Aldaulactone – An Original Phytotoxic Secondary Metabolite Involved in the Aggressiveness of Alternaria dauci on Carrot

Qualitative plant resistance mechanisms and pathogen virulence have been extensively studied since the formulation of the gene-for-gene hypothesis. The mechanisms involved in the quantitative traits of aggressivenessand plant partial resistance are less well-known. Nevertheless, they are prevalent in most plant-necrotrophic pathogen interactions, including the Daucus carota- Alternaria dauciinteraction. Phytotoxic metabolite production by the pathogen plays a key role in aggressivenessin these interactions. The aim of the present study was to explore the link between A. dauci aggressivenessand toxin production. We challenged carrot embryogenic cell cultures from a susceptible genotype (H1) and two partially resistant genotypes (I2 and K3) with exudatesfrom A. dauci strains with various aggressivenesslevels. Interestingly, A. dauci-resistant carrot genotypes were only affected by exudatesfrom the most aggressivestrain in our study (ITA002). Our results highlight a positive link between A. dauci aggressivenessand the fungal exudatecell toxicity. We hypothesize that the fungal exudatetoxicity was linked with the amount of toxic compounds produced by the fungus. Interestingly, organic exudateproduction by the fungus was correlated with aggressiveness. Hence, we further analyzed the apolar fungal exudatesusing HPLC, and correlations between the observed peak intensities and fungal aggressivenesswere measured. One observed peak was closely correlated with fungal aggressiveness. We succeeded in purifying this peak and NMR analysis revealed that the purified compound was a novel polyketidewith a 10-membered benzenediollactone, that we named ‘aldaulactone’. We used a new automated image analysis method and found that aldaulactone was toxic to in vitro cultured plant cells at those concentrations. The effects of both aldaulactone and fungal organic extracts were weaker on I2-resistant carrot cells compared to H1 carrot cells. Taken together, our results suggest that: (i) aldaulactone is a new phytotoxin, (ii) there is a relationship between the amount of aldaulactone produced and fungal aggressiveness, and (iii) carrot resistance to A. dauci involves mechanisms of resistance to aldaulactone.