Over-expression of sucrose phosphate synthase in Arabidopsis thaliana results in increased foliar sucrose/starch ratios and favours decreased foliar carbohydrate accumulation in plants after prolonged growth with CO2 enrichment

Arabidopsis thaliana ecotype Columbia was transformed with a maize sucrose phosphate synthase (SPS) cDNA under the control of the promoter for the small subunit of ribulose-l,5-bisphosphate carboxylase from tobacco (rbcS). The effects of SPS over-expression were compared in plants of the T-2 and T-3 generations grown either in air or with CO2 enrichment (700 mu l l(-1)) for either 4 or 10 weeks. Maximal extractable foliar SPS activities were three times those of the untransformed controls in the highest rbcS-SPS expressing line. In untransformed Arabidopsis 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 decrease in A,,, in the untransformed controls, but not in the high SPS expressors, was observed. There was a strong correlation between the sucrose-to-starch ratio of the leaves and their SPS activity in both growth conditions. The total foliar carbohydrate contents of 4-week-old plants was similar in all lines whether plants were grown in air or with CO2 enrichment, After 10 weeks growth the leaves of the high rbcS-SPS expressors accumulated much less total carbohydrate than untransformed control leaves in both growth conditions. It was concluded that SPS overexpression causes increased foliar sucrose/starch ratios in Arabidopsis leaves and favours decreased foliar carbohydrate contents when plants are grown for long periods with CO2 enrichment.