Abstract
The chromophore of phytochrome is a linear tetrapyrrole, phytochromobilin. Phycoerythrobilin is one of the chromophores of phycobilin which is a cyanobacterial light-harvesting protein. These are biosynthesized through a common pathway. Although apo-phytochrome can bind with phycoerythrobilin as a chromophore, it no longer shows photoreversibility in vitro, because phycoerythrobilin lacks a double bond between C15 and C16 which is important for photoconversion and maintenance of phytochrome structure. To understand the role of phytochrome chromophore structure on plant development, we introduced the genes for phycoerythrobilin biosynthesis into the hy2 mutant which lack phytochromobilin. The plants expressing phycoerythrobilin (PEB plants) show long hypocotyl under light. Some mutants for the tetrapyrrole biosynthetic genes show the gun (genome uncoupled) phenotype which lose cooperativeness between nuclear gene expression and plastid development. PEB plants also show the gun phenotype. We discuss the relationship between chromophore structure and phytochrome function, and between tetrapyrrole biosynthesis and gun phenotype.
