Rhizosphere Signaling: Insights into Plant-Rhizomicrobiome Interactions for Sustainable Agronomy

Abstract

Rhizospheric plant-microbe interactions have dynamic importance in sustainable agriculture systems that have a reduced reliance on agrochemicals. Rhizosphere signaling focuses on the interactions between plants and the surrounding symbiotic microorganisms that facilitate the development of rhizobiome diversity, which is beneficial for plant productivity. Plant-microbe communication comprises intricate systems that modulate local and systemic defense mechanisms to mitigate environmental stresses. This review deciphers insights into how the exudation of plant secondary metabolites can shape the functions and diversity of the root microbiome. It also elaborates on how rhizosphere interactions influence plant growth, regulate plant immunity against phytopathogens, and prime the plant for protection against biotic and abiotic stresses, along with some recent well-reported examples. A holistic understanding of these interactions can help in the development of tailored microbial inoculants for enhanced plant growth and targeted disease suppression.

Keywords: defense priming; microbial metabolites; phytohormones; plant–microbe signaling; quorum sensing; rhizosphere; sustainable agriculture.

Figure 1

Schematic representation of interactions between rhizo-microorganisms and plants.

Figure 2

Schematic representation of quorum sensing (QS) and quorum quenching (QQ) inhibition of signal perception pathways. The binding of QQ agents to the LuxR receptors either inactivates quorum sensing receptors or reduces the quantity of receptors in the QS molecules for targeted gene expression.

Figure 3

Overview of rhizosphere communication in intra- or interspecies signaling among microorganisms and interkingdom signaling between microbes and plants. Myc, mycorrhizal; LCOs, lipo-chitooligosaccharides; Nod, nodulation; VOCs, volatile organic compounds; Ais, autoinducers; AHLs, N-acyl homoserine lactone; QSMs, quorum sensing molecules.

Figure 4

Summary of significant factors of rhizosphere signaling. SAR, systemic acquired resistance; ISR, induced systemic resistance; LCOs, lipo-chitooligosaccharides; Nod, nodulation; Myc, mycorrhizal; PRR, pattern recognition receptor; PAMPs, pathogen-associated molecular patterns.

Figure 5

Schematic representation of plant immunity against phytopathogens through pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is activated through the recognition of pathogen-associated molecular patterns (PAMPs) by the pattern recognition receptors (PRRs) in plant cell walls. After PAMP recognition, the plant triggers a signal cascade that further induces multiple intracellular defense responses and activates defense genes. In response to PTI, pathogens secrete effectors that are recognized by nucleotide-binding (NB) and leucine-rich repeat (LRR) receptors (NLRs) and develop another defense layer called ETI. Activated NLRs with many intracellular signaling events trigger the hypersensitive response (HR).

Figure 6

Schematic representation of defense priming: (a) a picture depicting induced systemic resistance (ISR), which is induced by plant growth-promoting rhizobacteria that provide resistance to biotic stresses, and systemic acquired resistance (SAR), which is induced by phytopathogens that provide resistance to biotic and abiotic stresses; (b) a flowchart showing signal transduction pathway of ISR that are followed by jasmonate (JA) and ethylene (ET) plant hormones, which are encoded by the jar1 and etr1 genes, respectively, whereas SAR is followed by salicylic acid (SA), which is encoded by the sid1 and sid2 genes.