Regulation of chloroplast gene expression during light-induced development in barley

Abstract

Four plastid genes, psaA, psaB, psbD and psbC, were localized on the barley (Hordeum vulgare L.) plastid genome. PsaA was adjacent to psaB in one transcription unit and psbD was adjacent to psbC in a second transcription unit. Two 5.3 kb transcripts hybridize to psaA-psaB which differ at their 3' ends. In dark-grown barley, four large transcripts hybridize to psbD and psbC; two additional transcripts hybridize to psbC. Illumination of seedlings causes a decrease in the six psbD-psbC transcripts found in etioplasts, and the accumulation of two different psbD-psbC transcripts. The accumulation of the two psbD-psbC transcripts is regulated by blue light and is correlated with the maintenance of high rates of psbD gene product translation in mature chloroplasts. A nuclear mutant of barley, viridis-115, lacking Photosystem II activity, was characterized. Chloroplasts isolated from wild-type and viridis-115 seedlings illuminated for 1 h synthesized similar populations of radiolabeled polypeptides. After 16 h of illumination, mutant plastids exhibited reduced ability to radiolabel proteins encoded by psbA and psbB, and by 72 h synthesis of these proteins was undetectable although transcripts for these proteins were present. These results indicate that the nuclear mutation present in viridis-115 affects the translation and stability of the chloroplast-encoded psbA and psbB gene products, and its influence is expressed after the onset of light-induced chloroplast development. Illumination of barley with a brief pulse of red light induced the accumulation of the radiolabeled chlorophyll a-apoproteins as well as the rapid conversion of protochlorophyllide to chlorophyll a. Accumulation of the chlorophyll a-apoproteins was not reversed by a far-red light treatment given immediately after the pulse of red light. Chlorophyll-deficient mutants, xan-f^10 and xan-j^64, failed to accumulate the radiolabeled chlorophyll a-apoproteins upon illumination. These results demonstrate that the formation of chlorophyll $a$ is necessary for accumulation of the plastid-encoded chlorophyll a-apoproteins and one red light photoreceptor controlling this response is the protochlorophyllide holochrome.