THE FUNCTION OF PHYTOCHROME-A

Knowledge of the photoperceptive function of phytochrome A has improve d substantially thanks to the availability of mutants lacking phytochr ome A and transgenic plants transformed with the PHYA gene in sense or antisense orientation. In imbibed seeds, phytochrome A mediates very- low-fluence responses, In etiolated seedlings, phytochrome A mediates very-low-fluence responses, high-irradiance responses under continuous far-red light, responsivity amplification to phytochrome B and red-li ght enhancement of the phototropic response to blue light, In light-gr own seedings, phytochrome A modulates the extent of response to reduct ions in red/far-red ratio perceived by phytochrome B, perceives daylen gth extensions and night interruptions affecting flowering, and percei ves light treatments resetting endogenous rhythms, Under natural radia tion these abilities are manifested during seed germination and seedli ng de-etiolation under dense canopies or extremely low light fluences, and during early neighbour detection, but other processes await exper imental evaluation, Phytochrome A affects growth and development throu ghout the whole life cycle of angiosperms.