The Arabidopsis endosperm is a temperature-sensing tissue that implements seed thermoinhibition through phyB and PIF3

Seed thermoinhibition, the repression of germination under high temperatures, prevents seedling establishment under potentially fatal conditions. Thermoinhibition is relevant for ecology, phenology and agriculture, particularly in a warming globe. The temperature sensing mechanisms and signaling pathways sustaining thermoinhibition are unknown. We found that thermoinhibition in Arabidopsis thaliana is not autonomously controlled by the embryo but is rather implemented by the endosperm surrounding the embryo. High temperature is sensed through endospermic phyB by accelerating its reversion from the active signaling Pfr form into the inactive Pr form, as described in seedlings. This leads to stabilization of endospermic PIF3, which represses the expression of the endospermic ABA catabolic gene CYP707A1 and promotes endospermic ABA synthesis and release towards the embryo to block its growth. Furthermore, endospermic ABA represses embryonic PIF3 accumulation that would otherwise promote embryonic growth. Hence, under high temperatures PIF3 exerts opposite growth responses in the endosperm and embryo.