OVER-EXPRESSION OF SUCROSE-PHOSPHATE SYNTHASE IN ARABIDOPSIS-THALIANARESULTS IN INCREASED FOLIAR SUCROSE STARCH RATIOS AND FAVORS DECREASED FOLIAR CARBOHYDRATE ACCUMULATION IN PLANTS AFTER PROLONGED GROWTH WITH CO2 ENRICHMENT/

Arabidopsis thaliana ecotype Columbia was transformed with a maize suc rose phosphate synthase (SPS) cDNA under the control of the promoter f or the small subunit of ribulose-l,5-bisphosphate carboxylase from tob acco (rbcS). The effects of SPS over-expression were compared in plant s of the T-2 and T-3 generations grown either in air or with CO2 enric hment (700 mu l l(-1)) for either 4 or 10 weeks. Maximal extractable f oliar SPS activities were three times those of the untransformed contr ols in the highest rbcS-SPS expressing line. In untransformed Arabidop sis leaves SPS activity was not subject to light/dark regulation, but was modified by incubation with either the inhibitor, orthophosphate, or the activator, mannose. Photosynthesis (A(max)) values were similar in all lines grown in air. After 10 weeks of CO, enrichment a decreas e in A,,, in the untransformed controls, but not in the high SPS expre ssors, was observed. There was a strong correlation between the sucros e-to-starch ratio of the leaves and their SPS activity in both growth conditions. The total foliar carbohydrate contents of 4-week-old plant s was similar in all lines whether plants were grown in air or with CO 2 enrichment, After 10 weeks growth the leaves of the high rbcS-SPS ex pressors accumulated much less total carbohydrate than untransformed c ontrol leaves in both growth conditions. It was concluded that SPS ove rexpression causes increased foliar sucrose/starch ratios in Arabidops is leaves and favours decreased foliar carbohydrate contents when plan ts are grown for long periods with CO2 enrichment.