| Regulation through Photosynthesis (AMF plants in comparison to non-AMF plants exhibit less damage to their photosynthesis machinery under drought stress) | AMF plants exhibit improved photosystem II efficiency during episodes of drought stress in addition to increased transpiration rates following drought recovery. | Germ et al., 2005 ; Ruiz-Sánchez et al., 2010 |