Light and abscisic acid interplay in early seedling development

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.

HY5 Suppresses, Rather Than Promotes, Abscisic Acid-Mediated Inhibition of Postgermination Seedling Development

In contradiction with what has been reported before, the light signaling factor HY5 negatively regulates ABA-mediated inhibition of post-germination seedling growth by acting downstream to COP1.

Light signaling and UV-B-mediated plant growth regulation

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.

CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA-mediated inhibition of post-germination seedling establishment

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.

The B-Box-Containing MicroProtein miP1a/BBX31 Regulates Photomorphogenesis and UV-B Protection

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 fluence-dependent 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.

BBX31 promotes hypocotyl growth, primary root elongation and UV-B tolerance in Arabidopsis

Photomorphogenesis is an important developmental process that helps the seedlings adapt to external light conditions. B-Box proteins are a family of transcription factors that regulate photomorphogenic responses. BBX31 negatively regulates photomorphogenesis under visible light. In contrast, it promotes photomorphogenesis under UV-B and enhances tolerance to high doses of UV-B radiation. BBX31 and HY5 independently and oppositely regulate the ability of seedlings to adapt to varying light intensities. BBX31 also regulates primary root elongation under low intensities of white light. GC-MS and HPLC-based metabolite profiling identified differential accumulation of multiple primary and secondary metabolites in 35S:BBX31 that might enhance tolerance to UV-B.

The B-box bridge between light and hormones in plants

Plant development is meticulously modulated by interactions between the surrounding environment and the endogenous phytohormones. Light, as an external signal coordinates with the extensive networks of hormones inside the plant to execute its effects on growth and development. Several proteins in plants have been identified for their crucial roles in mediating light regulated development. Among these are the B-box (BBX) family of transcription factors characterized by the presence of zinc-finger B-box domain in their N-terminal region. In Arabidopsis there are 32 BBX proteins that are divided into five structural groups on the basis of the domains present. Several BBX proteins play important roles in seedling photomorphogenesis, neighbourhood detection and photoperiodic regulation of flowering. There is increasing evidence that besides light signaling BBX proteins also play integral roles in several hormone signaling pathways in plants. Here we attempt to comprehensively integrate the roles of multiple BBX proteins in various light and hormone signaling pathways. We further discuss the role of the BBX proteins in mediating crosstalk between the two signaling pathways to harmonize plant growth and development. Finally, we try to analyse the conservation of BBX genes across species and discuss the role of BBX proteins in regulating economically important traits in crop plants.

Two B-Box Proteins Regulate Photomorphogenesis by Oppositely Modulating HY5 through their Diverse C-Terminal Domains

The Arabidopsis (Arabidopsis thaliana) BBX family comprises several positive and negative regulators of photomorphogenesis. BBX24, a member of BBX structural group IV, acts as a negative regulator of photomorphogenesis, whereas another member from the same group, BBX21, is a positive regulator. The molecular basis for the functional diversity shown by these related BBX family members is unknown. Using domain-swap lines, we show that the C-terminal regions of BBX24 and BBX21 specify their function. Because both BBX21 and BBX24 work in close association with HY5, we hypothesized that these proteins differentially regulate the levels or activity of HY5 to fulfill their opposite roles. We show that BBX21 can regulate HY5 post-transcriptionally and the two proteins can coordinate to promote photomorphogenesis. By contrast, BBX24 interferes with the binding of HY5 to the promoter of an anthocyanin biosynthetic gene, possibly by heterodimerizing with HY5 and preventing it from binding DNA. Our finding that both BBX21 and BBX24 regulate HY5 activity post-transcriptionally, in opposite ways, suggests that closely related B-box proteins execute contrasting functions through differential regulation of HY5.

Opposite roles of group IV BBX proteins: Exploring missing links between structural and functional diversity

BBX proteins are a family of zinc finger transcription factors that are versatile regulators of plant development. The 32 BBX proteins in Arabidopsis are subdivided into five structural groups based on their domain structure. Members of group IV play important and diverse roles in light-regulated development. The N-terminal B-box domains mediate DNA binding and transcriptional regulation. The C-terminal region determines the functional diversity of the structurally similar group IV members as reported in our recent study investigating the basis of functional diversification between BBX21 and BBX24. We also found that multi-layered regulation of HY5 by the BBX proteins leads to a diverse repertoire of developmental effects. Here we provide a comprehensive structure-function analysis of the group IV BBX proteins.