The Chemistry of Plant-Microbe Interactions in the Rhizosphere and the Potential for Metabolomics to Reveal Signaling Related to Defense Priming and Induced Systemic Resistance - PubMed (original) (raw)

Review

Msizi I Mhlongo et al. Front Plant Sci. 2018.

Abstract

Plant roots communicate with microbes in a sophisticated manner through chemical communication within the rhizosphere, thereby leading to biofilm formation of beneficial microbes and, in the case of plant growth-promoting rhizomicrobes/-bacteria (PGPR), resulting in priming of defense, or induced resistance in the plant host. The knowledge of plant-plant and plant-microbe interactions have been greatly extended over recent years; however, the chemical communication leading to priming is far from being well understood. Furthermore, linkage between below- and above-ground plant physiological processes adds to the complexity. In metabolomics studies, the main aim is to profile and annotate all exo- and endo-metabolites in a biological system that drive and participate in physiological processes. Recent advances in this field has enabled researchers to analyze 100s of compounds in one sample over a short time period. Here, from a metabolomics viewpoint, we review the interactions within the rhizosphere and subsequent above-ground 'signalomics', and emphasize the contributions that mass spectrometric-based metabolomic approaches can bring to the study of plant-beneficial - and priming events.

Keywords: chemical communication; induced resistance; metabolites; metabolomics; plant–microbe interactions; priming; signalomics.

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Figures

FIGURE 1

FIGURE 1

Overview of physical barriers (waxes, suberin, callose, lignin, etc.) and innate immunity defenses (MTI and ETI) that presents obstacles to potential microorganisms in establishing a beneficial interaction with plant roots.

FIGURE 2

FIGURE 2

Types of chemical-based interactions affecting plants. The plant host plus all its symbiotic microbes can be regarded as a community or an ecological unit (holobiont). (1) Interactions between plant roots and beneficial bacteria within the rhizosphere, (2) competitive interactions between beneficial bacteria and potential pathogens, (3) attack by potential pathogens on plant roots, (4) counter defense responses against pathogen attack, (5) communication between plants roots and leaves, and (6) interplant communication through leaves (6a) and roots (6b).

FIGURE 3

FIGURE 3

Rhizosphere plant and microbial ‘signalomics’. Plants and rhizomicrobes secrete compounds beneficial to each other to establish mutual relationships. This below–ground interaction, in turn, primes plants against various environmental stimuli that includes abiotic as well as biotic stresses. Perception of the priming stimulus leads to activation of signaling molecules, primary metabolism regulation and gene activation of enzymes involved in the production of secondary defense metabolites. When a secondary stimulus is detected the same process as in the priming stage takes place but at an enhanced level to minimize impact on the plant. Plants are able to pass on the induced primed state to their progeny in a process known as _trans_-generational priming. In addition, plants communicate with each other using allelopathic molecules. Abbreviations: volatile organic compounds (VOCs), quorum sensing molecules (QSM), _N_-acyl homoserine lactones (AHL), SA, methylsalicylic acid (MeSA), methyljasmonic acid (MeJA) ET.

FIGURE 4

FIGURE 4

Background for metabolomics studies of signaling in the rhizosphere between plant hosts (left) and microorganisms within the rhizosphere (right). The inter-organismal communication affects the biological information flow from genome to metabolome. The metabolome is complementary to the transcriptome and proteome, captures the functional, or physiological state of the cell, and provides a communications link between genotype and phenotype. Metabolites also form part of the regulatory systems in an integrated manner (solid lines indicating regulatory loops). Altered gene expression is ultimately reflected in changes in the pattern and/or concentration of metabolites. It is through these interactions amongst the members of the central dogma components, that a cell acquires its full functionality of its cellular metabolism.

FIGURE 5

FIGURE 5

Flowchart for plant metabolomic studies. The three main steps of a metabolomic analysis are sample preparation, data acquisition, and data mining. These three steps are interrelated and lead to the discovery of signatory biomarkers, metabolite annotation, and biochemical interpretation.

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