ISOLATION AND INITIAL CHARACTERIZATION OF ARABIDOPSIS MUTANTS THAT ARE DEFICIENT IN PHYTOCHROME-A

Phytochrome, a red/far-red-light photoreceptor protein of plants, is e ncoded by a small gene family. Phytochrome A (PHYA), the product of th e PHYA gene, is the predominant molecular species of phytochrome in et iolated tissue and has been best characterized biochemically. To defin e a role for PHYA, we isolated new mutants, designated fre1 (far-red e longated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were i dentified on the basis of their long hypocotyl phenotype under continu ous far-red light. Although the fre1 mutants lacked the hypocotyl resp onse to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PH YA appears to play only a minor role in the regulation of hypocotyl el ongation under natural conditions. In contrast, the fre1 mutation affe cted greening; a fre1 mutant was less able than the wild type to deeti olate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed signifi cantly in the fre1 mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis developme nt.