SUGAR FEEDING MIMICS EFFECT OF ACCLIMATION TO HIGH CO2-RAPID DOWN-REGULATION OF RUBISCO SMALL-SUBUNIT TRANSCRIPTS BUT NOT OF THE LARGE SUBUNIT TRANSCRIPTS

The abundance of rbcS transcripts, derived from the nuclear gene-famil y coding for the small subunit of RuBisCO, was dramatically reduced in tomato plants exposed to high CO2 for 4 days or more whereas the decl ine in the rbcL RNA transcripts, from the chloroplast gene coding for the large subunit of RuBisCO, was less pronounced. The rate of decline in the abundance of the rbcs transcripts was enhanced when leaves wer e detached and supplied with water so as to deprive them of any major sink and simultaneously exposed to high CO2. The reduction in the abun dance of rbcS mRNA, but nor rbcL, was mimicked when sucrose or glucose was supplied to leaf tissue, whereas acetate or sorbitol had no effec t. Based on these and other published data a molecular model involving the repression of transcription of nuclear-encoded genes for chloropl ast proteins by photosynthetic end-products is proposed to account for photosynthetic acclimation to high CO2 in tomato and other species.