Jiancan Du

Seed germination and post-germination growth are strictly regulated by phytohormones. In Arabidopsis thaliana, jasmonate (JA) and auxin suppress these processes by boosting abscisic acid (ABA) signaling. However, whether there exists a transcriptional switch orchestrating the intricate interplay among these hormones and the underlying molecular mechanisms remain elusive. Herein, we demonstrated that endogenous JA signaling, involving the CORONATINE INSENSITIVE1 (COI1)–JASMONATE ZIM-DOMAIN (JAZ) module and the downstream MYC2, MYC3, and MYC4 transcription factors, potentiates auxin-induced ABA responses. MYC2, MYC3, and MYC4 interact with AUXIN RESPONSE FACTOR10 (ARF10) and ARF16. Consistently, MYC2 and ARF16 synergistically augment ABA signaling-delayed seed germination. Furthermore, MYC2 and MYC4 form a protein complex with ABSCISIC ACID INSENSITIVE5 (ABI5), a master activator of ABA signaling. Genetic analyses suggested that MYC2 positively facilitates ABA responses largely through ABI5. Notably, MYC2 collaborates with ARF16 to enhance the transcriptional activity and protein abundance of ABI5. Intriguingly, MYC2, ARF16, and ABI5 assemble into a ternary complex. Simultaneously disrupting MYC, ARF10/ARF16, and ABI5 renders the seed germination process insensitive to combined JA, auxin, and ABA. Collectively, our findings unveil the MYC–ARF10/ARF16–ABI5 module as a pivotal molecular switch that integrates JA, auxin, and ABA signaling pathways, thereby enabling plants to mount adaptive responses to challenging environmental conditions.

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.

Seed size is crucial for crop yield and plant ecological fitness. The phytohormone jasmonate regulates Arabidopsis thaliana seed size, but the underlying molecular mechanisms remain elusive. Here, we established that CORONATINE INSENSITIVE1 (COI1)-mediated jasmonate signaling acts maternally to repress seed growth. Accordingly, jasmonate signaling suppresses the expression of KLUH (KLU), encoding an inducer of integument cell proliferation. KLU regulates the effects of COI1-mediated signaling on seed size. The JASMONATE ZIM-DOMAIN (JAZ) repressors of jasmonate signaling interact with SUPPRESSOR OF DA1-1 (SOD7) and DEVELOPMENT-RELATED PcG TARGET IN THE APEX4 (DPA4), two transcription factors that directly repress KLU. Overexpression of SOD7 largely rescues the seed size phenotype of coi1 mutants and JAZ1-overexpressing JAZ1-ΔJas plants. Furthermore, SOD7 associates with MYC2 and MYC4, two master transcriptional regulators of jasmonate signaling. SOD7 and MYC2 synergistically decrease KLU transcription and inhibit seed growth, while JAZ1 interferes with their transcriptional activities and physical interaction. Notably, jasmonate signaling considerably impacts seed size under salinity stress, primarily through SOD7 and KLU. Collectively, our findings suggest that the JAZ repressors and MYC transcription factors of the jasmonate signaling pathway coordinate with the SOD7/DPA4–KLU pathway to incorporate jasmonate signals into seed development.

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.

The phytohormone jasmonate (JA) coordinates stress and growth responses to increase plant survival in unfavorable environments. Although JA can enhance plant UV‐B stress tolerance, the mechanisms underlying the interaction of UV‐B and JA in this response remain unknown. In this study, we demonstrate that the UV RESISTANCE LOCUS 8 ‐ TEOSINTE BRANCHED1, Cycloidea and PCF 4 ‐ LIPOXYGENASE2 (UVR8‐TCP4‐LOX2) module regulates UV‐B tolerance dependent on JA signaling pathway in Arabidopsis thaliana. We show that the nucleus‐localized UVR8 physically interacts with TCP4 to increase the DNA‐binding activity of TCP4 and upregulate the JA biosynthesis gene LOX2. Furthermore, UVR8 activates the expression of LOX2 in a TCP4‐dependent manner. Our genetic analysis also provides evidence that TCP4 acts downstream of UVR8 and upstream of LOX2 to mediate plant responses to UV‐B stress. Our results illustrate that the UV‐B‐dependent interaction of UVR8 and TCP4 serves as an important UVR8‐TCP4‐LOX2 module, which integrates UV‐B radiation and JA signaling and represents a new UVR8 signaling mechanism in plants.

CONSTANS (CO) is a critical regulator of flowering that combines photoperiodic and circadian signals in Arabidopsis (Arabidopsis thaliana). CO is expressed in multiple tissues, including seedling roots and young leaves. However, the roles and underlying mechanisms of CO in modulating physiological processes outside of flowering remain obscure. Here, we show that the expression of CO responds to salinity treatment. CO negatively mediated salinity tolerance under long-day (LD) conditions. Seedlings from co mutants were more tolerant to salinity stress, whereas overexpression of CO resulted in plants with reduced tolerance to salinity stress. Further genetic analyses revealed the negative involvement of GIGANTEA (GI) in salinity tolerance requires a functional CO. Mechanistic analysis demonstrated that CO physically interacts with four critical basic leucine zipper (bZIP) transcription factors; ABSCISIC ACID-RESPONSIVE ELEMENT BINDING FACTOR1 (ABF1), ABF2, ABF3, and ABF4. Disrupting these ABFs made plants hypersensitive to salinity stress, demonstrating that ABFs enhance salinity tolerance. Moreover, ABF mutations largely rescued the salinity-tolerant phenotype of co mutants. CO suppresses the expression of several salinity-responsive genes and influences the transcriptional regulation function of ABF3. Collectively, our results show that the LD-induced CO works antagonistically with ABFs to modulate salinity responses, thus revealing how CO negatively regulates plant adaptation to salinity stress.

Abscisic acid (ABA) represses seed germination and postgerminative growth in Arabidopsis thaliana. Auxin and jasmonic acid (JA) stimulate ABA function; however, the possible synergistic effects of auxin and JA on ABA signaling and the underlying molecular mechanisms remain elusive. Here, we show that exogenous auxin works synergistically with JA to enhance the ABA-induced delay of seed germination. Auxin biosynthesis, perception, and signaling are crucial for JA-promoted ABA responses. The auxin-dependent transcription factors AUXIN RESPONSE FACTOR10 (ARF10) and ARF16 interact with JASMONATE ZIM-DOMAIN (JAZ) repressors of JA signaling. ARF10 and ARF16 positively mediate JA-increased ABA responses, and overaccumulation of ARF16 partially restores the hyposensitive phenotype of JAZ-accumulating plants defective in JA signaling in response to combined ABA and JA treatment. Furthermore, ARF10 and ARF16 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a critical regulator of ABA signaling, and the ability of ARF16 to stimulate JA-mediated ABA responses is mainly dependent on ABI5. ARF10 and ARF16 activate the transcriptional function of ABI5, whereas JAZ repressors antagonize their effects. Collectively, our results demonstrate that auxin contributes to the synergetic modulation of JA on ABA signaling, and explain the mechanism by which ARF10/16 coordinate with JAZ and ABI5 to integrate the auxin, JA, and ABA signaling pathways.

CONSTANS (CO) is a master flowering-time regulator that integrates photoperiodic and circadian signals in Arabidopsis thaliana. CO is expressed in multiple tissues, including young leaves and seedling roots, but little is known about the roles and underlying mechanisms of CO in mediating physiological responses other than flowering. Here, we show that CO expression is responsive to jasmonate. CO negatively modulated jasmonate-imposed root-growth inhibition and anthocyanin accumulation. Seedlings from co mutants were more sensitive to jasmonate, whereas overexpression of CO resulted in plants with reduced sensitivity to jasmonate. Moreover, CO mediated the diurnal gating of several jasmonate-responsive genes under long-day conditions. We demonstrate that CO interacts with JASMONATE ZIM-DOMAIN (JAZ) repressors of jasmonate signaling. Genetic analyses indicated that CO functions in a CORONATINE INSENSITIVE1 (COI1)-dependent manner to modulate jasmonate responses. Furthermore, CO physically associated with the basic helix-loop-helix (bHLH) subgroup IIId transcription factors bHLH3 and bHLH17. CO acted cooperatively with bHLH17 in suppressing jasmonate signaling, but JAZ proteins interfered with their transcriptional functions and physical interaction. Collectively, our results reveal the crucial regulatory effects of CO on mediating jasmonate responses and explain the mechanism by which CO works together with JAZ and bHLH subgroup IIId factors to fine-tune jasmonate signaling.