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A. dauci is a fungal species with an ability to cause symptoms on a large range of dicotyledonous plants. This fungal species harbors a strong diversity of fungal strain with highly variable aggressivity levels (Boedo et al., 2008). Known resistant carrot genotypes show classical horizontal resistance, i.e. the resistance is quantitative, has a multigenic determinism involving QTLs (Le Clerc et al., 2009; Le Clerc et al., 2015b), and does not show cultivar x strain interaction (Le Clerc et al., 2015a).

Confronted with an interaction where the nature of plant resistance and fungal aggressivity is mainly quantitative, we first set up different phenotyping methods enabling a better measure of disease severity (Boedo et al., 2010; Lecomte et al., 2014). Methods tested included different inoculation methods, qPCR quantification of fungal biomass, and challenging plant cell suspension with fungal toxins. Results obtained led us to the hypothesis that different resistance mechanisms were variously involved in the macroscopically observed resistance level (Boedo et al., 2010). The fact that many favorable allele for the resistance QTLs are genotype-specific (Le Clerc et al., 2015b) fits in such a hypothesis.

Three possible resistance mechanisms were more thoroughly investigated:

* The jasmonates pathway activation: when the fungus is present, it was observed in some, but not all, resistant genotypes (Lecomte et al., 2011).

* Defense molecules: they probably play a role in nonhost resistance, but their role in observed host resistance level could not be confirmed (Lecomte et al., 2012).

* Fungal toxins: Carrot cell resistance to fungal toxins is correlated with whole plant resistance to A. dauci (Lecomte et al., 2014; Figure 1).

Today, we are pursuing this line of research in two complementary directions:

* Deepening: to better understand toxin resistance, we are studying A. dauci toxins, their nature, their synthesis pathways and plant resistance mechanisms (ResTox project), and

* Broadening: QTLs for the accumulation of plant metabolites (mQTLs) will be mapped on the same populations than known resistance QTLs. The purpose is to select mQTL co-locating with resistance QTLs (MetaboCar project).