Regulation of the expression of photosynthetic nuclear genes by CO 2 is mimicked by regulation by carbohydrates: a mechanism for the acclimation of photosynthesis to high CO 2 ?

The abundance of transcripts of cab‐7 and cab‐3 C, which code for the chlorophyll a/b binding proteins of the light‐harvesting complexes I and II, respectively, and the abundance of transcripts of Rca , which encodes Rubisco activase, were reduced in tomato plants exposed to high CO 2 for up to 9d, whereas the abundance of mRNA from psa A– psa B and psb A, which encode the proteins of the core complex of PSI and the D1 protein 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 pyrophosphorylase was increased after 1 d at elevated CO 2 . The chlorophyll a/b ratio decreased significantly over 9 d of exposure to elevated CO 2 . The responses of the nuclear genes to high CO 2 were enhanced when leaves were detached so as to deprive them of any major sink. The responses of these transcripts to high CO 2 were mimicked when sucrose or glucose was supplied to the leaf tissue, whereas acetate or sorbitol had no effect. Carbohydrate analyses of leaves grown in high CO 2 or supplied with sucrose revealed that major increases occurred in the amount of glucose and fructose. Based on these and other published data, a molecular model involving the repression or activation of the transcription of nuclear genes coding for chloroplast proteins by photosynthetic end‐products is proposed to account for photosynthetic acclimation to high CO 2 in tomato plants and other species.