EFFECTS OF SHORT-TERM AND LONG-TERM ELEVATED CO2 ON THE EXPRESSION OFRIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE GENES AND CARBOHYDRATE ACCUMULATION IN LEAVES OF ARABIDOPSIS-THALIANA (L) HEYNH/

To investigate the proposed molecular characteristics of sugar-mediate d repression of photosynthetic genes during plant acclimation to eleva ted CO2, we examined the relationship between the accumulation and met abolism of,nonstructural carbohydrates and changes in ribulose-1,5-bis phosphate carboxylase/oxygenase (Rubisco) gene expression in leaves of Arabidopsis thaliana exposed to elevated CO2. Long-term growth of Ara bidopsis at high CO2 (1000 mu L L-1) resulted in a 2-fold increase in nonstructural carbohydrates, a large decrease in the expression of Rub isco protein and in the transcript of rbcL, the gene encoding the larg e subunit of Rubisco (approximately 35-40%), and an even greater decli ne in mRNA of rbcS, the gene encoding the small subunit (approximately 60%). This differential response of protein and mRNAs suggests that t ranscriptional/posttranscriptional processes and protein turnover may determine the final amount of leaf Rubisco protein at high CO2. Analys is of mRNA levels of individual rbcS genes indicated that reduction in total rbcS transcripts was caused by decreased expression of all four rbcS genes. Short-term transfer of Arabidopsis plants grown at ambien t CO2 to high CO2 resulted in a decrease in total rbcS mRNA by d 6, wh ereas Rubisco content and rbcL mRNA decreased by d 9. Transfer to high CO2 reduced the maximum expression level of the primary rbcS genes (1 A and, particularly, 3B) by limiting their normal pattern of accumulat ion through the night period. The decreased nighttime levels of rbcS m RNA were associated with a nocturnal increase in leaf hexoses. We sugg est that prolonged nighttime hexose metabolism resulting from exposure to elevated CO2 affects rbcS transcript accumulation and, ultimately, the level of Rubisco protein.