Partial Resistance of Carrot to Alternaria dauci Correlates with In Vitro Cultured Carrot Cell Resistance to Fungal Exudates

Abstract

Although different mechanisms have been proposed in the recent years, plant pathogen partial resistance is still poorly understood. Components of the chemical warfare, including the production of plant defense compounds and plant resistance to pathogen-produced toxins, are likely to play a role. Toxins are indeed recognized as important determinants of pathogenicity in necrotrophic fungi. Partial resistance based on quantitative resistance loci and linked to a pathogen-produced toxin has never been fully described. We tested this hypothesis using the Alternaria dauci – carrot pathosystem. Alternaria dauci, causing carrot leaf blight, is a necrotrophic fungus known to produce zinniol, a compound described as a non-host selective toxin. Embryogenic cellular cultures from carrot genotypes varying in resistance against A. dauci were confronted with zinniol at different concentrations or to fungal exudates (raw, organic or aqueous extracts). The plant response was analyzed through the measurement of cytoplasmic esterase activity, as a marker of cell viability, and the differentiation of somatic embryos in cellular cultures. A differential response to toxicity was demonstrated between susceptible and partially resistant genotypes, with a good correlation noted between the resistance to the fungus at the whole plant level and resistance at the cellular level to fungal exudates from raw and organic extracts. No toxic reaction of embryogenic cultures was observed after treatment with the aqueous extract or zinniol used at physiological concentration. Moreover, we did not detect zinniol in toxic fungal extracts by UHPLC analysis. These results suggest that strong phytotoxic compounds are present in the organic extract and remain to be characterized. Our results clearly show that carrot tolerance to A. dauci toxins is one component of its partial resistance.

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

“…Toxins were also found to be heavily involved in specific partial resistance in the Corynespora cassiicola /rubber tree pathosystem ( Barthe et al, 2007 ). More recently, toxin resistance was found to be correlated with QDR in several pathosystems, including the Stemphylium solani /garlic pathosystem ( Zheng et al, 2010 ) and, in our laboratory, the Alternaria dauci / Daucus carota pathosystem ( Lecomte et al, 2014 ).…”

“…Potato dextrose broth ( Atlas, 2010 ) was used unless otherwise specified. Carrot juice medium ( Lecomte et al, 2014 ) was used in some specified cases. Liquid medium was inoculated with either conidial suspension to reach a final concentration of 5 ⋅ 10 3 conidia.mL -1 or 10 colonized agar plugs of 5 mm dia.…”

“…We thus showed that plant secondary metabolites, such as the phytoanticipin falcarindiol or the phytoalexin 6-methoxymellein, could be involved in partial resistance, but perhaps also in host specificity, since Alternaria species that are pathogenic in carrot ( A. dauci , A. radicina ) were much less affected by 6-methoxymellein than A. brassicicola which is pathogenic in Brassicaceae ( Lecomte et al, 2012 ). More recently, we detected a link between toxins and partial resistance, i.e., partial resistance to A. dauci was correlated with in vitro cultured carrot cell resistance to fungal exudates ( Lecomte et al, 2014 ). Hydrophobic compounds from these exudates were strongly phytotoxic when applied to cells of A. dauci -susceptible carrot genotypes ( Lecomte et al, 2015 ).…”

Link between carrot leaf secondary metabolites and resistance to Alternaria dauci

“… 21 identified differences in genetic backgrounds between H1, I2 and K3. Numerous additional data suggested different resistance mechanisms between Boléro, I2 and K3 against A. dauci : induction of PR4, a gene from the jasmonic acid pathway, in K3 but not in I2 after A. dauci inoculation 42 , I2 being more resistant to fungal extract than Boléro and K3 43 , complementary favorable alleles for different rQTLs in I2 or K3 21 . From these previous results, we hypothesize that Boléro, I2 and K3 hosted different resistant mechanisms against A. dauci that could originate from differences in their secondary metabolite profile.…”

Identification and Quantification of a Phytotoxic Metabolite from Alternaria dauci

“…Previous studies on phytotoxin production by A. dauci have reported the identification of some non-host-selective phytotoxins (non-HST′s), including alternariol, alternariol monomethyl ether, and zinniol [ 5 , 7 , 8 , 9 , 10 , 11 ]. Recently, it was demonstrated that zinniol is not responsible for the phytotoxic effect caused by the organic extract of A. dauci [ 12 ]; these results, which strongly suggested that the phytotoxic effect of the organic extract of cultures from A. dauci was caused by other lipophilic metabolites, were confirmed with the identification of aldaulactone as a new phytotoxin involved in the aggressiveness of A. dauci against carrot cells [ 5 ]. Given these findings, and as part of our continuing interest in the isolation and identification of phytotoxic metabolites produced by A. dauci and their role in the infection process, we wish to report here on the bioassay-guided purification and identification of α -acetylorcinol ( 1 ) and p -hydroxybenzoic acid ( 2 ) as two additional lipophilic phytotoxic metabolites produced by the fungal pathogen.…”