Chloroplast gene expression in lettuce grown under different irradiances.

Chloroplast DNA sequences have been used as hybridisation probes to measure the levels of RNA transcripts present in low- and high-light-grown lettuce (Lactuca sativa L.) plants. The transcript levels for rbc L, psa A, psb A and rDNA are not different between these two light regimes. In contrast, transcript levels for atp BE and pet BD are increased in high-light as a proportion of the chloroplast RNA. Three days after transfer from low-light into high-light, increased transcript levels were found for atp BE, although no change was detected for the psa A or psb A transcripts. In addition, young plants in high-light contain twofold more chloroplast RNA per unit chlorophyll than do low-light plants of equivalent age. Therefore, in these young high-light plants the absolute transcript levels per unit chlorophyll are much greater. With increasing leaf age the RNA per chlorophyll becomes similar for both light conditions. These results are discussed in relation to the photoregulation of chloroplast-encoded gene expression.