Plant developmental processes are controlled by both endogenous programs and environmental stimuli. As a photomorphogenetic mutant, hy5 of Arabidopsis has been isolated and characterized. Our detailed characterization has revealed that the mutant is deficient in a variety of stimulus responses, including gravitropic response and waving growth of roots, as well as light-dependent hypocotyl elongation. In the roots and hypocotyl, the hy5 mutation also affects greening and specific cell proliferation such as lateral root formation and secondary thickening. Those phenotypes indicate that the HY5 gene is responsible for the regulation of fundamental developmental processes of the plant cell: cell elongation, cell proliferation, and chloroplast development. Molecular cloning of the HY5 gene using a T-DNA-tagged mutant has revealed that the gene encodes a protein with a bZIP motif, one of the motifs found in transcriptional regulators. Nuclear localization of the HY5 protein strongly suggests that the HY5 gene modulates the signal transduction pathways under the HY5-related development by controlling expression of genes downstream of these pathways.

UV-A͞blue light acts to regulate a number of physiological processes in higher plants. These include light-driven chloroplast movement and phototropism. The NPH1 gene of Arabidopsis encodes an autophosphorylating protein kinase that functions as a photoreceptor for phototropism in response to low-intensity blue light. However, nph1 mutants have been reported to exhibit normal phototropic curvature under high-intensity blue light, indicating the presence of an additional phototropic receptor. A likely candidate is the nph1 homologue, npl1, which has recently been shown to mediate the avoidance response of chloroplasts to high-intensity blue light in Arabidopsis. Here we demonstrate that npl1, like nph1, noncovalently binds the chromophore flavin mononucleotide (FMN) within two specialized PAS domains, termed LOV domains. Furthermore, when expressed in insect cells, npl1, like nph1, undergoes light-dependent autophosphorylation, indicating that npl1 also functions as a light receptor kinase. Consistent with this conclusion, we show that a nph1 npl1 double mutant exhibits an impaired phototropic response under both low-and highintensity blue light. Hence, npl1 functions as a second phototropic receptor under high fluence rate conditions and is, in part, functionally redundant to nph1. We also demonstrate that both chloroplast accumulation in response to low-intensity light and chloroplast avoidance movement in response to high-intensity light are lacking in the nph1 npl1 double mutant. Our findings therefore indicate that nph1 and npl1 show partially overlapping functions in two different responses, phototropism and chloroplast relocation, in a fluence rate-dependent manner.L ight is an important environmental factor controlling plant growth and development. In particular, wavelengths in UV-A (320-390 nm) and blue (390-500 nm) regions of the electromagnetic spectrum act to regulate a range of different plant responses. These processes include de-etiolation, photoentrainment of the circadian clock, floral initiation, phototropic curvature, chloroplast relocation, and stomatal opening (1-3). Much of our understanding of blue light perception in higher plants has come from the isolation of blue-light-response mutants of Arabidopsis thaliana. Indeed, molecular genetic studies have shown that the effects of blue light on plant development are mediated by at least four different blue-light receptors in Arabidopsis: cryptochrome 1 (cry1), cryptochrome 2 (cry2), phototropin (nph1, for non-phototropic hypocotyl 1), and the npl1 (nph1-like 1) protein.The phototropin photoreceptor, nph1, mediates both root and hypocotyl phototropism in response to low-fluence-rate unilateral blue light (Ͻ1 mol⅐m Ϫ2 ⅐s Ϫ1 ) (4, 5). Nph1 is a 120-kDa plasma-membrane-associated protein that contains a serine͞ threonine kinase domain located within its C terminus. Furthermore, the N-terminal region of nph1 contains a repeated motif of 110 aa, designated LOV1 and LOV2, that belong to the PAS domain (found in PER, ARNT, and SIM proteins) superfamil...

Arabidopsis COP1 acts as a light-inactivable repressor of photomorphogenic development, but its molecular mode of action remains unclear. Here, we show that COP1 negatively regulates HY5, a bZIP protein and a positive regulator of photomorphogenic development. Both in vitro and in vivo assays indicate that COP1 interacts directly and specifically with HY5. The hyperphotomorphogenic phenotype caused by the over-expression of a mutant HY5, which lacks the COP1-interactive domain, supports the regulatory role of HY5-COP1 interaction. Further, HY5 is capable of directly interacting with the CHS1 minimal promoter and is essential for its light activation. We propose that the direct interaction with and regulation of transcription factors by COP1 may represent the molecular mechanism for its control of gene expression and photomorphogenic development.

Trichome patterning in Arabidopsis is a model for the generation of a spacing pattern from initially equivalent cells. We show that the TRIPTYCHON gene that functions in lateral inhibition encodes a single-repeat MYB-related transcription factor that lacks a recognizable activation domain. It has high sequence similarity to the root hair patterning gene CAPRICE. Both genes are expressed in trichomes and act together during lateral inhibition. We further show that TRIPTYCHON and CAPRICE act redundantly in the position-dependent cell fate determination in the root epidermis. Thus, the same lateral inhibition mechanism seems to be involved in both de novo patterning and position-dependent cell determination. We propose a model explaining trichome and root hair patterning by a common mechanism.