Resistant carrots to Alternaria dauci or tasty carrots: should we choose?

Abstract

The carrot ideotype for carrot breeders could be a variety resistant to Alternaria leaf blight-the main foliar disease on carrot worldwide-with a low level of bitterness as consumers consider it as an undesirable taste of carrots. Our previous investigation on carrot resistance to Alternaria dauci allowed the identification of resistance QTLs (rQTLs). Based on these results, breeding for a higher level of resistance is possible but we could interrogate about the impact of selecting these genomic regions on carrot bitterness. To answer this question, we combined chemical and sensorial analyses of a set of resistant and susceptible carrot genotypes evaluated in different environments to identify the metabolites involved in bitterness. The genetic control and heritability field experimentation and colocalizations between rQTLs and metabolite QTLs (mQTLs) were searched in order to evaluate the link between bitterness and resistance traits. Our results showed that selecting the genomic regions mainly involved in carrot resistance is possible without compromising carrot taste. However, the colocalization of rQTLs and mQTLs suggest that some genomic regions are involved in the expression of both traits and should be avoided or even counter selected. INTRODUCTION While breeding for varieties highly resistant to Alternaria dauci is a top priority for carrot seed companies, consumers are more and more demanding on gustative quality, especially low bitter carrots. Amon others, terpens and polyacetylenes are considered key compounds for bitterness in carrots (Simon et al. 1980; Seljasen et al. 2001). Relying on sensory and chemical analyses, Kreutzmann et al. (2008) highlighted that a small number of metabolites, among them eight terpens, could predict bitterness. However, while the genetic control of carrot resistance to A. dauci has already been investigated, leading to the identification of rQTLs (Quantitative Trait Loci involved in resistance) in different genetic backgrounds (Le Clerc et al., 2009; 2015), no information were available except very recently published by Keilwagen at al. (2017) on terpens and polyacetylenes. Identifying the genomic regions involved in resistance and/or bitterness will be very helpful for breeders to develop resistant varieties while avoiding increasing bitterness. To answer this question, chemical and sensory analyses were performed on a set of five carrot genotypes during three years, in four environments to identify the terpens involved in bitterness. These metabolites were characterized during two consecutive years in field trials in a population already segregating for resistance to Alternaria dauci and their genetic control was determined. The colocalization between previously identified rQTLS and mQTLS (QTLs involved in the accumulation of metabolites) were examined and recommendations were made for breeders.