PLANT HOLOBIONT ENHANCES ENVIRONMENTAL STRESS TOLERANCE
Plant-associated microbial communities are known to impart tolerance against salt (Chakraborty et al., 2021; H. Li et al., 2021) and drought (Bazany et al., 2022; de Vries et al., 2020) stress conditions. Salt-induced decreases in root nodulation (Singh & Valdes-Lopez, 2023) involve the expression of a glycogen synthase kinase 3 (GSK3)-like kinase, GmSK2-8, which phosphorylates GmNSP1a, preventing the binding of symbiotic genes such as GmNINa and reducing nodulation (C. He et al., 2021). The salt-inducible GmNAC181 TF activates GmNINaexpression and maintains soybean nodulation under salt stress (X. Wang et al., 2022). Drought triggers a compartment-specific restructuring of rice root microbiota in rice, particularly in the endosphere. Drought increases the prevalence of monoderm bacteria, which lack an outer cell membrane and contain thick cell walls (Santos-Medellin et al., 2021; Xu et al., 2018). Apart from providing drought adaptation, the microbial communities also contribute to drought-induced stress memory in rice, through compositional shifts that help plants to survive better upon re-watering (Fig. 2B) (Santos-Medellin et al., 2021). In the bioenergy model grass Panicum hallii , AMFs inoculation under water-limiting conditions has been shown to impart resistance to sensitive bacterial communities in the hydrosphere, indicating fungal-bacterial synergy (Hestrin et al., 2022). However, these findings need more careful investigation, while designing microbe-based solutions for realistic field scenarios. For instance, drought-induced increased prevalence of beneficial microbes in rhizosheaths has been demonstrated to induce risk factors for harmful fungi (Lei, Ding, Xu, & Zhang, 2023).
Stress as well as nutrient-limiting conditions have been shown to alter innate immunity, which helps in recruiting beneficial microbes. Pi-starvation induced transcriptional regulator PHR1 activates the expression of RALF (Rapid alkalinization factor) genes. RALF peptides hijack the FERONIA kinase to suppress plant immunity and allow colonization by specialized root microbiota such as Pseudomonas fluorescens (Fig. 2C) (Tang et al., 2022). FERONIA inhibitors, including reversine and staurosporine, enhance innate immunity against soil-borne diseases in tobacco, tomato, and rice without imposing growth penalties (H. B. Liu et al., 2023). Feronia mutants show low ROS accumulation and harbor elevated levels of rhizosphere pseudomonads, suggesting that FERONIA participates in the discrimination of beneficial and harmful microbes (Song et al., 2021). Additionally, the water-deficit conditions release flavonoids that reshape the root microbiome by attracting Aeromonas species that enhance dehydration resistance in plants (D. He et al., 2022). Apart from plant exudates, the core bacterial commensals and host tryptophan-derived specialized metabolites also function to control the overgrowth of fungal species (Wolinska et al., 2021), thereby maintaining host-microbe as well as microbe-microbe homeostasis. Taken together, the role of stress in shaping the plant microbial signature is clear; however, the cause-effect relationship is yet to be established.