Abstract
The diverse biological activities of glucosinolate (GSL) hydrolysis products play significant biological and economical roles in the defense system and nutritional qualities of Brassica napus (oilseed rape). Yet, genomic-based study of the B. napus GSL regulatory mechanisms are scarce due to the complexity of working with polyploid species. To address these challenges, we used transcriptome-based GWAS approach, Associative Transcriptomics (AT), across a diversity panel of 288 B. napus genotypes to uncover the underlying genetic basis controlling quantitative variation of GSLs in B. napus vegetative tissues. Single nucleotide polymorphism (SNP) markers and gene expression markers (GEMs) associations identify orthologues of MYB28/HAG1 (AT5G61420), specifically the copies on chromosome A9 and C2, to be the key regulators of aliphatic GSL variation in leaves. We show that the positive correlation observed between aliphatic GSLs in seed and leaf is due to the amount synthesized, as controlled by Bna.HAG1.A9 and Bna.HAG1.C2, rather than by variation in the transport processes. In addition, AT and differential expression analysis in root tissues implicate an orthologue of MYB29/HAG3 (AT5G07690), Bna.HAG3.A3, as controlling root aromatic GSL variation. Based on the root expression data we also propose Bna.MAM3.A3 to have a role in controlling phenylalanine chain elongation for aromatic GSL biosynthesis. This work uncovers a regulator of homophenylalanine-derived aromatic GSLs and implicates the shared biosynthetic pathways between aliphatic and aromatic GSLs.
Highlights
The identification of Bronowski, the low-GSL Polish spring rape cultivar of B. napus, in the 1970s has provided the genetic source for all commercial B. napus cultivars of low seed GSLs through selective breeding (Rosa et al, 1997)
Past studies reported no significant correlation of GSL levels between seeds and leaves in canola cultivars, B. napus with low seed GSLs (Porter et al, 1991; Fieldsen and Milford, 1994), leading to the assumption that inhibition of GSL transport processes could have given rise to the low-seed GSL trait in B. napus
This hypothesis was supported by a report on the role of controlling GSL accumulation in A. thaliana seeds by GTR1 and GTR2, two members of the nitrate/peptide transporter family (Nour-Eldin et al, 2012)
Summary
The GSL profiles differ extensively between the leaves and roots in both type and amount Both the single nucleotide polymorphism and gene expression marker associations identify the MYB28/HAG1 orthologues on chromosomes A9 and C2 as key regulators for aliphatic GSLs in leaves. Datasets with large spreadsheets are uploaded with this thesis as zip files. Markers and genomic regions showing single nucleotide polymorphism association with variation for aliphatic GSL content in the leaf tissues. This data has been published as Appendix 9 in Kittipol et al (2019b). This data has been published as Appendix 11 in Kittipol et al (2019b)
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