Comparative Transcriptomics Analysis and Functional Study Reveal Important Role of High-Temperature Stress Response Gene GmHSFA2 During Flower Bud Development of CMS-Based F(1) in Soybean

High-temperature (HT) is one of the most important environmental factors that negatively impact the yield of some soybean cytoplasmic male sterility (CMS)-based hybrid (F(1)) combinations. The response of soybean to HT, especially at the male organ development stage, is poorly understood. To investigate the molecular mechanisms of the response from soybean CMS-based F(1) male organ to HT, a detailed transcriptomics analysis was performed during flower bud development of soybean HT-tolerant and HT-sensitive CMS-based F(1) combinations (NF(1) and YF(1)) under normal-temperature and HT conditions. Obvious HT damage was observed by subjecting YF(1) with HT, such as indehiscent anthers and decreased pollen fertility, whereas the male fertility of NF(1) was normal. In total, 8,784 differentially expressed genes (DEGs) were found to respond to HT stress, which were mainly associated with anther/pollen wall development, carbohydrate metabolism and sugar transport, and auxin signaling. The quantitative real-time PCR (qRT-PCR) analysis and substance content detection also revealed that HT caused male fertility defects in YF(1) by altering pectin metabolism, auxin, and sugar signaling pathways. Most importantly, the sugar signaling-PIF-auxin signaling pathway may underlie the instability of male fertility in YF(1) under HT. Furthermore, HT induced the expression of heat shock factor (HSF) and heat shock protein (HSP) gene families. Overexpression of GmHSFA2 in Arabidopsis can promote the expression of HT protective genes (such as HSP20) by binding to the HSE motifs in their promoters, so as to improve the HT tolerance during flowering. Our results indicated that GmHSFA2 acted as a positive regulator, conferring HT tolerance improvement in soybean CMS-based F(1). GmHSFA2 may be directly involved in the activation of male fertility protection mechanism in the soybean CMS-based F(1) under HT stress.