Light-Dependent Accumulation of Radiolabeled Plastid-Encoded Chlorophyll a-Apoproteins Requires Chlorophyll a

Abstract

The accumulation of radiolabeled plastid-encoded chlorophyll a-apoproteins is light dependent and is controlled at a posttranscriptional level. Illumination of dark-grown barley (Hordeum vulgare L.) with a brief pulse of red light induced the accumulation of radiolabeled chlorophyll a-apoproteins in subsequent protein synthesis assays. The induction of radiolabeled chlorophyll a-apoprotein accumulation was not affected by pretreatment of leaves with cycloheximide. Fluence response studies showed that a red light photoreceptor controls the accumulation of radiolabeled chlorophyll a-apoproteins with a threshold fluence of approximately 50 to 100 microeinsteins per square meter. While red light initiated chlorophyll a-apoprotein accumulation, this process was not reversed by a far red light treatment given immediately after the pulse of red light. The light pulse which initiated the accumulation of radiolabeled chlorophyll a-apoproteins also induced the rapid conversion of protochlorophyllide to chlorophyll a. A chlorophyll-deficient mutant, xan-f10, which is blocked in chlorophyll biosynthesis prior to protochlorophyllide formation, failed to accumulate radiolabeled chlorophyll a-apoproteins in the light even though transcripts for these apoproteins were present. A second mutant, xan-j64, which accumulates chlorophyllide in the light but only low levels of chlorophyll a, also showed reduced accumulation of radiolabeled chlorophyll a-apoproteins upon illumination. These results suggest that the light-unduced conversion of protochlorophyllide to 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.