Blue light-induced phosphorylation of Arabidopsis cryptochrome 1 is essential for its photosensitivity

Liquid-liquid phase separation of TZP promotes PPK-mediated phosphorylation of the phytochrome A photoreceptor

Phytochrome A (phyA) is the plant far-red (FR) light photoreceptor and plays an essential role in regulating photomorphogenic development in FR-rich conditions, such as canopy shade. It has long been observed that phyA is a phosphoprotein in vivo; however, the protein kinases that could phosphorylate phyA remain largely unknown. Here we show that a small protein kinase family, consisting of four members named PHOTOREGULATORY PROTEIN KINASES (PPKs) (also known as MUT9-LIKE KINASES), directly phosphorylate phyA in vitro and in vivo. In addition, TANDEM ZINC-FINGER/PLUS3 (TZP), a recently characterized phyA-interacting protein required for in vivo phosphorylation of phyA, is also directly phosphorylated by PPKs. We reveal that TZP contains two intrinsically disordered regions in its amino-terminal domain that undergo liquid-liquid phase separation (LLPS) upon light exposure. The LLPS of TZP promotes colocalization and interaction between PPKs and phyA, thus facilitating PPK-mediated phosphorylation of phyA in FR light. Our study identifies PPKs as a class of protein kinases mediating the phosphorylation of phyA and demonstrates that the LLPS of TZP contributes significantly to more production of the phosphorylated phyA form in FR light.

Light signaling in plants-a selective history

In addition to providing the radiant energy that drives photosynthesis, sunlight carries signals that enable plants to grow, develop and adapt optimally to the prevailing environment. Here we trace the path of research that has led to our current understanding of the cellular and molecular mechanisms underlying the plant's capacity to perceive and transduce these signals into appropriate growth and developmental responses. Because a fully comprehensive review was not possible, we have restricted our coverage to the phytochrome and cryptochrome classes of photosensory receptors, while recognizing that the phototropin and UV classes also contribute importantly to the full scope of light-signal monitoring by the plant.

Photoregulatory protein kinases fine-tune plant photomorphogenesis by directing a bifunctional phospho-code on HY5 in Arabidopsis

Photomorphogenesis is a light-dependent plant growth and development program. As the core regulator of photomorphogenesis, ELONGATED HYPOCOTYL 5 (HY5) is affected by dynamic changes in its transcriptional activity and protein stability; however, little is known about the mediators of these processes. Here, we identified PHOTOREGULATORY PROTEIN KINASE 1 (PPK1), which interacts with and phosphorylates HY5 in Arabidopsis, as one such mediator. The phosphorylation of HY5 by PPK1 is essential to establish high-affinity binding with B-BOX PROTEIN 24 (BBX24) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), which inhibit the transcriptional activity and promote the degradation of HY5, respectively. As such, PPKs regulate not only the binding of HY5 to its target genes under light conditions but also HY5 degradation when plants are transferred from light to dark. Our data identify a PPK-mediated phospho-code on HY5 that integrates the molecular mechanisms underlying the regulation of HY5 to precisely control plant photomorphogenesis.

Arabidopsis hypocotyl growth in darkness requires the phosphorylation of a microtubule-associated protein

Rapid hypocotyl elongation allows buried seedlings to emerge, where light triggers de-etiolation and inhibits hypocotyl growth mainly by photoreceptors. Phosphorylation/dephosphorylation events regulate many aspects of plant development. Only recently we have begun to uncover the earliest phospho-signaling responders to light. Here, we reported a large-scale phosphoproteomic analysis and identified 20 proteins that changed their phosphorylation pattern following a 20 min light pulse compared to darkness. Microtubule-associated proteins were highly overrepresented in this group. Among them, we studied CIP7 (COP1-INTERACTING-PROTEIN 7), which presented microtubule (MT) localization in contrast to the previous description. An isoform of CIP7 phosphorylated at Serine915 was detected in etiolated seedlings but was undetectable after a light pulse in the presence of photoreceptors, while CIP7 transcript expression decays with long light exposure. The short hypocotyl phenotype and rearrangement of MTs in etiolated cip7 mutants are complemented by CIP7-YFP and the phospho-mimetic CIP7S915D -YFP, but not the phospho-null CIP7S915A -YFP suggesting that the phosphorylated S915 CIP7 isoform promotes hypocotyl elongation through MT reorganization in darkness. Our evidence on Serine915 of CIP7 unveils phospho-regulation of MT-based processes during skotomorphogenic hypocotyl growth.

The dual-action mechanism of Arabidopsis cryptochromes

Photoreceptor cryptochromes (CRYs) mediate blue-light regulation of plant growth and development. It has been reported that Arabidopsis CRY1and CRY2 function by physically interacting with at least 84 proteins, including transcription factors or co-factors, chromatin regulators, splicing factors, messenger RNA methyltransferases, DNA repair proteins, E3 ubiquitin ligases, protein kinases and so on. Of these 84 proteins, 47 have been reported to exhibit altered binding affinity to CRYs in response to blue light, and 41 have been shown to exhibit condensation to CRY photobodies. The blue light-regulated composition or condensation of CRY complexes results in changes of gene expression and developmental programs. In this mini-review, we analyzed recent studies of the photoregulatory mechanisms of Arabidopsis CRY complexes and proposed the dual mechanisms of action, including the "Lock-and-Key" and the "Liquid-Liquid Phase Separation (LLPS)" mechanisms. The dual CRY action mechanisms explain, at least partially, the structural diversity of CRY-interacting proteins and the functional diversity of the CRY photoreceptors.

Diversification of plant SUPPRESSOR OF MAX2 1 (SMAX1)-like genes and genome-wide identification and characterization of cotton SMXL gene family

BACKGROUND

Strigolactones (SLs) are a recently discovered class of plant hormones. SUPPRESSOR OF MAX2 1 (SMAX1)-like proteins, key component of the SL signaling pathway, have been studied extensively for their roles in regulating plant growth and development, such as plant branching. However, systematic identification and functional characterization of SMXL genes in cotton (Gossypium sp.), an important fiber and oil crop, has rarely been conducted.

RESULTS

We identified 210 SMXL genes from 21 plant genomes and examined their evolutionary relationships. The structural characteristics of the SMXL genes and their encoded proteins exhibited both consistency and diversity. All plant SMXL proteins possess a conserved Clp-N domain, P-loop NTPase, and EAR motif. We identified 63 SMXL genes in cotton and classified these into four evolutionary branches. Gene expression analysis revealed tissue-specific expression patterns of GhSMXL genes, with some upregulated in response to GR24 treatment. Protein co-expression network analysis showed that GhSMXL6, GhSMXL7-1, and GhSMXL7-2 mainly interact with proteins functioning in growth and development, while virus-induced gene silencing revealed that GhSMAX1-1 and GhSMAX1-2 suppress the growth and development of axillary buds.

CONCLUSIONS

SMXL gene family members show evolutionary diversification through the green plant lineage. GhSMXL6/7-1/7-2 genes play critical roles in the SL signaling pathway, while GhSMXL1-1 and GhSMXL1-2 function redundantly in growth of axillary buds. Characterization of the cotton SMXL gene family provides new insights into their roles in responding to SL signals and in plant growth and development. Genes identified in this study could be used as the candidate genes for improvement of plant architecture and crop yield.

The Role of Light Quality in Regulating Early Seedling Development

It is well-established that plants are sessile and photoautotrophic organisms that rely on light throughout their entire life cycle. Light quality (spectral composition) is especially important as it provides energy for photosynthesis and influences signaling pathways that regulate plant development in the complex process of photomorphogenesis. During previous years, significant progress has been made in light quality's physiological and biochemical effects on crops. However, understanding how light quality modulates plant growth and development remains a complex challenge. In this review, we provide an overview of the role of light quality in regulating the early development of plants, encompassing processes such as seed germination, seedling de-etiolation, and seedling establishment. These insights can be harnessed to improve production planning and crop quality by producing high-quality seedlings in plant factories and improving the theoretical framework for modern agriculture.