Summary
Under acute stress conditions, precocious seedling development may result in the premature death of young seedlings, before they switch to autotrophic growth. The phytohormone abscisic acid (ABA) inhibits seed germination and post‐germination seedling establishment under unfavorable conditions. Various environmental signals interact with the ABA pathway to optimize these early developmental events under stress. Here, we show that light availability critically influences ABA sensitivity during early seedling development. In dark conditions, the ABA‐mediated inhibition of post‐germination seedling establishment is strongly enhanced. COP1, a central regulator of seedling development in the dark, is necessary for this enhanced post‐germination ABA sensitivity in darkness. Despite their slower germination, cop1 seedlings establish faster than wild type in the presence of ABA in both light and dark. PHY and CRY photoreceptors that inhibit COP1 activity in light modulate ABA‐mediated inhibition of seedling establishment in light. Genetically, COP1 acts downstream to ABI5, a key transcriptional regulator of ABA signaling, and does not influence the transcriptional and protein levels of ABI5 during the early post‐germination stages. COP1 promotes post‐germination growth arrest independent of the antagonistic interaction between ABA and cytokinin signaling pathways. COP1 facilitates the binding of ABI5 on its target promoters and the ABA‐mediated upregulation of these target genes is reduced in cop1‐4. Together, our results suggest that COP1 positively regulates ABA signaling to inhibit post‐germination seedling establishment under stress.
Cotyledon opening is a key morphological change that occurs in seedlings during de-etiolation. Brassinosteroids (BR) inhibit opening of the cotyledons in darkness while light promotes cotyledon opening. The molecular regulation of the interplay between light and BR to regulate cotyledon opening is not well understood. Here, we show the B-box protein BBX32 negatively regulates light signaling and promotes BR signaling to inhibit cotyledon opening in Arabidopsis (Arabidopsis thaliana). BBX32 is highly expressed in the cotyledons of seedlings during de-etiolation. bbx32 and 35S:BBX32 seedlings exhibit enhanced and reduced cotyledon opening, respectively, in response to both light and brassinazole (BRZ) treatment in the dark, suggesting that BBX32 mediates cotyledon opening through both light and BR signaling pathways. BBX32 expression is induced by exogenous BR and is upregulated in bzr1-1D (BRASSINAZOLE RESISTANT1-1D). Our in vitro and in vivo interaction studies suggest that BBX32 physically interacts with BZR1. Further, we found that PIF3 (PHYTOCHROME INTERACTING FACTOR 3) interacts with BBX32 and promotes BR-mediated cotyledon closure. BBX32, BZR1, and PIF3 regulate the expression of common target genes that modulate the opening and closing of cotyledons. Our work suggests BBX32 integrates light and BR signals to regulate cotyledon opening during de-etiolation.
HighlightThis study demonstrates that overexpression of POTH15 (a class-I KNOX in potato) can alter multiple morphological traits, and identifies numerous POTH15 targets involved in diverse developmental processes in potato.
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