Downstream effectors of light- and phytochrome-dependent regulation of hypocotyl elongation in Arabidopsis thaliana. Plant Molecular Biology 81

Abstract Arabidopsis, like most plants, exhibits tissue-specific, light-dependent growth responses. Cotyledon and leaf growth and the accumulation of photosynthetic pig-ments are promoted by light, whereas hypocotyl growth is inhibited. The identification and characterization of distinct phytochrome-dependent molecular effectors that are asso-ciated with these divergent tissue-specific, light-dependent growth responses are limited. To identify phytochrome-dependent factors that impact the photoregulation of hypocotyl length, we conducted comparative gene expression studies using Arabidopsis lines exhibiting dis-tinct patterns of phytochrome chromophore inactivation and associated disparate hypocotyl elongation responses under far-red (FR) light. A large number of genes was misregulated in plants lacking mesophyll-specific phyto-chromes relative to constitutively-deficient phytochrome lines. We identified and characterized genes whose expres-sion is impacted by light and by phyA and phyB that have roles in the photoregulation of hypocotyl length. We characterized the functions of several identified target genes by phenotyping of T-DNA mutants. Among these genes is a previously uncharacterized LHE (LIGHT-INDUCED HYPOCOTYL ELONGATION) gene, which we show impacts light- and phytochrome-mediated regulation of hypocotyl elongation under red (R) and FR illumination. We describe a new approach for identifying genes involved in light- and phytochrome-dependent, tissue-specific growth regulation and confirmed the roles of three such genes in the phyto-chrome-dependent photoregulation of hypocotyl length