Evening complex proteins antagonize ABI3 and ABI5 to temporally regulate abscisic acid signaling and seed germination

Abstract Seed germination and post-germinative growth are precisely regulated by multiple signals. In Arabidopsis thaliana, the phytohormone abscisic acid (ABA) suppresses these processes and several circadian clock-associated proteins mediate ABA responses. Nevertheless, whether seed germination is controlled by temporal signals under diurnal conditions remains obscure, as do the associated underlying molecular mechanisms. Here, we found that the germination of wild-type seeds varies with time of release from cold stratification (i.e., transferred to 22°C) under diurnal conditions upon ABA, salinity or osmotic stress exposure. Additionally, the evening complex (EC) components EARLY FLOWERING 3 (ELF3), ELF4, and LUX ARRHYTHMO (LUX) attenuate ABA signaling. Notably, time-dependent seed germination relies on these EC components and other core clock proteins. ELF3, ELF4, and LUX physically interact with and act genetically upstream of ABSCISIC ACID INSENSITIVE3 (ABI3) and ABI5, two crucial transcriptional activators of ABA signaling. ELF3, ELF4, and LUX repress the function and accumulation of ABI3 and ABI5. Consistent with these results, ABI3 and ABI5 are essential for the time-based modulation of seed germination. Our findings highlight the critical effects of temporal signals on seed germination, and clarify the mechanism through which the EC components antagonize ABI3 and ABI5 to facilitate the crosstalk between the clock and ABA signaling pathways.