Photoregulation of psbA transcript levels in mustard cotyledons.

We have investigated the photoreceptors potentially involved in the light regulation of the transcript levels of the psbA gene coding for D1, the 32 kD QB-binding protein of PSII. In cotyledons of 4 day old mustard seedlings, increasing fluence rates of continuous white light from ca. 0.1 to 250 μmol m(-2)s(-1) (400-700 nm) lead to a five-fold increase in transcript level from ca. 0.7 to 2.8 mg/g total RNA. The blue (<500 nm) component of this light did not contribute substantially to this effect, thus ruling out cryptochrome as the receptor responsible. Although phytochrome involvement was apparent from red/far-red reversibility, even multiple red pulses failed to elicit a comparable increase in transcript level to that seen under continuous white light. Although DCMU successfully inhibited delayed fluorescence quenching, it had no effect on transcript levels, thus ruling out photoregulation via electron transport and later components of the photosynthetic system. By contrast, Norflurazon, which leads to photobleaching of chlorophyll and hence disruption of thylakoid membrane assembly, completely abolished the light effect on psbA transcript level. We infer that photoregulation of the psbA transcript is principally related to thylakoid development, which is in turn critically dependent on photoconversion of protochlorophyllide to chlorophyll, but also associated with other processes such as phytochrome-regulated LHCP availability. Photocontrol of psbA expression is discussed in relation to that of the nuclear cab and rbcS genes.