Red light‐regulated interaction of Per‐Arnt‐Sim histidine kinases with partner histidine‐containing phosphotransfer proteins in Physcomitrium patens

Multi‐step phosphorelay (MSP) is a broadly distributed signaling system in organisms. In MSP, histidine kinases (HKs) receive various environmental signals and transmit them by autophosphorylation followed by phosphotransfer to partner histidine‐containing phosphotransfer proteins (HPts). Previously, we reported that Per‐Arnt‐Sim (PAS) domain‐containing HK1 (PHK1) and PHK2 of the moss Physcomitrium (Physcomitrella) patens repressed red light‐induced protonema branching, a critical step in the moss life cycle. In plants, PHK homolog‐encoding genes are conserved only in early‐diverging lineages such as bryophytes and lycophytes. PHKs‐mediated signaling machineries attract attention especially from an evolutionary viewpoint, but they remain uninvestigated. Here, we studied the P. patens PHKs focusing on their subcellular patterns of localization and interaction with HPts. Yeast two‐hybrid analysis, a localization assay with a green fluorescent protein, and a bimolecular fluorescence complementation analysis together showed that PHKs are localized and interact with partner HPts mostly in the nucleus, as unprecedented features for plant HKs. Additionally, red light triggered the interactions between PHKs and HPts in the cytoplasm, and light co‐repressed the expression of PHK1 and PHK2 as well as genes encoding their partner HPts. Our results emphasize the uniqueness of PHKs‐mediated signaling machineries, and functional implications of this uniqueness are discussed. Two histidine kinases (HKs) of a moss plant showed nuclear localizations and interactions with their partner proteins, as unprecedented features in land plant HKs. Moreover, red light triggered cytoplasmic interactions of these HKs with the partner proteins. Functional implications of the uniqueness of these HKs are discussed, mainly from the evolutionary viewpoint.