The effect of light intensities on the transcript level of proteins involved in photosynthesis in mustard plants

Abstract

Summary The influence of light quantity on the steady-state levels of plastid encoded transcripts was examined during the development of primary leaves from mustard ( Sinapis alba ) plants. RbcL mRNA, petA mRNA, and psbA mRNA, which encode the large subunit (LSU) of Rubisco, Cyt f of the Cyt b6/f complex, and D1 protein of PS II were investigated in leaves grown under high-light (HL) or low-light (LL) conditions. Additionally, the nuclear encoded 25 S rRNA was quantified. As a proportion of total RNA, the levels of 25 S rRNA, rbcL mRNA, petA mRNA, and psbA mRNA did not differ substantially in the HL versus LL plants. During leaf ontogenesis, though, the proportion of psbA mRNA in total RNA increased drastically, in contrast to the other transcripts. The results calculated per unit fresh weight, dry weight, leaf area, and chlorophyll closely resembled each other and gave an idea of the absolute prevalence of the transcripts in the cell. The light regime had a remarkable influence on the absolute levels of 25 S rRNA, rbcL mRNA, and petA mRNA: Paralleling total RNA content, the amounts of the three transcripts per unit fresh weight at the beginning of leaf development were, in HL plants, more than twice the amounts found in LL plants. Furthermore, during leaf ontogenesis the decrease in total RNA, 25 S rRNA, rbcL mRNA, and petA mRNA was more pronounced in HL than in LL, while the psbA mRNA content per unit fresh weight was only slightly higher in HL than in LL leaves and did not decline significantly during HL and LL leaf ontogenesis. Comparison of the absolute levels of rbcL mRNA, petA mRNA, and psbA mRNA with the amounts of the corresponding proteins revealed that the prevalence of the transcripts contributes principally to the differences in the protein content in HL and LL leaves.