The structure of Arabidopsis phytochrome A reveals topological and functional diversification among the plant photoreceptor isoforms

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.

Phytochrome B phosphorylation expanded: site-specific kinases are identified

The phytochrome B (phyB) photoreceptor is a key participant in red and far-red light sensing, playing a dominant role in many developmental and growth responses throughout the whole life of plants. Accordingly, phyB governs diverse signaling pathways, and although our knowledge about these pathways is constantly expanding, our view about their fine-tuning is still rudimentary. Phosphorylation of phyB is one of the relevant regulatory mechanisms, and - despite the expansion of the available methodology - it is still not easy to examine. Phosphorylated phytochromes have been detected using various techniques for decades, but the first phosphorylated phyB residues were only identified in 2013. Since then, concentrated attention has been turned toward the functional role of post-translational modifications in phyB signaling. Very recently in 2023, the first kinases that phosphorylate phyB were identified. These discoveries opened up new research avenues, especially by connecting diverse environmental impacts to light signaling and helping to explain some long-term unsolved problems such as the co-action of Ca2+ and phyB signaling. This review summarizes our recent views about the roles of the identified phosphorylated phyB residues, what we know about the enzymes that modulate the phospho-state of phyB, and how these recent discoveries impact future investigations.

Phytochrome-Interacting Proteins

Phytochromes are photoreceptors of plants, fungi, slime molds bacteria and heterokonts. These biliproteins sense red and far-red light and undergo light-induced changes between the two spectral forms, Pr and Pfr. Photoconversion triggered by light induces conformational changes in the bilin chromophore around the ring C-D-connecting methine bridge and is followed by conformational changes in the protein. For plant phytochromes, multiple phytochrome interacting proteins that mediate signal transduction, nuclear translocation or protein degradation have been identified. Few interacting proteins are known as bacterial or fungal phytochromes. Here, we describe how the interacting partners were identified, what is known about the different interactions and in which context of signal transduction these interactions are to be seen. The three-dimensional arrangement of these interacting partners is not known. Using an artificial intelligence system-based modeling software, a few predicted and modulated examples of interactions of bacterial phytochromes with their interaction partners are interpreted.