Biotic stress-induced changes in root exudation confer plant stress tolerance by altering rhizospheric microbial community

Abstract

Every organism on the earth maintains some kind of interaction with its neighbours. As plants are sessile, they sense the varied above-ground and below-ground environmental stimuli and decipher these dialogues to the below-ground microbes and neighbouring plants via root exudates as chemical signals resulting in the modulation of the rhizospheric microbial community. The composition of root exudates depends upon the host genotype, environmental cues, and interaction of plants with other biotic factors. Crosstalk of plants with biotic agents such as herbivores, microbes, and neighbouring plants can change host plant root exudate composition, which may permit either positive or negative interactions to generate a battlefield in the rhizosphere. Compatible microbes utilize the plant carbon sources as their organic nutrients and show robust co-evolutionary changes in changing circumstances. In this review, we have mainly focused on the different biotic factors responsible for the synthesis of alternative root exudate composition leading to the modulation of rhizosphere microbiota. Understanding the stress-induced root exudate composition and resulting change in microbial community can help us to devise strategies in engineering plant microbiomes to enhance plant adaptive capabilities in a stressful environment.

Guanidine production by plant homoarginine-6-hydroxylases

“…In soil, a greater proportion of bacteria contains genes for guanidine import and assimilation than among bacteria associated with human skin or the gut, which preferentially contain genes for guanidine exporters ( Sinn et al, 2021 ). Since bacteria differ in their responses to guanidine, it is possible that guanidine accumulation or even secretion is a way for plants to modulate their microbiomes ( Sharma et al, 2023 ). A similar function was proposed for hydroxyguanidine that was secreted by Pseudomonas canavaninivorans after degradation of plant-produced canavanine by a PLP-dependent canavanine-γ-lyase ( Hauth et al, 2022 ).…”

Guanidine Production by Plant Homoarginine-6-hydroxylases

“…In soil, a greater proportion of bacteria contains genes for guanidine import and assimilation than among bacteria associated with human skin or the gut, which preferentially contain genes for guanidine exporters (Sinn et al, 2021 ). Since bacteria differ in their responses to guanidine, it is possible that guanidine accumulation or even secretion is a way for plants to modulate their microbiomes (Sharma et al, 2023 ). A similar function was proposed for hydroxyguanidine that was secreted by Pseudomonas canavaninivorans after degradation of plant-produced canavanine by a PLPdependent canavanine-γ-lyase (Hauth et al, 2022 ).…”

Root Exudate Metabolites Alter Food Crops Microbiomes, Impacting Plant Biocontrol and Growth

“…Root exudates are a chemically diverse class of substances that plants, throughout their lifetime, secrete into their surrounding environment (rhizosphere) via their root system. It is estimated that plant roots secrete about 5-40% of their photosynthetic assimilated carbon as root exudates [21,24]. Plant root exudates are comprised of high-and low-molecularweight compounds belonging to organic acids, sugars, phenols, amino acids, hormones, flavonoids, and growth regulators [60].…”

“…Many factors influence the composition of root exudates and shifts in root exudate patterns, with changes in climatic conditions being the most pivotal. Other factors that have been reported to change root exudation patterns include soil pH, soil type, soil temperature, soil oxygen content (aeration), soil physio-chemical properties, plant developmental stage, and plant genotype [24,64,[66][67][68]. Intercropping has also been reported to alter the pattern of root exudates and to help food crops (e.g., tomato) to recruit beneficial microbiota to the rhizosphere [69].…”

“…The largest numbers of bacteria are generally found in the plant rhizosphere, i.e., the portion of the soil that is present in the immediate vicinity of plant roots [5]. The reason for the high level of bacteria around plant roots is because of the exudation of a significant fraction (often 5-40%) of the carbon that is fixed by the plant through photosynthesis [21][22][23][24]. In addition to their presence in the rhizosphere, many plant growth-promoting bacteria (PGPB) are also able to colonize a plant's interior tissues (i.e., they are endophytic bacteria and are typically found in the spaces between plant cells).…”