Metabolomic Evaluation of Tissue-Specific Defense Responses in Tomato Plants Modulated by PGPR-Priming against Phytophthora capsici Infection.

Msizi I Mhlongo,Paul A Steenkamp,Lizelle A Piater,Nico Labuschagne,Ian A Dubery

doi:10.3390/plants10081530

Msizi I Mhlongo, Paul A Steenkamp + Show 3 more

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https://doi.org/10.3390/plants10081530

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Abstract

Plant growth-promoting rhizobacteria (PGPR) can stimulate disease suppression through the induction of an enhanced state of defense readiness. Here, untargeted ultra-high performance liquid chromatography–mass spectrometry (UHPLC–MS) and targeted ultra-high performance liquid chromatography coupled to triple-quadrupole mass spectrometry (UHPLC–QqQ-MS) were used to investigate metabolic reprogramming in tomato plant tissues in response to priming by Pseudomonas fluorescens N04 and Paenibacillus alvei T22 against Phytophthora capsici. Roots were treated with the two PGPR strains prior to stem inoculation with Ph. capsici. Metabolites were methanol-extracted from roots, stems and leaves at two–eight days post-inoculation. Targeted analysis by UHPLC–QqQ-MS allowed quantification of aromatic amino acids and phytohormones. For untargeted analysis, UHPLC–MS data were chemometrically processed to determine signatory biomarkers related to priming against Ph. capsici. The aromatic amino acid content was differentially reprogrammed in Ps. fluorescens and Pa. alvei primed plants responding to Ph. capsici. Furthermore, abscisic acid and methyl salicylic acid were found to be major signaling molecules in the tripartite interaction. LC–MS metabolomics analysis showed time-dependent metabolic changes in the primed-unchallenged vs. primed-challenged tissues. The annotated metabolites included phenylpropanoids, benzoic acids, glycoalkaloids, flavonoids, amino acids, organic acids, as well as oxygenated fatty acids. Tissue-specific reprogramming across diverse metabolic networks in roots, stems and leaves was also observed, which demonstrated that PGPR priming resulted in modulation of the defense response to Ph. capsici infection.

Highlights

In addition to facilitating the absorption of water and minerals, roots secrete organic exudates into the rhizosphere [1,2]
In the case of Ps. fluorescens primed-Ph. capsici challenged plants, signs of wilting on the edges of leaves were observed at 8 d.p.i. (Figure 1F), whereas Pa. alvei primed and Ph. capsici challenged plants displayed no sign of wilting on 8 d.p.i. (Figure 1L)
The aromatic amino acid content was found to undergo differential reprogramming in plants pre-treated with Ps. fluorescens and Pa. alvei and subsequently inoculated with to Ph. capsici

Summary

Introduction

In addition to facilitating the absorption of water and minerals, roots secrete organic exudates into the rhizosphere [1,2]. The composition of root exudates plays an important role in microbial population attraction and selection of the microbial community colonizing the rhizosphere [1,3,4]. The latter comprises of neutral, pathogenic and beneficial microbes. Among these are the plant growth-promoting rhizobacteria (PGPR) that have beneficial effects on plant growth, development and production via direct (by synthesizing phytohormones, or facilitating the uptake of certain nutrients from the environment) or indirect (by producing antagonistic substances or inducing resistance to pathogens) mechanisms [5,6,7]. The utilization of beneficial microbes is considered a “green”/non-agrochemical approach to augment plant immunity and to control diseases and is referred to as “bio-priming” with PGPR

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