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

Abstract

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 aggressiveness and plant partial resistance are less well-known. Nevertheless, they are prevalent in most plant-necrotrophic pathogen interactions, including the Daucus carota–Alternaria dauci interaction. Phytotoxic metabolite production by the pathogen plays a key role in aggressiveness in these interactions. The aim of the present study was to explore the link between A. dauci aggressiveness and toxin production. We challenged carrot embryogenic cell cultures from a susceptible genotype (H1) and two partially resistant genotypes (I2 and K3) with exudates from A. dauci strains with various aggressiveness levels. Interestingly, A. dauci-resistant carrot genotypes were only affected by exudates from the most aggressive strain in our study (ITA002). Our results highlight a positive link between A. dauci aggressiveness and the fungal exudate cell toxicity. We hypothesize that the fungal exudate toxicity was linked with the amount of toxic compounds produced by the fungus. Interestingly, organic exudate production by the fungus was correlated with aggressiveness. Hence, we further analyzed the fungal organic extract using HPLC, and correlations between the observed peak intensities and fungal aggressiveness were 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 10-membered benzenediol lactone, a polyketid 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.

Recent Advances in Alternaria Phytotoxins: A Review of Their Occurrence, Structure, Bioactivity, and Biosynthesis

“…It was also indicated that aldaulactone was involved in both fungal pathogenicity and plant resistance mechanisms. In phytotoxicity, aldaulactone was toxic to carrot cells, inducing a delay in embryonic development and a decrease in cell viability [142]. The biological function and biosynthetic pathway of aldaulactone has not yet been defined.…”

Characterization of NRPS and PKS genes involved in the biosynthesis of SMs in Alternaria dauci including the phytotoxic polyketide aldaulactone

“…On the contrary, in vitro carrot cell toxicity of aldaulactone was pointed out at concentrations observed in A. dauci cultures. In an original way, this toxicity is negatively correlated with carrot cell resistance 39 . Interestingly, it is only the second evidence, with SS-toxin in the Stemphylium solani - Allium sativum pathosystem, of a link between fungal SMs and partial resistance 37 , 39 , 46 .…”

“…Remarkably, numerous phytopathogenic Alternaria species produce HSTs that cause high specificity for their host 8 . So far at least seven phytotoxins have been isolated and characterized from A. dauci cultures: zinniol, alternariol, alternariol monomethyl-ether, α -acetylorcinol, p -hydroxybenzoic acid and aldaulactone 39 – 43 . To our knowledge, little is known about their biosynthetic pathways, except for the alternariol biosynthetic cluster that was recently predicted by sequence homology 44 .…”

“…To our knowledge, little is known about their biosynthetic pathways, except for the alternariol biosynthetic cluster that was recently predicted by sequence homology 44 . The contribution of these toxins to A. dauci pathogenicity has been shown only for zinniol and aldaulactone 39 , 40 . However, recent studies highlighted that only high concentrations of zinniol were toxic to carrot cells 37 , 45 .…”

“…Interestingly, it is only the second evidence, with SS-toxin in the Stemphylium solani - Allium sativum pathosystem, of a link between fungal SMs and partial resistance 37 , 39 , 46 . Aldaulactone seems to be the key component of this link in the A. dauci-D. carota pathosystem, although it was shown that other toxins might be involved in A. dauci pathogenicity 39 .…”