REGULATION OF THE EXPRESSION OF PHOTOSYNTHETIC NUCLEAR GENES BY CO2 IS MIMICKED BY REGULATION BY CARBOHYDRATES - A MECHANISM FOR THE ACCLIMATION OF PHOTOSYNTHESIS TO HIGH CO2

The abundance of transcripts of cab-7 and cab-3C, which code for the c hlorophyll a/b binding proteins of the light-harvesting complexes I an d II, respectively, and the abundance of transcripts of Rca, which enc odes Rubisco activase, were reduced in tomato plants exposed to high C O2 for up to 9d, whereas the abundance of mRNA from psa A-psa B and ps b A, which encode the proteins of the core complex of PSI and the D1 p rotein of PSII, respectively, and the abundance of glycolate oxidase, which is involved in photorespiration, were not affected. However, the abundance of the transcript for the B subunit of ADP-glucose pyrophos phorylase was increased after 1 d at elevated CO2. The chlorophyll am ratio decreased significantly over 9 d of exposure to elevated CO2. Th e responses of the nuclear genes to high CO2 were enhanced when leaves were detached so as to deprive them of any major sink. The responses of these transcripts to high CO2 were mimicked when sucrose or glucose was supplied to the leaf tissue, whereas acetate or sorbitol had no e ffect. Carbohydrate analyses of leaves grown in high CO2 or supplied w ith sucrose revealed that major increases occurred in the amount of gl ucose and fructose. Based on these and other published data, a molecul ar model involving the repression or activation of the transcription o f nuclear genes coding for chloroplast proteins by photosynthetic end- products is proposed to account for photosynthetic acclimation to high CO2 in tomato plants and other species.