Aida Jalloul

Activity of Class III Peroxidases in the Defense of Cotton to Bacterial Blight

Cotton cotyledons displayed a hypersensitive reaction (HR) in the cultivar Réba B50 after infiltration with the avirulent race 18 from Xanthomonas campestris pv. malvacearum. Two sets of peroxidases were associated with the HR time course. Early but transient accumulation of peroxidase in material encapsulating the bacteria in intercellular areas was observed by immunocytochemistry at 3 h postinfection and coincided with the oxidative burst. Total guaiacol-peroxidase activity was highly increased in cells undergoing HR, from 12 h after treatment. Molecular characterization of seven cloned peroxidase genes revealed highly conserved B, D, and F domains, with similarities to plant class III peroxidases. Analysis of gene expression showed variation in transcript accumulation during both compatible (race 20) and incompatible interactions for four of these genes: pod2, pod3, pod4, and pod6. Pod4 and pod6 were more intensely up-regulated during resistance than during disease and in the control, while pod3 was specifically down-regulated during the HR after the oxidative burst. Pod2 was induced by pathogen infection and weakly stimulated in the control. These data suggest that cotton peroxidases may have various functions in the defense response to Xanthomonas infections.

Identification of coffee WRKY transcription factor genes and expression profiling in resistance responses to pathogens

In plants, WRKY proteins are a group of transcription factors existing as a gene superfamily that play important roles in regulation of defense response pathways. To assess the diversity of this protein family in coffee (Coffea spp.), data mining methods were used on a set of around 200,000 coffee expressed sequence tags. A total of 53 different putative WRKY genes were obtained, but only 22 unigenes encoding a protein with a WRKY domain were identified, eight of which are supported by full-length cDNA sequences. Alignment of WRKY domain sequences of the coffee unigenes and 72 Arabidopsis thaliana WRKY genes showed that the 22 coffee WRKY members were distributed among the main A. thaliana WRKY subgroups and shared conserved peptide domains. To assess the involvement of WRKY genes in coffee defense response pathways, their expression was analyzed under biotic (nematode and rust fungus infection), hormonal (salicylic acid, methyl-jasmonate), and wounding treatments, leaf senescence, and fruit development. Five members of WRKY groups IId and III were regulated only by pathogens and hormone treatments. Although a significant correlation of WRKY genes expression after MeJA and rust treatments was observed, expression of coffee genes involved in JA biosynthesis and lipoxygenase (EC 1.13.11.12) activity assays did not support the involvement of JA in the early coffee resistance responses to the rust pathogen. The five WRKY transcription factor members identified might play important roles as regulators of pathogen resistance responses and could be useful for improving coffee tolerance to various biotic stresses.

A novel patatin-like protein from cotton plant, GhPat1, is co-expressed with GhLox1 during Xanthomonas campestris-mediated hypersensitive cell death

In cotton plant, Xanthomonas-induced hypersensitive response (HR) is accompanied by a lipid peroxidation process involving a 9-lipoxygenase (LOX), GhLox1. Initiation of this oxidative metabolism implies the release of the LOX substrates, or polyunsaturated fatty acids. Since patatin-like proteins (PLPs) are likely candidates for mediating the latter step, we searched for genes encoding such enzymes, identified and cloned one of them that we named GhPat1. Biochemical and molecular studies showed that GhPat1 expression was up-regulated during the incompatible interaction, prior to the onset of the corresponding galactolipase activity and cell death symptoms in tissues. Protein sequence analysis and modelling also revealed that GhPat1 catalytic amino acids and fold were conserved across plant PLPs. Based on these results and our previous work (Jalloul et al. in Plant J 32:1–12, 2002), a role for GhPat1, in synergy with GhLox1, during HR-specific lipid peroxidation is discussed.

GhERF‐IIb3 regulates the accumulation of jasmonate and leads to enhanced cotton resistance to blight disease

The phytohormone jasmonic acid (JA) and its derivatives, collectively referred to as jasmonates, regulate many developmental processes, but are also involved in the response to numerous abiotic/biotic stresses. Thus far, powerful reverse genetic strategies employing perception, signalling or biosynthesis mutants have broadly contributed to our understanding of the role of JA in the plant stress response and development, as has the chemical gain-of-function approach based on exogenous application of the hormone. However, there is currently no method that allows for tightly controlled JA production in planta. By investigating the control of the JA synthesis pathway in bacteria-infected cotton (Gossypium hirsutum L.) plants, we identified a transcription factor (TF), named GhERF-IIb3, which acts as a positive regulator of the JA pathway. Expression of this well-conserved TF in cotton leaves was sufficient to produce in situ JA accumulation at physiological concentrations associated with an enhanced cotton defence response to bacterial infection.