Cryptochrome 1, Cryptochrome 2, and Phytochrome A Co-Activate the Chloroplast psbD Blue Light–Responsive Promoter

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Abstract
The reaction center core of photosystem II is composed of two chlorophyll binding proteins, D1 and D2, that are encoded by the chloroplast genes psbA and psbD. These chlorophyll binding proteins are damaged during photochemistry, especially under high irradiance. Photosystem II function is maintained under these conditions through turnover and resynthesis of D1 and D2. Blue light–activated transcription of psbD from a special light-responsive promoter is part of the repair system. In this study, light-activated chloroplast and psbD transcription were studied after dark adaptation of 21-day-old light-grown Arabidopsis plants. Illumination of dark-adapted plants with red light increased chloroplast transcription activity and transcription from the psbD light-responsive promoter. Blue light further increased chloroplast transcription activity and stimulated differential transcription from the psbD light-responsive promoter. Photoreceptor mutants showed that blue light–specific activation of chloroplast transcription and the psbD light-responsive promoter involve cryptochrome 1 (cry1) or cryptochrome 2 (cry2) and phytochrome A (phyA). Blue light–induced activation of the psbD light-responsive promoter was normal in det2-1 and hy5-1 but attenuated in det3-1. Therefore, cry1/cry2/phyA–mediated blue light activation of the psbD light-responsive promoter in 21-day-old Arabidopsis plants does not involve hy5, a transcription factor that mediates other phyA and blue light–induced responses.