Modulation of photosynthesis and chloroplast biogenesis, by low and high temperatures, was studied in 12-d-old pea (Pisum sativum) plants grown at 25 degrees C and subsequently exposed to 7 degrees C or 40 degrees C up to 48 h. The decline in variable
chlorophyll a fluorescence/maximum
chlorophyll a fluorescence and estimated electron transport rate in temperature-stressed plants was substantially restored when they were transferred to room temperature. The
ATP-driven import of precursor of small subunit of
Rubisco (pRSS) into plastids was down-regulated by 67% and 49% in heat-stressed and chill-stressed plants, respectively. Reduction in binding of the pRSS to the chloroplast envelope membranes in heat-stressed plants could be due to the down-regulation of Toc159 gene/
protein expression. In addition to impaired binding, reduced
protein import into chloroplast in heat-stressed plants was likely due to decreased gene/
protein expression of certain components of the TOC complex (Toc75), the
TIC complex (Tic20, Tic32, Tic55, and Tic62), stromal Hsp93, and
stromal processing peptidase. In chill-stressed plants, the gene/
protein expression of most of the components of
protein import apparatus other than Tic110 and Tic40 were not affected, suggesting the central role of Tic110 and Tic40 in inhibition of
protein import at low temperature. Heating of intact chloroplasts at 35 degrees C for 10 min inhibited
protein import, implying a low thermal stability of the
protein import apparatus. Results demonstrate that in addition to decreased gene and
protein expression, down-regulation of photosynthesis in temperature-stressed plants is caused by reduced posttranslational import of plastidic
proteins required for the replacement of impaired
proteins coded by nuclear genome.