Abstract
Plant signaling mechanisms are not completely understood in plant–fungal biotrophic pathogen interactions. Further how such interactions are influenced by compatible rhizosphere microbes are also not well-studied. Therefore, we explored the pea-Erysiphe pisi (obligate biotroph) system to understand the interaction and applied compatible rhizospheric bio-agents Trichoderma asperellum (T42) and Pseudomonas fluorescens (OKC) singly or in combination to assess their influence on the host while under the pathogen challenge. Transcript accumulation pattern of some vital genes in the lignin biosynthetic pathway in pea under E. pisi challenge indicated enhanced activation of the pathway. Interestingly, transcript accumulations were even higher in the bio-agent treated plants compared to untreated plants after pathogen inoculation particularly in co-inoculated treatments. Further, down regulation of the lignifications-associated ABC transporter gene in the pathogen challenged plants possibly is an indication of passive diffusion of monolignols across the membrane from symplast. Additionally, up regulation of NADPH oxidase gene revealed ROS generation in the challenged plants which was confirmed through spectrophotometric estimation of H2O2. Up regulation of laccase and peroxidase along with higher H2O2 generation points out their involvement in lignifications which was further confirmed through cross section analysis of pea stems that showed increased lignifications in pathogen challenged plants co-inoculated with the bioagents. Interestingly, pathogen responsive MAPK homologs MAPK3/MAPK6 and the enzyme serine threonine kinase that activates MAPKs were down regulated and the results possibly indicate non-participation of the MAPK cascade in this interaction. Therefore, it can be concluded that the microbial treatments enhanced pea resistance to E. pisi by generation of ROS and lignifications.
Highlights
Plants recognize potential pathogens by perceiving elicitors commonly known as microbe-associated molecular patterns (MAMPs)
In order to understand the host responses during interaction with an obligate fungal pathogen we addressed the questions (i) whether NADPH oxidase is involved in generation of reactive oxygen species (ROS) during the interaction? (ii) whether the canonical MAP kinase cascade is involved in the interaction following ROS generation? and (iii) what is the pattern of lignifications during the host-pathogen interactions? We used pea and E. pisi as a model system to understand the issues
Transcript accumulation pattern of NADPH oxidase (NOX) showed that its accumulation was highest (2.5 fold) in plants co-inoculated with the soil microbes and challenged with the pathogen E. pisi followed by pathogen challenged plants treated with only Trichoderma (2.25 fold) (Figure 1A)
Summary
Plants recognize potential pathogens by perceiving elicitors commonly known as microbe-associated molecular patterns (MAMPs). ROS as intracellular secondary messenger triggers a cascade of mitogen-activated protein kinases (MAPKs) (Jalmi and Sinha, 2015). The serine threonine kinase (STK) is activated which phosphorylates the MAPKs as observed in the case of interactions between the hemi-biotroph pathogen Pseudomonas syringae pv. MAPK cascade is known to activate following perception of stimuli from pathogens by plant heterotrimeric G-protein leading to dissociation of Gα unit from the Gβγ unit (Su et al, 2015). Not much is known about the defense responses generated during plant– fungal biotroph interactions in relation to G-protein mediated signaling and specific roles of ROS, MAP kinases and the impact on lignifications
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