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Signal Transduction of plant photoreceptors Plants respond to changes in the light environment by altering their developmental programs. For example, seedlings of dicotyledonous plants (such as pea, tomato, or Arabidopsis) grown in the dark develop elongated hypocotyl (primary stem) and small unopened cotyledons (primary leaf). Upon exposure to light, hypocotyl elongation is inhibited and resources are devoted to cotyledon expansion, chloroplast development, changes in gene expression, as well as other developmental processes, which eventually result in the establishment of the seedling as a photoautotrophic organism. Although it is known that these photomorphogenic responses of plants rely on the combined action of two photosensory receptor systems: the red-light receptor phytochrome and blue light receptor cryptochrome, the molecular mechanisms of neither receptor systems is understood. We are interested in deciphering the signal transduction mechanisms of the blue-light receptor cryptochrome, and we use Arabidopsis as our model organism in our study. Recently, we have isolated the Arabidopsis gene encoding the apoprotein of a cryptochrome, CRY2, and found that it involves in mediating blue-light induction of cotyledon expansion, as well as hypocotyl inhibition response. We are currently employing molecular genetic approaches to dissecting the signal perception and transduction mechanisms of CRY2.