Integrated Transcriptome and Metabolome Insights Into Floral Buds Fertility and Adaptive Mechanisms Under Long-Term Thermal Stress in Brassica napus L.

Rapeseed (Brassica napus L.) is sensitive to high-temperature events, particularly during the reproductive stage, which significantly affects yield. Climate change is predicted to be associated with high temperatures lasting longer than a few hours or days, and the effect of long-term temperature stress is still poorly documented. In the current study, we investigated the impact of long-term high-temperature stress (HTS) on floral buds. We revealed that long-term HTS (7-14 days) severely affects floral bud development with distinct alterations of the sporophytic anther tissue, such as tapetum, epidermis, endothecium, and stomium, with dramatic consequences on pollen viability and stigma receptivity. Comparative transcriptome and metabolome profiling upon exposure to HTS, 25°C/33°C for 11 days and control 18°C/25°C, showed 8194 and 10,786 DEGs and 636 and 696 DAMs during microsporogenesis (before late microspore, S1) and microgametogenesis (after microspore, S2), respectively. The results highlighted that prolonged HTS triggered a cascade of regulatory processes, altering the normal function and expression of genes involved in transcription regulation, signal transduction, photosynthesis and photosystem, cellular organization, and primary and secondary metabolites processing, leading to disruption of anther development and impaired pollen and stigma fertility. As a response, plants synthesize fatty acids, store them in wax, and activate phenylpropanoid and flavonoid biosynthesis pathways and amino acid metabolism as adaptive mechanisms. Thus, enhancing these pathways could help plants withstand HTS. Further investigation of genes and metabolites involved in these pathways could pave the way to develop thermotolerance B. napus toward genetic improvement.