PHYSIOLOGICAL AND ECOLOGICAL FUNCTION WITHIN THE PHYTOCHROME FAMILY

The phytochrome family of photoreceptors provides plants with a battery of sensors of the natural radiation environment. Each phytochrome can exist in two photoconvertible isomeric forms: The Pr form absorbs maximally in the red (R = 600-700 nm) and is photoconverted to the Pfr form, which absorbs maximally in the far-red (FR = 700-800 nm), thereby being converted to PI. Investigations of mutant and transgenic plants with altered levels of expression of the genes for phytochromes A and B (i.e. the PHPA and PHYB genes) have begun to provide detailed information on the physiological functions of individual members of the family. Current evidence indicates that phytochrome A (phyA) mediates the far-red high-irradiance-response (FR-HIR) and functions differentially throughout the life cycle. Phytochrome B (phyB) is the principal phytochrome responsible for the classical red/far-red (R/FR) reversible responses, and for the responses of light-grown plants to the R:FR ratio. Other phytochromes have minor functions in these responses. Evidence is accumulating that the Pr form of phyB (PrB) has biological activity and that responses mediated by phyB may be the result of antagonistic actions of PrB and PfrB. For both phyA and phyB, response is related to gene dosage, indicating that tight control of PHY gene expression throughout the life cycle is required for the proper operation of the phytochrome family. The advent of molecular genetic techniques has begun to provide approaches for testing the fitness correlates of phytochrome-mediated responses, and thus to provide a comprehensive view of the adaptive value of photomophogenesis. The same approaches are also being used in attempts to improve crop plant performance through genetic engineering of the PHY genes.