Recovery of plastids from photooxidative damage: Significance of a plastidic factor

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Abstract
It was inferred from previous findings that a plastid-derived factor (plastidic factor) is involved in the transcriptional control of nuclear genes coding for proteins destined for the chloroplast. Photooxidative damage to the plastid destroys the ability of the organelle to give off this factor. Cytosolic enzyme levels are not impaired if plastids are damaged, and morphogenesis of seedlings is normal. The only exception found so far is nitrate reductase, a cytosolic enzyme, which is regulated by the cellas if it were a plastidic protein. In the present study we have shown that the plastids in the mesophyll of mustard (Sinapis alba L.) cotyledons, damaged by 3 h photooxidation in red light (6.8 W·m-2) and then returned to darkness or to continuous, non-photooxidative far-red light (cFR), recover from photooxidative damage. The rate of recovery is stimulated by phytochrome (operationally, cFR). Since the cytosolic enzyme nitrate reductase is affected by the different treatments in principally the same way as the levels of plastidic enzymes, we conclude that it is recovery of the plastids' ability to give off the plastidic factor rather than structural recovery which leads to recovery of gene expression and protein (and chlorophyll) re-accumulation. The extent of recovery varied according to the enzyme and this variation could be explained by different plastidic-factor requirements for gene expression. This explanation was confirmed by measurements of translatable mRNAs. It was found that LHCP-gene expression (light-harvesting chlorophyll a/b-binding protein of photosystem II) is far more sensitive to photooxidative damage of the plastids than SSU-gene expression (small subunit of ribulose-1.5-bisphosphate carboxylase). Correspondingly, recovery is expressed to a much greater extent in the latter than in the former case.

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