2 DOMINANT PHOTOMORPHOGENIC MUTATIONS OF ARABIDOPSIS-THALIANA IDENTIFIED AS SUPPRESSOR MUTATIONS OF HY2

By screening suppressor mutants of the hy2 mutation of Arabidopsis tha liana, two dominant photomorphogenic mutants, shy1-1D and shy2-1D, for two genetic loci designated as SHY1 and SHY2 (suppressor of hy2 mutat ion) have been isolated. Both of these non-allelic, extragenic suppres sor mutations of hy2 are located on chromosome 1 of the Arabidopsis ge nome. Both mutations suppress the elongated hypocotyl phenotype of hy2 by light-independent inhibition of hypocotyl growth as well as by inc reasing the effectiveness of light inhibition of hypocotyl elongation. The shy1-1D mutation is partially photomorphogenic in darkness with a pical hook opening and reduced hypocotyl elongation. The shy2-1D mutan t displays highly photomorphogenic characteristics in darkness such as true leaf development, cotyledon expansion, and extremely reduced hyp ocotyl growth. In regard to hypocotyl elongation, however, the shy2-1D mutation is still light sensitive. Examination of red/far-red light r esponses shows that the shy1-1D mutation suppresses the hypocotyl elon gation of the hy2 mutation effectively in red light but not effectivel y in far-red light. The shy2-1D suppresses hypocotyl elongation of the hy2 mutation effectively in both red and far-red light. Both mutation s can also suppress the early-flowering phenotype of hy2 and have a di stinct pleiotropic effect on leaf development such as upward leaf roll ing. The data obtained suggest that SHY1 and SHY2 represent a novel cl ass of components involved in the photomorphogenic pathways of Arabido psis. This is the first report on the identification of dominant mutat ions in the light signal transduction pathway of plants.