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Aldaulactone, a new phytotoxin involved in Alternaria dauci - Daucus carota interaction

Julia Courtial: University of Angers

<div>The mechanisms of quantitative aggressiveness and partial resistance have not been extensively studied and are prevalent in most plant – necrotrophic pathogen interactions, including the <em>Daucus carota</em> – <em>Alternaria dauci</em> interaction. In those, pathogens often produce phytotoxic secondary metabolites. In the work presented here, we explored the link between <em>A. dauci</em> aggressiveness and toxin production. In a first set of experiments, we uncovered a link between aggressiveness and fungal exudates toxicity on <em>in vitro</em> cultured carrot cells. These results were obtained with cells from the<em> A. dauci</em> susceptible genotype H1. Cells from the partially resistant genotypes K3 and I2 were only affected by exudates from the most aggressive <em>A. dauci</em> strain, ITA002. We made the hypothesis that toxicity of the fungal exudates was correlated with the amount of toxic compounds produced by the fungus. Interestingly, apolar fungal exudates production was correlated with strain aggressiveness. We analyzed the apolar fungal exudates using HPLC, and correlations between observed peaks intensity and fungal aggressiveness were measured. One of the observed peaks was strongly correlated with fungal aggressiveness. NMR analysis revealed that the purified compound was a novel polyketide with a decalactone ring. We named it “aldaulactone”. In a new set of experiments, involving automated image analysis, we found that aldaulactone is toxic to <em>in vitro</em> cultured plant cells at concentrations observed in fungal exudates. Similarly, the effect of both aldaulactone and fungal extracts was weaker on I2 plant cells than on H1 plant cells. Taken together, these results suggest that (i) aldaulactone is a phytotoxin, (ii) variations in aldaulactone production affect fungal aggressiveness and (iii) resistance to aldaulactone is a mechanism of carrot resistance to <em>A. dauci.</em></div>

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