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.