Abstract
Flavonoids, the compounds that impart color to fruits, flowers, and seeds, are the most widespread secondary metabolites in plants. However, a systematic analysis of these loci has not been performed in Brassicaceae. In this study, we isolated 649 nucleotide sequences related to flavonoid biosynthesis, i.e., the Transparent Testa (TT) genes, and their associated amino acid sequences in 17 Brassicaceae species, grouped into Arabidopsis or Brassicaceae subgroups. Moreover, 36 copies of 21 genes of the flavonoid biosynthesis pathway were identified in Arabidopsis thaliana, 53 were identified in Brassica rapa, 50 in Brassica oleracea, and 95 in B. napus, followed the genomic distribution, collinearity analysis and genes triplication of them among Brassicaceae species. The results showed that the extensive gene loss, whole genome triplication, and diploidization that occurred after divergence from the common ancestor. Using qRT-PCR methods, we analyzed the expression of 18 flavonoid biosynthesis genes in 6 yellow- and black-seeded B. napus inbred lines with different genetic background, found that 12 of which were preferentially expressed during seed development, whereas the remaining genes were expressed in all B. napus tissues examined. Moreover, 14 of these genes showed significant differences in expression level during seed development, and all but four of these (i.e., BnTT5, BnTT7, BnTT10, and BnTTG1) had similar expression patterns among the yellow- and black-seeded B. napus. Results showed that the structural genes (BnTT3, BnTT18, and BnBAN), regulatory genes (BnTTG2 and BnTT16) and three encoding transfer proteins (BnTT12, BnTT19, and BnAHA10) might play an crucial roles in the formation of different seed coat colors in B. napus. These data will be helpful for illustrating the molecular mechanisms of flavonoid biosynthesis in Brassicaceae species.
Highlights
Rapeseed (Brassica napus) is the most important source of edible vegetable oil and protein-rich meal in the world diet, and the seeds, which store protein and oil, are the major harvested plant organ (Nesi et al, 2008)
The number of gene families varied by species; for example, only 10 copies of PAL3 were found in 6 species, but 43 cinnamate 4-hydroxylase (C4H) gene copies occurred in 17 species with as many as 10 orthologs (BnC4H) in Boechera stricta (Bs). napus (Supplementary Table S2)
36 copies of 21 genes of the flavonoid biosynthesis pathway were identified in A. thaliana, but 53 were identified in B. rapa, 50 in B. oleracea, and 95 in B. napus, in accordance with the fact that sequences present as a single copy in the A. thaliana were present in 2–8 copies in B. napus (Cavell et al, 1998), and with the observation that excessive gene loss is typical after polyploidization in eukaryotes (Sankoff et al, 2010; Wang et al, 2011)
Summary
Rapeseed (Brassica napus) is the most important source of edible vegetable oil and protein-rich meal in the world diet, and the seeds, which store protein and oil, are the major harvested plant organ (Nesi et al, 2008). Flavonoid biosynthesis pathway play important roles in the coloration of fruits, flowers, and seeds, and numerous evidences had showed that TT-type genes and their homologs are crucial for the accumulation of flavonoids and their derivatives (Nesi et al, 2001; Winkel-Shirley, 2002; Xie et al, 2003; Baudry et al, 2004; Hoffmann et al, 2006; Lepiniec et al, 2006; Kasai et al, 2007). BrTT8 was recently shown to regulate the accumulation of proanthocyanidins (PAs) in the seed coat and to regulate the expression of the late biosynthetic genes (LBGs) of the flavonoid pathway in B. rapa, and an analysis in the “sarson” line of B. rapa showed that the yellow-seeded trait was caused by loss of BrTT8 function (Li X. et al, 2012). The molecular mechanism underlying the yellow seed coat trait is unclear in Brassica species
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