Arabidopsis Cryptochrome Research Articles

Antagonistic actions of Arabidopsis cryptochromes and phytochrome B in the regulation of floral induction.

The Arabidopsis photoreceptors cry1, cry2 and phyB are known to play roles in the regulation of flowering time, for which the molecular mechanisms remain unclear. We have previously hypothesized that phyB mediates a red-light inhibition of floral initiation and cry2 mediates a blue-light inhibition of the phyB function. Studies of the cry2/phyB double mutant provide direct evidence in support of this hypothesis. The function of cryptochromes in floral induction was further investigated using the cry2/cry1 double mutants. The cry2/cry1 double mutants showed delayed flowering in monochromatic blue light, whereas neither monogenic cry1 nor cry2 mutant exhibited late flowering in blue light. This result suggests that, in addition to the phyB-dependent function, cry2 also acts redundantly with cry1 to promote floral initiation in a phyB-independent manner. To understand how photoreceptors regulate the transition from vegetative growth to reproductive development, we examined the effect of sequential illumination by blue light and red light on the flowering time of plants. We found that there was a light-quality-sensitive phase of plant development, during which the quality of light exerts a profound influence on flowering time. After this developmental stage, which is between approximately day-1 to day-7 post germination, plants are committed to floral initiation and the quality of light has little effect on the flowering time. Mutations in either the PHYB gene or both the CRY1 and CRY2 genes resulted in the loss of the light-quality-sensitive phase manifested during floral development. The commitment time of floral transition, defined by a plant's sensitivity to light quality, coincides with the commitment time of inflorescence development revealed previously by a plant's sensitivity to light quantity - the photoperiod. Therefore, the developmental mechanism resulting in the commitment to flowering appears to be the direct target of the antagonistic actions of the photoreceptors.

Chimeric proteins between cry1 and cry2 Arabidopsis blue light photoreceptors indicate overlapping functions and varying protein stability.

A blue light (cryptochrome) photoreceptor from Arabidopsis, cry1, has been identified recently and shown to mediate a number of blue light-dependent phenotypes. Similar to phytochrome, the cryptochrome photoreceptors are encoded by a gene family of homologous members with considerable amino acid sequence similarity within the N-terminal chromophore binding domain. The two members of the Arabidopsis cryptochrome gene family (CRY1 and CRY2) overlap in function, but their proteins differ in stability: cry2 is rapidly degraded under light fluences (green, blue, and UV) that activate the photoreceptor, but cry1 is not. Here, we demonstrate by overexpression in transgenic plants of cry1 and cry2 fusion constructs that their domains are functionally interchangeable. Hybrid receptor proteins mediate functions similar to cry1 and include inhibition of hypocotyl elongation and blue light-dependent anthocyanin accumulation; differences in activity appear to be correlated with differing protein stability. Because cry2 accumulates to high levels under low-light intensities, it may have greater significance in wild-type plants under conditions when light is limited.

Expression of an Arabidopsis cryptochrome gene in transgenic tobacco results in hypersensitivity to blue, UV-A, and green light.

The Arabidopsis HY4 gene, required for blue-light-induced inhibition of hypocotyl elongation, encodes a 75-kDa flavoprotein (CRY1) with characteristics of a blue-light photoreceptor. To investigate the mechanism by which this photoreceptor mediates blue-light responses in vivo, we have expressed the Arabidopsis HY4 gene in transgenic tobacco. The transgenic plants exhibited a short-hypocotyl phenotype under blue, UV-A, and green light, whereas they showed no difference from the wild-type plant under red/far-red light or in the dark. This phenotype was found to cosegregate with overexpression of the HY4 transgene and to be fluence dependent. We concluded that the short-hypocotyl phenotype of transgenic tobacco plants was due to hypersensitivity to blue, UV-A, and green light, resulting from over-expression of the photoreceptor. These observations are consistent with the broad action spectrum for responses mediated by this cryptochrome in Arabidopsis and indicate that the machinery for signal, transduction required by the CRY1 protein is conserved among different plant species. Furthermore, the level of these photoresponses is seen to be determined by the cellular concentration of this photoreceptor.