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PhD defended december 18th, 2014
Direction: Florence Val

Abstract:

Using quantitative resistance is a major option for the design of more sustainable and environmentally friendly methods in crop protection. If such resistance is considered durable, the underlying mechanisms are not yet fully elucidated. The aim of the present work is to check the hypothesis that quantitative resistance is directly related to quantitative differences (in intensity and/or kinetics) of the defense reactions of plants induced by PAMPs (Pathogen Associated Molecular Patterns). This hypothesis was tested on four potato genotypes with varying levels of resistance to Phytophthora infestans (downy mildew).

To mimic the pathogen, a concentrated culture filtrate (CCF) of the oomycete was used as PAMP. The chemical characterization of the filtrate identified three elicitins (INF1, and INF4 INF5) and a complex polysaccharide containing galactose as active molecules. The potato genotypes were classified according to their level of resistance after inoculation with P. infestans. Simultaneously,induction of defense responses by CCF was evaluated by quantitative PCR of transcipts. This required the development of a molecular tool including fifty markers representative of general defense pathways in potato. The induction of transcripts was then confirmed by quantification of phytohormones and phenolic compounds. Untargeted profiling of all metabolites induced by the CCF was assessed to complete the study.

Transcripts and metabolites showed differential induction of signaling pathways (salicylic acid (SA), jasmonic acid (JA) and ethylene (Et)) in the four genotypes. A strong induction of the transcripts of the SA pathway (pr-1, pr-2, wrky1) and a transient accumulation (12h) of SA was observed in one of the two resistant genotypes. JA and ET pathways seemed not to be induced. The complete analysis of transcripts discriminate the four genotypes depending on the level of induction of the defense pathways, but regardless of their level of resistance. The most expressed genes, regardless of the genotype, are those involved in the signaling, PR proteins (pr-2, pr-4), the hydroxycinnamic acid amides (tht) and peroxidase (pox). Some genes such as ein3, ert1, chs2, pect are conversely repressed in all genotypes. Nevertheless, differences were observed in the kinetics and the intensity of the induction of gene expression. The preliminary results of the metabolic profiling separate susceptible genotypes from the resistant ones, but this analysis takes into account both the constitutive and induced metabolites. The differences between resistant and susceptible genotypes may be explained by the presence of high amounts of constitutive antimicrobial compounds, chlorogenic acid, quercetin, and rutin, in resistant genotypes compared to susceptible ones.

Altogether these results underline the genotype-dependent nature of the defense responses induced by the CCF. They suggest that the quantitative resistance could be explained by the combined action of constitutive and induced defenses.