Non‐angiosperm phytochromes and the evolution of vascular plants

Abstract

The phytochromes, a class of plant light‐sensing pigments, are a gene family with a long, complex evolutionary history. Angiosperms each have five or more phytochromes (designated A to E in Arabidopsis) with distinct functions as light receptors and only moderate sequence identities for different types within a species. The long‐term challenge taken up here is to trace the origin and function of the various motifs within the angiosperm phytochromes through gymnosperm phytochromes (types N, O and P) and lower plant phytochromes, sometimes reaching even to bacterial progenitor molecules. Particularly intriguing are the findings of homology of a C‐terminal region of phytochromes with bacterial transmitter modules and of a large N‐terminal region with a protein encoded by a gene from the cyanobacterum Synechocystis. Phylogenetic analysis helps to answer general questions such as the times of divergence of mono‐ and dicotyledons, of groups of gymnosperms or of ferns. Phytochrome sequences suggest (1) that mono‐ and dicotyledons became separated 150‐200 million years earlier than indicated by the fossil record and (2) that Ginkgo and Cycas have been separated unexpectedly late from the lineage giving rise to the Pinidae. (3) The status of Psilotum as a close relative of the primeval vascular plants is not supported. Phytochrome gene sequences additionally reveal that (4) moss and fern phytochromes have erratically acquired C‐termini which, though kinase‐like, are different from the common ones and that (5) introns have been lost, gained or shifted in position from algae to angiosperms. Phytochromes promise to be a rich source of phylogenetic information into the future as more sequences and functional data emerge, not least from studies of lower plants.