The Function of Root Exudates in the Root Colonization by Beneficial Soil Rhizobacteria

Abstract

Soil-beneficial microbes in the rhizosphere play important roles in improving plant growth and health. Root exudates play key roles in plant-microbe interactions and rhizobacterial colonization. This review describes the factors influencing the dynamic interactions between root exudates and the soil microbiome in the rhizosphere, including plant genotype, plant development, and environmental abiotic and biotic factors. We also discuss the roles of specific metabolic mechanisms, regulators, and signals of beneficial soil bacteria in terms of colonization ability. We highlight the latest research progress on the roles of root exudates in regulating beneficial rhizobacterial colonization. Organic acids, amino acids, sugars, sugar alcohols, flavonoids, phenolic compounds, volatiles, and other secondary metabolites are discussed in detail. Finally, we propose future research objectives that will help us better understand the role of root exudates in root colonization by rhizobacteria and promote the sustainable development of agriculture and forestry.

Keywords: rhizobacteria; root colonization; root exudate–rhizobacteria interaction; root exudation.

Figure 1

Colonization process of beneficial bacteria in the rhizosphere. Colonization by nonsymbiotic beneficial rhizobacteria consists of several steps, including chemotaxis, root surface attachment, and biofilm formation [11]. Root exudates serve as signals, nutrients, and antimicrobial compounds during the colonization process. The receptors and regulators from bacteria are represented in the gray boxes. The specific metabolic mechanisms, regulators, and signals of beneficial bacteria and tolerance to antimicrobial compounds determine the colonization ability of beneficial rhizobacteria. Successful colonization by beneficial bacteria provides various benefits to plants.

Figure 2

The influencing factors on root exudation and rhizosphere bacteria composition. The root exudation–rhizobacteria interaction is a dynamic process impacted by plant genotype (A), plant development (B), environmental abiotic factors (C), and biotic factors (D): (A) The process of root exudation displays variability across different plant genotypes. The diverse signaling receptors of bacteria and their ability to utilize and decrypt substances contribute to changes in the rhizosphere microbiome in the rhizosphere with different compositions of root exudates. (B) The blue arrow represents dynamic root exudation at different plant developmental stages. The green and red arrows indicate the attraction and repulsion of rhizobacteria, respectively. (C,D) In response to abiotic or biotic stress, plants can recruit some beneficial bacteria through exudation.