Phytochrome-Deficient hy1 and hy2 Long Hypocotyl Mutants of Arabidopsis Are Defective in Phytochrome Chromophore Biosynthesis.

Abstract

The hy1 and hy2 long hypocotyl mutants of Arabidopsis contain normal levels of immunochemically detectable phytochrome A, but the molecule is photochemically nonfunctional. We have investigated the biochemical basis for this lack of function. When the hy1 and hy2 mutants were grown in white light on a medium containing biliverdin IX[alpha], a direct precursor to phytochromobilin, the phytochrome chromophore, the seedlings developed with a morphological phenotype indistinguishable from the light-grown wild-type control. Restoration of a light-grown phenotype in the hy1 mutant was also accomplished by using phycocyanobilin, a tetrapyrrole analog of phytochromobilin. Spectrophotometric and immunochemical analyses of the rescued hy1 and hy2 mutants demonstrated that they possessed wild-type levels of photochemically functional phytochrome that displayed light-induced conformational changes in the holoprotein indistinguishable from the wild type. Moreover, phytochrome A levels declined in vivo in response to white light in rescued hy1 and hy2 seedlings, indicative of biliverdin-dependent formation of photochemically functional phytochrome A that was then subject to normal selective turnover in the far-red-light-absorbing form. Combined, these data suggest that the hy1 and hy2 mutants are inhibited in chromophore biosynthesis at steps prior to the formation of biliverdin IX[alpha], thus potentially causing a global functional deficiency in all members of the phytochrome photoreceptor family.

Photoresponses of Transgenic Arabidopsis Overexpressing the Fern Adiantum capillus-veneris PHY1

“…Their hypocotyl lengths were not reduced by the application of biliverdin but in fact were even longer than their negative controls. In contrast, the phytochrome chromophore mutant hyl (Koornneef et al, 1980) was reduced to a similar leve1 to that of wild-type seedlings, as shown previously (Parks and Quail, 1991). These results indicate that the phenotype of the PHYl transgenic plants is not a consequence of reduced chromophore availability for the endogenous PHYA in the transgenic plants.…”

CUE1: A Mesophyll Cell-Specific Positive Regulator of Light-Controlled Gene Expression in Arabidopsis.

“…The mutant screen is based on the selection for survivors of allyl alcohol treatment of mutagenized Arabidopsis lines containing a CAB3 promoter fused to the ADH gene, thus allowing the identification of mutants-independent of phenotype-in which a tightly regulated light-controlled promoter is expressed at low levels in the light. Our data indicate that the screen is robust, resulting in mutations in at least 11 independent cue loci (E. Lopez and J. Chory, unpublished results), including loci encoding the apoprotein component of phytochrome 6 (Reed et al, 1993), and the synthesis of the linear tetrapyrrole common to all phytochromes, hyl (Koornneef et al, 1980;Parks and Quail, 1991). The fact that we found photoreceptor mutations indicates that we might recover alleles in all of the positively acting components of phytochrome signaling of CA5 gene expression.…”

A New Class of Arabidopsis Constitutive Photomorphogenic Genes Involved in Regulating Cotyledon Development.

“…These mutants, including six hy loci and three blu loci, develop long hypocotyls in the light. Mutants in three of these loci (hyl, hy2, and hy6) are deficient in functional phytochrome due to a defect in the biosynthesis of the phytochrome chromophore (Parks and Quail, 1991;Chory, 1992). Mutations at the hy3 locus cause a specific reduction in type B phytochrome (Somers et al, 1991).…”