Temporal transcriptomes unravel the effects of heat stress on seed germination during wheat grain filling

Promoting seed germination after short episodes of heat stress during the wheat grain filling stage is a serious problem that results in pre-harvest sprouting. The plant hormones abscisic acid (ABA), gibberellins (GAs), and ethylene (ETH) are well known to be involved in germination control. However, the genes associated with the metabolism and responsiveness of these hormones to heat stress during wheat grain filling are not well understood. Transcriptomic analysis was carried out to explore the mechanisms controlling seed germination under five days (15-20 days after flowering, DAF) of heat stress (20, 24, 28, and 32 degrees C) in wheat grains at 15-30 DAF using comparative RNA sequencing. A dataset of 2073 differentially regulated genes was used to help elucidate the molecular mechanisms that respond to heat stress and affect seed germination in wheat. Some genes related to ABA, GA, and ETH biosynthesis, transport, and signaling had significantly different expression levels under heat stress. Among these genes, the transcriptional alterations of plant hormone-related genes, such as NCED9, AAO3, CYP707A2, GA20ox, and SAM1 uncovered here, provide a foundation for identifying key players involved in determining seed dormancy and germination. The expression levels of many germination-related genes did not linearly increase with increasing temperature. In this study, 28 degrees C is a threshold of temperature tolerance during the grain filling stage. Heat stress, especially extremely high temperature (>28 degrees C), represses ABA-related gene expression and promotes seed germination.