Resistance against Ralstonia solanacearum in tomato depends on the methionine cycle and the γ‐aminobutyric acid metabolic pathway

Abstract

Summary Bacterial wilt caused by Ralstonia solanacearum is a complex and destructive disease that affects over 200 plant species. To investigate the interaction of R. solanacearum and its tomato (Solanum lycopersicum) plant host, a comparative proteomic analysis was conducted in tomato stems inoculated with highly and mildly aggressive R. solanacearum isolates (RsH and RsM, respectively). The results indicated a significant alteration of the methionine cycle (MTC) and downregulation of γ‐aminobutyric acid (GABA) biosynthesis. Furthermore, transcriptome profiling of two key tissues (stem and root) at three stages (0, 3 and 5 days post‐inoculation) with RsH in resistant and susceptible tomato plants is presented. Transcript profiles of MTC and GABA pathways were analyzed. Subsequently, the MTC‐associated genes SAMS2, SAHH1 and MS1 and the GABA biosynthesis‐related genes GAD2 and SSADH1 were knocked‐down by virus‐induced gene silencing and the plants’ defense responses upon infection with R. solanacearum RsM and RsH were analyzed. These results showed that silencing of SAHH1, MS1 and GAD2 in tomato leads to decreased resistance against R. solanacearum. In summary, the infection assays, proteomic and transcriptomic data described in this study indicate that both MTC and GABA biosynthesis play an important role in pathogenic interaction between R. solanacearum and tomato plants.

γ‐Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

“…Moreover, GAD is known to be activated following cytosol acidification (Shelp et al, ; Snedden et al, ). In fact, recognition of pathogens such as bacteria and fungi was proven to trigger GABA accumulation in several plant species, such as bean ( Phaseolus vulgaris L.), tomato ( Solanum lycopersicon L.) and soybean ( Glycine max L.) (Copley, Aliferis, Kliebenstein, & Jabaji, ; O'Leary et al, ; Wang et al, ) (Table ). The accumulated GABA is presumed to be transported to the mitochondria, where it undergoes deamination via GABA transaminase (GABAT) to succinic semialdehyde (SSA) (Shelp & Zarei, ).…”

“…Metabolomic analysis of Lr34, a wheat ( Triticum aestivum L.) variety highly resistant towards the biotrophic fungus Puccinia triticina , revealed an overactivation of several processes related to primary metabolism compared to a susceptible variety during infection, including the GABA shunt (Bolton, Kolmer, Xu, & Garvin, ). GAD Knock Out (KO) tomato plants exhibited increased susceptibility towards the hemi‐biotroph Ralstonia solanacearum , the causal agent of bacterial wilt (Wang et al, ), evidencing that GABA contributes to plant defence against R. solanacearum . However, following R. solanacearum inoculation, GABA biosynthesis via GAD, alongside the methionine cycle, is suppressed while GABA catabolism is rapidly stimulated (Wang et al, ).…”

“…GAD Knock Out (KO) tomato plants exhibited increased susceptibility towards the hemi‐biotroph Ralstonia solanacearum , the causal agent of bacterial wilt (Wang et al, ), evidencing that GABA contributes to plant defence against R. solanacearum . However, following R. solanacearum inoculation, GABA biosynthesis via GAD, alongside the methionine cycle, is suppressed while GABA catabolism is rapidly stimulated (Wang et al, ). Such data might indicate that suppression of GABA accumulation is a pathogen strategy to weaken the host during the early infection phases.…”

“…We supported this hypothesis by using a pharmacological approach based on the use of the ET signalling inhibitor 1‐methylcyclopropene (1‐MCP) (Tarkowski, Van de Poel, Höfte & Van den Ende, ). Wang et al () proposed that the accumulation of S ‐adenosyl‐methionine, produced by the inhibition of the methionine cycle, is important to sustain the ET biosynthesis. Together, these findings may suggest that the methionine cycle is inhibited, causing ET accumulation which in turn enhances the positive effect of GABA on plant defence.…”

“…To date, important roles in plant defence were established for several primary metabolites, such as sucrose (Suc) and glutamate (Glu) (Seifi, Van Bockhaven, et al, ; Tauzin & Giardina, ). Among such metabolites, γ‐aminobutyric acid (GABA) stands out for the vast body of literature linking it to positive roles in plant stress physiology (Bown & Shelp, ; Mahmud et al, ; Wang et al, ).…”