Deciphering the role of three specific flavonoids produced by carrot as potential breeding markers for resistance against Alternaria dauci

Carrot (Daucus carota) is a high nutritional value crop as a natural source of antioxidants and pro-Vitamin A carotenoids. However, the world carrot production is constantly threatened by a foliar disease caused by Alternaria dauci fungus, impacting the harvestability of the product. As described in recent studies, the resistance to A. dauci is governed by several resistance quantitative trait loci (rQTLs), whose underlying mechanisms remain to be unveiled for future resistance breeding. Therefore, the aim of this study was to characterize one of these mechanisms, by highlighting the role played by three flavonoids differentially accumulated in carrot leaves. We showed that: i) two candidate genes (ST1 and TF1) underlying rQTLs were differentially expressed between resistant and susceptible lines; ii) the correlation between the content of the three candidate flavonoids and the carrot disease resistance level was evident, not only among parental lines used for the candidate identification, but also within other sources of resistance; iii) two of the three flavonoids, extracted and purified from Mentha × piperita and D. carota leaves, strongly impacted A. dauci conidial germination in vitro; and iv) finally, the kinetics of the flavonoid accumulation in the leaves showed that the earliest discriminant carrot developmental stage where resistance and accumulation of these flavonoids are quantitatively correlated, is the two-leaf stage. These results are promising for the development of an early breeding tool for breeders.