A POSSIBLE CONTROL BY PHYTOCHROME and OTHER PHOTORECEPTORS OF PROTEIN ACCUMULATION IN THE GREEN ALGA Ulva rigida

Antagonistic blue- and red-light regulation of cab-gene expression during photosynthetic adaptation in Scenedesmus obliquus

During adaptation of the photosynthetic apparatus of the green alga Scenedesmus obliquus to various light qualities, the accumulation of chlorophylls and pigment-protein complexes (with specific consideration of chlorophyll a/b-binding (Cab) proteins) and cab-gene expression were determined. The fluence rate dependences for chlorophyll accumulation and cab-gene expression were very different. Very low fluence rates of violet (404 nm), blue (461 nm) and red (650 nm) light below the photosynthetic threshold, i.e. between 10(-3) and 10(-1) mumol m-2 s-1, inhibited all of these reactions in cells grown under heterotrophic conditions. At elevated fluence rates (above 1 mumol m-2 s-1), red light retained its negative regulation, whereas blue light stimulated pigment accumulation. Under autotrophic conditions the pattern was more complex, because chlorophyll accumulation was unaffected by light below the photosynthetic threshold. However, the expression of cab-genes was inhibited by red light but stimulated by blue light. Cells adapted to fluence rates, which ensured photosynthetic energy supply (above 1 mumol m-2 s-1), showed an increase in chlorophyll accumulation, blue light being more effective than red light. The results confirm and extend our previous discovery of two antagonistically acting photoreceptors in Scenedesmus which mediate and coordinate the complex functional and structural changes associated with photosynthetic adaptation. One of these receptor pigments is a blue-light receptor with positive action; the other is a violet-red-light receptor which can operate far below the photosynthetic threshold and exerts a negative regulation.