Identification of metabolic QTLs and candidate genes for glucosinolate synthesis in Brassica oleracea leaves, seeds and flower buds

Abstract

Glucosinolates are major secondary metabolites found in the Brassicaceae family. These compounds play an essential role in plant defense against biotic and abiotic stresses, but more interestingly they have beneficial effects on human health. We performed a genetic analysis in order to identify the genome regions regulating glucosinolates biosynthesis in a DH mapping population of Brassica oleracea. In order to obtain a general overview of regulation in the whole plant, analyses were performed in the three major organs where glucosinolates are synthesized (leaves, seeds and flower buds). Eighty two significant QTLs were detected, which explained a broad range of variability in terms of individual and total glucosinolate (GSL) content. A meta-analysis rendered eighteen consensus QTLs. Thirteen of them regulated more than one glucosinolate and its content. In spite of the considerable variability of glucosinolate content and profiles across the organ, some of these consensus QTLs were identified in more than one tissue. Consensus QTLs control the GSL content by interacting epistatically in complex networks. Based on in silico analysis within the B. oleracea genome along with synteny with Arabidopsis, we propose seven major candidate loci that regulate GSL biosynthesis in the Brassicaceae family. Three of these loci control the content of aliphatic GSL and four of them control the content of indolic glucosinolates. GSL-ALK plays a central role in determining aliphatic GSL variation directly and by interacting epistatically with other loci, thus suggesting its regulatory effect.

Figure 1. Formation of the core structure of the three major groups of glucosinolates in A.thaliana, including the genes controlling this process , .

(A). A biochemical genetic model of the biosynthesis of aliphatic glucosinolates in Brassicaceae including the major genes controlling this process (B).

Figure 2. Framework map of DH population showing eighty-two metabolic quantitative trait loci (QTL) for individual GSLs and sums of GSLs.

Linkage groups were labeled by following the nomenclature of Iñiguez-Luy et al. . Bars represent the LOD confidence interval of each QTL. QTLs are in different colors depending on the plant organ: leaves (green), flower buds (red) and seeds (blue). After the name of each QTL, -P₁ indicates allele from DH rapid cycling of Chinese kale (TO1000DH3) and -P₂ indicates allele from DH broccoli line ‘Early Big’.

Figure 3. An epistatic network including all the significant relationships of QTL9.2 (GSL-ALK) with other QTLs.

Aliphatic glucosinolates: GIV, Glucoiberverin; GIB, Glucoiberin; SIN, Sinigrin; GER, Glucoerucin; GRA, Glucoraphanin; GNA, Gluconapin; PRO, Progoitrin; ALY, Glucoalyssin; GBN, Glucobrassicanapin; ALIPH: sum of aliphatic GSLs; Indolic glucosinolate: GBS, Glucobrassicin; TOTAL: sum of total GSLs. Organs: L, Leaves; F: Flower buds; S: seeds. Continuous lines represent positive epistatic interactions while dashed lines represent negative epistatic interactions.