• Work performed
1) We identified endosperm as a critical tissue that perceives temperature signal to control seed germination and seedling growth;
2) We identified that bent cotyledon stage is the precise stage of seed development in which Brassica seeds are most sensitive to temperature.
3) We constructed transcriptomic networks for endosperm and embryo in response to temperature shifts during seed development;
4) We demonstrated that ABA plays a crucial role in regulating endosperm-dependant seed germination in response to temperature changes.
5) We used TILLING to produce key mutants in Brassica oleracea which are involved in sensing temperature.
• Main results achieved so far
First, we constructed the transcriptomic networks in specific seed tissues responding to temperature by performing the time-series RNA-seq during seed maturation and seed germination. A specific cluster was identified in endosperm, which includes previously reported temperature or seed dormancy regulators, such as bZIP67, FUS3, MFT and SPT, ZOU. bZIP67 directly binds to the promoter of DOG1 and activates its expression. FUS3 is a LEFL transcription factor that plays important roles in seed maturation. In wheat MFT was identified as a temperature sensor to regulate seed germination. MFT also regulates seed dormancy with SPT in Arabidopsis as well. ZOU was recently shown to determine seed dormancy depth with another transcription factor ICE1. The similar transcriptomic pattern of these regulators indicates that there are core genes responds to temperature and controls seed germination in endosperm. Furthermore, we demonstrated a contrasting response of endospermic and embryonic DOG1. In high temperatures, endospermic DOG1 increased, while embryonic DOG1 decreased. This is a consequence of seed development, which is driven by high temperature.
Second, we found that ABA plays a key role in seed maturation process. ABA was reduced in endosperm and embryo when plants were in heat shock for only one day. The differential ABA levels further contribute to the final seed germination difference. By examining the time-series transcriptomic data, we found CYP707As were upregulated in high temperature. This accelerated ABA catabolism. Furthermore, we showed that the seed dormancy phenotype of Arabidopsis mutants aba2 and cyp707a1/2 was not dependant on temperature. Further, we demonstrated that absence of ABA attenuates the temperature effect on MFT and DOG1.
