期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2014
卷号：111
期号：44
页码：15827-15832
DOI：10.1073/pnas.1416751111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificancePhytochromes are photosensory signaling proteins widely distributed in unicellular organisms and multicellular land plants. Best known for their global regulatory roles in photomorphogenesis, plant phytochromes are often assumed to have arisen via gene transfer from the cyanobacterial endosymbiont that gave rise to photosynthetic chloroplast organelles. Our analyses support the scenario that phytochromes were acquired prior to diversification of the Archaeplastida, possibly before the endosymbiosis event. We show that plant phytochromes are structurally and functionally related to those discovered in prasinophytes, an ecologically important group of marine green algae. Based on our studies, we propose that these phytochromes share light-mediated signaling mechanisms with those of plants. Phytochromes presumably perform critical acclimative roles for unicellular marine algae living in fluctuating light environments. Phytochrome photosensors control a vast gene network in streptophyte plants, acting as master regulators of diverse growth and developmental processes throughout the life cycle. In contrast with their absence in known chlorophyte algal genomes and most sequenced prasinophyte algal genomes, a phytochrome is found in Micromonas pusilla, a widely distributed marine picoprasinophyte (<2 {micro}m cell diameter). Together with phytochromes identified from other prasinophyte lineages, we establish that prasinophyte and streptophyte phytochromes share core light-input and signaling-output domain architectures except for the loss of C-terminal response regulator receiver domains in the streptophyte phytochrome lineage. Phylogenetic reconstructions robustly support the presence of phytochrome in the common progenitor of green algae and land plants. These analyses reveal a monophyletic clade containing streptophyte, prasinophyte, cryptophyte, and glaucophyte phytochromes implying an origin in the eukaryotic ancestor of the Archaeplastida. Transcriptomic measurements reveal diurnal regulation of phytochrome and bilin chromophore biosynthetic genes in Micromonas. Expression of these genes precedes both light-mediated phytochrome redistribution from the cytoplasm to the nucleus and increased expression of photosynthesis-associated genes. Prasinophyte phytochromes perceive wavelengths of light transmitted farther through seawater than the red/far-red light sensed by land plant phytochromes. Prasinophyte phytochromes also retain light-regulated histidine kinase activity lost in the streptophyte phytochrome lineage. Our studies demonstrate that light-mediated nuclear translocation of phytochrome predates the emergence of land plants and likely represents a widespread signaling mechanism in unicellular algae.
