Interaction between phenylpropane metabolism and oil accumulation in the developing seed of Brassica napus revealed by high temporal-resolution transcriptomes.

BACKGROUND: Brassica napus is an important oilseed crop providing high-quality vegetable oils for human consumption and non-food applications. However, the regulation between embryo and seed coat for the synthesis of oil and phenylpropanoid compounds remains largely unclear. RESULTS: Here, we analyzed the transcriptomes in developing seeds at 2-day intervals from 14 days after flowering (DAF) to 64 DAF. The 26 high-resolution time-course transcriptomes are clearly clustered into five distinct groups from stage I to stage V. A total of 2217 genes including 136 transcription factors, are specifically expressed in the seed and show high temporal specificity by being expressed only at certain stages of seed development. Furthermore, we analyzed the co-expression networks during seed development, which mainly included master regulatory transcription factors, lipid, and phenylpropane metabolism genes. The results show that the phenylpropane pathway is prominent during seed development, and the key enzymes in the phenylpropane metabolic pathway, including TT5, BAN, and the transporter TT19, were directly or indirectly related to many key enzymes and transcription factors involved in oil accumulation. We identified candidate genes that may regulate seed oil content based on the co-expression network analysis combined with correlation analysis of the gene expression with seed oil content and seed coat content. CONCLUSIONS: Overall, these results reveal the transcriptional regulation between lipid and phenylpropane accumulation during B. napus seed development. The established co-expression networks and predicted key factors provide important resources for future studies to reveal the genetic control of oil accumulation in B. napus seeds.

Transcriptome shock in interspecific F1 allotriploid hybrids between Brassica species.

Interspecific hybridization drives the evolution of angiosperms and can be used to introduce novel alleles for important traits or to activate heterosis in crop breeding. Hybridization brings together gene expression networks from two different species, potentially causing global alterations of gene expression in the F1 plants which is called 'transcriptome shock'. Here, we explored such a transcriptome shock in allotriploid Brassica hybrids. We generated interspecific F1 allotriploid hybrids between the allotetraploid species Brassica napus and three accessions of the diploid species Brassica rapa. RNA-seq of the F1 hybrids and the parental plants revealed that 26.34-30.89% of genes were differentially expressed between the parents. We also analyzed expression level dominance and homoeolog expression bias between the parents and the F1 hybrids. The expression-level dominance biases of the Ar, An, and Cn subgenomes was genotype and stage dependent, whereas significant homoeolog expression bias was observed among three subgenomes from different parents. Furthermore, more genes were involved in trans regulation than in cis regulation in allotriploid F1 hybrids. Our findings provide new insights into the transcriptomic responses of cross-species hybrids and hybrids showing heterosis, as well as a new method for promoting the breeding of desirable traits in polyploid Brassica species.

DELLA proteins BnaA6.RGA and BnaC7.RGA negatively regulate fatty acid biosynthesis by interacting with BnaLEC1s in Brassica napus.

Seed oil content (SOC) and fatty acid (FA) composition determine the quality and economic value of rapeseed (Brassica napus). Little is known about the role of gibberellic acid (GA) in regulating FA biosynthesis in B. napus. Here, we discovered that four BnaRGAs (B. napus REPRESSOR OF GA), encoding negative regulators of GA signalling, were suppressed during seed development. Compared to the wild type, SOC was reduced in gain-of-function mutants bnaa6.rga-D and ds-3, which also showed reduced oleic acid and increased linoleic acid contents. By contrast, the loss-of-function quadruple mutant bnarga displayed higher SOC during early seed development than the wild type, with increased oleic acid and reduced linoleic acid contents. Notably, only BnaA6.RGA and BnaC7.RGA physically interacted with two BnaLEC1s, which function as essential transcription factors in FA biosynthesis. The FA composition did not significantly differ between bnarga bnalec1 sextuple mutants and bnalec1, suggesting that BnaLEC1s are epistatic to BnaRGAs in the regulation of FA composition. Furthermore, BnaLEC1-induced activation of BnaABI3 expression was repressed by BnaA6.RGA, indicating that GA triggers the degradation of BnaRGAs to relieve their repression of BnaLEC1s, thus promoting the transcription of downstream genes to facilitate oil biosynthesis. Therefore, we uncovered a developmental stage-specific role of GA in regulating oil biosynthesis via the GA-BnaRGA-BnaLEC1 signalling cascade, providing a novel mechanistic understanding of how phytohormones regulate FA biosynthesis in seeds. BnaRGAs represent promising targets for oil crop improvement.