Regulation of Plastid Gene Expression during Photooxidative Stress

We have used the carotenoid biosynthesis inhibitor norflurazon to study the relationship between chloroplast and nuclear gene expression and the mechanisms by which plastid mRNA accumulation is regulated in response to photooxidative stress. By treating 4-week-old hydroponic spinach plants (Spinacea oleracea), we were able to determine the response at two distinct stages of chloroplast development. For all parameters studied, differences were found between the norflurazon-treated young and mature leaves. Young leaves lost essentially all pigment content in the presence of norflurazon, whereas mature leaves retained more than 60% of their chlorophyll and carotenoids. The accumulation of plastid mRNA was determined for several genes, and we found a decrease in mRNA levels for all genes except psbA in herbicide-treated young leaves. For genes such as atpB, psbB, and psaA, there was a corresponding change in the relative level of transcription, but for psbA and rbcL, transcription and mRNA accumulation were uncoupled. In norflurazon-treated mature leaves, all plastid mRNAs except psaA accumulated to normal levels, and transcription levels were either normal or higher than corresponding controls. This led to the conclusion that plastid mRNA accumulation is regulated both transcriptionally and posttranscriptionally in response to photooxidative stress. Although direct photooxidative damage is confined to the plastid and peroxisome, there is a feedback of information controlling the transcription of nuclear-encoded plastid proteins. Considerable evidence has accumulated implicating a "plastid factor" in this control. Therefore, the expression of several nuclear-encoded plastid proteins and the corresponding mRNAs were determined. Although the levels of both the small subunit of ribulose-1,5-bisphosphate carboxylase and the light harvesting chlorophyll a/b-binding protein and corresponding mRNAs were reduced, a 28-kilodalton chloroplast RNA-binding protein and corresponding mRNA were at normal levels in norflurazon-treated plants. Changes in mRNA and protein levels were not the result of a general loss due to photooxidation but rather the result of selective stabilization of certain components. The response of both genomes to photooxidative stress is discussed in terms of the postulated plastid factor.