Zhichong Huang

Seed germination and postgerminative 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.

Abscisic acid (ABA) suppresses Arabidopsis (Arabidopsis thaliana) seed germination and post-germinative growth. Nitrate stimulates seed germination, but whether it directly regulates ABA signaling and the associated underlying molecular mechanisms remain unknown. Here, we showed that nitrate alleviates the repressive effects of ABA on seed germination independently of the nitric oxide (NO) pathway. Moreover, nitrate attenuates ABA signaling activated by ABSCISIC ACID INSENSITIVE3 (ABI3) and ABI5, two critical transcriptional regulators of the ABA pathway. Mechanistic analyses demonstrated that ABI3 and ABI5 physically interact with the nitrate signaling-related core transcription factor NIN-LIKE PROTEIN 8 (NLP8). After ABA treatment, NLP8 suppresses ABA responses during seed germination without affecting ABA content. Notably, nitrate represses ABA signaling mainly through NLP8. Genetic analyses showed that NLP8 acts upstream of ABI3 and ABI5. Specifically, NLP8 inhibits the transcriptional functions of ABI3 and ABI5, as well as their ABA-induced accumulation. Additionally, NLP8 overexpression largely suppresses the ABA hypersensitivity of mutant plants exhibiting impaired NO biosynthesis or signaling. Collectively, our study reveals that nitrate counteracts the inhibitory effects of ABA signaling on seed germination and provides mechanistic insights into the NLP8–ABI3/ABI5 interactions and their antagonistic relationships in ABA signaling.