30Once sentence summary: The contrasting roles of BBX21 and BBX24 in photomorphogenesis arise 31 through their different C-terminal regions and how they alter HY5 function at the post-transcriptional level. 33 Abstract 34The Arabidopsis thaliana BBX family comprises several positive and negative regulators of 35 photomorphogenesis. BBX24, a member of BBX structural group IV, acts as a negative regulator of 36 photomorphogenesis, while another member from the same group, BBX21, is a positive regulator. The 37 molecular basis for the functional diversity shown by these related BBX family members is unknown. 38Using domain-swap lines, we show that the C-terminal regions of BBX24 and BBX21 specify their

The bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) represents a major hub in the light-signaling cascade both under visible and UV-B light. The mode of transcriptional regulation of HY5, especially under UV-B light, is not well characterized. B-BOX (BBX) transcription factors regulate HY5 transcription and also posttranscriptionally modulate HY5 to control photomorphogenesis under white light. Here, we identify BBX31 as a key signaling intermediate in visible and UV-B light signal transduction in Arabidopsis (Arabidopsis thaliana). BBX31 expression is induced by UV-B radiation in a fluencedependent manner. HY5 directly binds to the promoter of BBX31 and regulates its transcript levels. Loss-and gain-of-function mutants of BBX31 indicate that it acts as a negative regulator of photomorphogenesis under white light but is a positive regulator of UV-B signaling. Genetic interaction studies suggest that BBX31 regulates photomorphogenesis independent of HY5. We found no evidence for a direct BBX31-HY5 interaction, and they primarily regulate different sets of genes in white light. Under high doses of UV-B radiation, BBX31 promotes the accumulation of UV-protective flavonoids and phenolic compounds. It enhances tolerance to UV-B radiation by regulating genes involved in photoprotection and DNA repair in a HY5-dependent manner. Under UV-B radiation, overexpression of BBX31 enhances HY5 transcriptional levels in a UV RESISTANCE LOCUS8-dependent manner, suggesting that BBX31 might regulate HY5 transcription.

Light plays an important role in plants’ growth and development throughout their life cycle. Plants alter their morphological features in response to light cues of varying intensity and quality. Dedicated photoreceptors help plants to perceive light signals of different wavelengths. Activated photoreceptors stimulate the downstream signaling cascades that lead to extensive gene expression changes responsible for physiological and developmental responses. Proteins such as ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) act as important factors which modulate light‐regulated gene expression, especially during seedling development. These factors function as central regulatory intermediates not only in red, far‐red, and blue light pathways but also in the UV‐B signaling pathway. UV‐B radiation makes up only a minor fraction of sunlight, yet it imparts many positive and negative effects on plant growth. Studies on UV‐B perception, signaling, and response in plants has considerably surged in recent times. Plants have developed different strategies to use UV‐B as a developmental cue as well as to withstand high doses of UV‐B radiation. Plants’ responses to UV‐B are an integration of its cross‐talks with both environmental factors and phytohormones. This review outlines the current developments in light signaling with a major focus on UV‐B‐mediated plant growth regulation.

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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.

Abscisic acid (ABA) plays a crucial role in plant development, regulating germination, seedling development and stomatal movements, especially under adverse conditions. Light interacts with the ABA signalling pathway to fine tune these processes. Here, we provide an overview of the recent investigations on ABA-light interplay during early plant development after germination. We discuss the multilayered and reciprocal interactions between ABA signalling components and several light signalling modulators, including photoreceptors, transcription factors and posttranslational modifiers. ABSCISIC ACID INSENSITIVE5 acts as a central convergence point for these interactions during postgermination seedling development. ABA also regulates the adaptation of seedlings to challenging light environments. Furthermore, we enlist the role of ABA-light cross-talk in regulating seedling establishment in crops and highlight open questions for future investigations.

Letters HY5 suppresses, rather than promotes, ABA-mediated inhibition of post-germination seedling development Dear Editor, Seed germination and post-germination seedling establishment are crucial early developmental events in angiosperms. Although the demarcations between these two successive events appear to be elusive, they have been defined as distinct developmental processes and identified to involve separate regulatory mechanisms at the molecular level. Seed germination is marked by the protrusion of embryonic root out of the seed coat. Post-germination seedling establishment denotes the developmental window after germination that involves the opening, greening and expansion of cotyledons or foliar leaves, marking the switch to autotrophic development (Lopez

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