Comprehensive Transcriptome Profiling in Relation to Seed Storage Compounds in Tetralocular Brassica rapa

Tetralocular type of Brassica rapa has a wide silique, containing more seeds than in the bilocular type. To elucidate physiological and molecular mechanisms underlying seed development of the tetralocular silique, we defined the embryo development of tetralocular B. rapa (‘Sarsyun’) from 10 to 49 days after flowering (DAF) and measured total C, soluble sugars, lipid content, and changes in genome-wide transcript abundance during the seed filling phase. Hexoses transiently accumulated in young seeds at higher levels than sucrose, whereas a high sucrose level was observed in mature seeds. In contrast, the silique wall exhibited high levels of total soluble sugars at 14–35 DAF, but their levels dissipated after 35 DAF. The gradual decrease in total soluble sugars in the silique wall was likely correlated with seed lipid accumulation. Lipid accumulation in seeds began at approximately 21 DAF, reaching the maximum (45% dry matter) at 35–42 DAF. High lipid accumulation between 28 and 35 DAF was strongly correlated with a rapid increase in seed dry weight. In parallel, 34,423 contigs from four different developing seeds (21, 28, 35, and 42 DAF) were analyzed for transcriptional profiling of the developing seeds. Most genes involved in seed photosynthesis and carbohydrate metabolism were highly expressed at 21 or 28 DAF and were subsequently downregulated. The expression of genes coding for oleosins and fatty acid synthesis and elongation markedly increased at 28 DAF through 35 DAF, respectively, remaining high thereafter. The expression of major storage protein genes increased at 28 or 35 DAF. The accumulation of seed storage compounds in tetralocular B. rapa was highly dependent on photoassimilates in seeds and the silique wall. Transcriptional changes and transcript abundances in relation to seed storage compounds differed across seed development stages. Overall, dynamic changes to transcript abundance of most genes in relation to seed storage products occurred between 28 and 35 DAF. We proposed that the tetralocular ovary in B. rapa may be a useful trait for improving seed yield and yield contributing traits in Brassica species.