Heat Stress Suppresses Brassica napus Seed Oil Accumulation by Inhibition of Photosynthesis and BnWRI1 Pathway

Heat stress during Brassica napus seed filling severely impairs yield and oil content. However, the mechanisms underlying heat-stress effects on B. napus seed photosynthesis and oil accumulation remain elusive. In this study, we showed that heat stress resulted in reduction of seed oil accumulation, whereas the seed sugar content was enhanced, which indicated that incorporation of carbohydrates into triacylglycerols was impaired. Photosynthesis and respiration rates, and the maximum quantum yield of photosystem II in developing seeds were inhibited by heat stress. Transcriptome analysis revealed that heat stress led to up-regulation of genes associated with high light response, providing evidence that photoinhibition was induced by heat stress. BnWRI1 and its downstream genes, including genes involved in de novo fatty acid biosynthesis pathway, were down-regulated by heat stress. Overexpression of BnWRI1 with a seed-specific promoter stabilized both oil accumulation and photosynthesis under the heat-stress condition, which suggested BnWRI1 plays an important role in mediating the effect of heat stress on fatty acid biosynthesis. A number of sugar transporter genes were inhibited by heat stress, resulting in defective integration of carbohydrates into triacylglycerols units. The results collectively demonstrated that disturbances of the seed photosynthesis machinery, impairment of carbohydrates incorporation into triacylglycerols and transcriptional deregulation of the BnWRI1 pathway by heat stress might be the major cause of decreased oil accumulation in the seed.