Regulation of Seed Germination at High Temperature in Arabidopsis thaliana

Correct timing of the transition from embryonic to post-embryonic development, which in plants coincides with the break of seed dormancy and the onset of germination, is critical for plant survival. Endogenous signals and environmental cues dictate the dormancy state of the seed and the timing of germination. Two hormones play a pivotal role in regulating this phase transition; the stress hormone abscisic acid (ABA) promotes dormancy and inhibits germination, while gibberellins (GA) stimulate germination. Previously, it was shown that unfavorable environmental conditions, such as supraoptimal temperature, stimulate ABA metabolism to induce secondary dormancy and inhibit germination. In this study, we characterized the mechanisms that regulate seed responses to high temperature. Microarray analysis revealed that the seed transcriptome is highly altered during prolonged heat stress. Genes involved in hormone metabolism and signaling, stress tolerance and seed maturation were enriched, a subset of which are targets of B3 domain transcription factors such as FUSCA3 (FUS3). We showed that FUS3, a master regulator of seed maturation and dormancy, is degraded during germination at optimal temperature. However, FUS3 degradation is inhibited in seeds imbibed at high temperature due to the high ABA/GA ratio. Overexpression of FUS3 increases seed sensitivity to high temperature by inducing de novo ABA synthesis, and confers greater seedling survival upon exit from heat stress. During supraoptimal temperature imbibition, the activity of the ubiquitin proteasome system (UPS) is suppressed in an ABA-dependent manner; both protein ubiquitination, and proteasome activity are strongly decreased. Similarly, UPS function is inhibited in seeds exposed to ABA at optimal temperature, but only within 2 days post-germination, a developmental window that governs seed sensitivity to ABA. Inhibition of proteasome activity arrested germination of excised, matured embryos, suggesting that protein degradation is required for germination. Supraoptimal temperature or ABA treatment do not alter transcription of most UPS genes, instead the ratio of various proteasomal complexes changes under these conditions, suggesting that ABA and high temperature affect proteasome activity posttranslationally. This study shows that supraoptimal temperature reshapes the transcriptome of imbibed seeds to induced stress tolerance, seed maturation, and hormone metabolism genes. A high ABA level inhibits UPS activity, leading to the accumulation of dormancy and stress regulators, ultimately inducing thermoinhibition. These findings provide a novel perspective on how hormones integrate environmental cues to regulate germination through modulation of UPS function.Ph.D