Blue light regulates the accumulation of two psbD‐psbC transcripts in barley chloroplasts.

Synthesis of D2, a Photosystem II reaction center protein encoded by psbD, is differentially maintained during light‐induced chloroplast maturation. The continued synthesis of D2 is paralleled by selective light‐induced accumulation of two psbD‐psbC transcripts which share a common 5′ terminus. In the present study, we examine the nature of the photoreceptor and the fluence requirement for psbD‐psbC transcript induction. The light‐induced change in psbD‐psbC RNA population can be detected between 1 and 2 h after 4.5 day old dark‐grown barley seedlings are transferred to the light. Light‐induced transcript accumulation occurs normally in the chlorophyll‐deficient barley mutant, xan‐f10, indicating that light‐activated chlorophyll formation and photosynthesis are not required for RNA induction. High fluence blue light fully induces psbD‐psbC transcript accumulation; low or high fluence red or far‐red light do not. However, psbD‐psbC transcript accumulation elicited by blue light pulses can be partially attenuated if far‐red light is given immediately following the blue light treatment. Thus, although blue light is needed to initiate transcript accumulation, phytochrome modulates the amplitude of the response. Pretreatment of dark‐grown plants with cycloheximide blocks light‐induced psbD‐psbC transcript accumulation. This could implicate a blue‐light responsive nuclear gene in the light‐induced accumulation of the two psbD‐psbC transcripts.