REDUCED SEDOHEPTULOSE-1,7-BISPHOSPHATASE LEVELS IN TRANSGENIC TOBACCOLEAD TO DECREASED PHOTOSYNTHETIC CAPACITY AND ALTERED CARBOHYDRATE ACCUMULATION

Transgenic tobacco (Nicotiana tabacum L. cv. Samsun) plants with reduc ed levels of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase; EC 3.1.3.37) were produced using an antisense construct in wh ich the expression of a tobacco SBPase cDNA clone was driven by the ca uliflower mosaic virus (CaMV) promoter. The reduction in SBPase protei n levels observed in the primary transformants correlated with the pre sence of the antisense construct and lower levels of the endogenous SB Pase mRNA. No changes in the amounts of other Calvin cycle enzymes wer e detected using Western blot analysis. The SBPase antisense plants wi th less than 20% of wild-type SBPase activity were observed to display a range of phenotypes, including chlorosis and reduced growth rates. Measurements of photosynthesis, using both light-dosage response and C O2 response curves, of T1 plants revealed a reduction in carbon assimi lation rates, which was apparent in plants retaining 57% of wild-type SBPase activity. Reductions were also observed in the quantum efficien cy of photosystem II. This decrease in photosynthetic capacity was ref lected in a reduction in the carbohydrate content of leaves. Analysis of carbohydrate status in fully expanded source leaves showed a shift in carbon allocation away from starch, whilst sucrose levels were main tained in all but the most severely affected plants. Plants with less than 15% of wild-type SBPase activity were found to contain less than 5% of wild-type starch levels. The results of this preliminary analysi s indicate that SBPase activity may limit the rate of carbon assimilat ion.