Abstract
Yellow seed is a desirable quality trait of the Brassica oilseed species. Previously, several seed coat color genes have been mapped in the Brassica species, but the molecular mechanism is still unknown. In the present investigation, map-based cloning method was used to identify a seed coat color gene, located on A9 in B. rapa. Blast analysis with the Arabidopsis genome showed that there were 22 Arabidopsis genes in this region including at4g09820 to at4g10620. Functional complementation test exhibited a phenotype reversion in the Arabidopsis thaliana tt8-1 mutant and yellow-seeded plant. These results suggested that the candidate gene was a homolog of TRANSPARENT TESTA8 (TT8) locus. BrTT8 regulated the accumulation of proanthocyanidins (PAs) in the seed coat. Sequence analysis of two alleles revealed a large insertion of a new class of transposable elements, Helitron in yellow sarson. In addition, no mRNA expression of BrTT8 was detected in the yellow-seeded line. It indicated that the natural transposon might have caused the loss in function of BrTT8. BrTT8 encodes a basic/helix-loop-helix (bHLH) protein that shares a high degree of similarity with other bHLH proteins in the Brassica. Further expression analysis also revealed that BrTT8 was involved in controlling the late biosynthetic genes (LBGs) of the flavonoid pathway. Our present findings provided with further studies could assist in understanding the molecular mechanism involved in seed coat color formation in Brassica species, which is an important oil yielding quality trait.
Highlights
B. rapa (AA) is an important crop and a model plant for studying Brassica genome evolution
A total of 1183 individuals were obtained from the BC4 black-seeded individuals, of which 565 plants were yellow-seeded and 618 plants were black-seeded. This segregation of yellow and black was consistent with the expected Mendelian ratio of 1:1 (x2 = 2.28, x20.05 = 3.84), confirming that only one seed color gene is present in the BC5 population
Seed color formation is due to the accumulation of flavonoids, mainly consists of flavonols, anthocyanins, phlobaphenes, isoflavones, and proanthocyanidins [30]
Summary
B. rapa (AA) is an important crop and a model plant for studying Brassica genome evolution. Yellow seed is a desirable quality trait of the Brassica oilseed species. Compared with their dark-seeded counterparts, yellow seeds of Brassica have inherent advantages, such as higher oil content [1,2,3]. Yellow sarson (Brassica rapa var.trilocularis) is a valuable yellow-seeded variety of B. rapa in India. It has been documented that there are two genes involved in controlling the seed color in B. rapa [6,7,8]. Seed color genes have been mapped and cloned in B. rapa during the past decade [9,10,11], but the molecular mechanism of the seed color formation has not been elaborated in B. rapa
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