Breeding for carrot resistance to Alternaria dauci without compromising taste

Developing carrot varieties highly resistant to Alternaria dauci is a top priority for breeders. Meanwhile, consumers are increasingly demanding as regards final product quality, particularly taste. Bitterness is one of the five common taste sensations, but it is rejected by most carrot consumers. Therefore, providing tools for efficient plant breeding of resistant, low bitter carrots would be helpful. While resistance QTLs (rQTLs) have already been identified for carrot resistance to A. dauci, the genetic control and mapping of the metabolites involved in bitterness perception have not been addressed so far. We identified the metabolites most involved in bitterness by combining chemical and sensory analyses of a set of resistant and susceptible carrot genotypes grown in different environments. We evaluated their genetic control and heritability in a segregating F2:3 population over 2 years of field trials and searched for colocalizations between rQTLs and metabolite QTLs (mQTLs) to evaluate the link between bitterness and resistance traits. Our results suggest that it is possible to increase resistance while favoring low bitter varieties by selecting genomic regions involved in the expression of one or the other trait and counter-selecting others when r- and mQTL colocalization is unfavorable

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

International audienceThe 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

Influence of fungal exudates of Alternaria dauci on carrot partial resistance

Alternaria leaf blight, caused by the necrotrophic fungus Alternaria dauci, is the most damaging foliar disease in carrot production. Fungicide use has variable efficiency, and presents economic and ecological costs. Popular partially resistant cultivars exist, but their resistance level is still unsatisfactory. A better knowledge on carrot A. dauci resistance mechanisms may help to produce more resistant cultivars faster. More generally, partial resistance mechanisms in plants are still poorly understood. In order to better understand partial resistance to leaf blight in carrot, we implemented an a priori approach, investigating the role of different resistance mechanisms in the carrot-A. dauci interaction. It has been suggested that Alternaria dauci produces toxins, such as zinniol. The work presented here is centered on the role played by these toxins in the plant-fungus interaction, and more especially plant resistance. Plant cell suspensions from several genotypes were treated with raw fungal extracts. A good correlation was found between whole plant resistance to the fungus and metabolic activity of the cell suspensions after 48 h of exposure. Similar results were obtained using different techniques and different exposure times. Additionally, it was found that the toxicity of fungal extracts was due to secreted apolar compounds, which did not include zinniol. Zinniol is only produced when the fungus is grown in anoxic conditions. Nevertheless, fungal growth condition and zinniol content do not impact extract toxicity. Moreover, zinniol was not toxic to plant cells at physiological concentration