Carbohydrate partitioning and photosynthesis in Arabidopsis thaliana
To characterize carbohydrate partitioning and photosynthesis in Arabidopsis thaliana we examined three different lines: wild type Columbia ecotype, immutans, and T-DNA knockouts of vacuolar acid invertase. Wild type plants displayed decreasing photosynthesis from the beginning of leaf expansion through senescence. Chlorophyll, total soluble protein, and expression of LhcB also declined in a similar fashion. Pulse-label feeding of 14C showed a peak flux into glucose and fructose coincident with full leaf expansion a large decrease in LhcB expression. However, flux of 14C into the different carbohydrate fractions did not reflect carbohydrate pool sizes. The conclusion was that flux into the different carbohydrate pools, specifically the hexoses, may correlate with the decrease in LhcB expression and the decline in photosynthesis. The immutans variegation mutant had higher photosynthetic parameters in the green sectors when compared to wild type leaves. These sectors also produced more sucrose and had greater sucrose phosphate synthase activity. White sectors had a higher cell wall invertase activity indicating movement of sucrose toward those cells. These observations led to the conclusion that increased sink demand by white sectors signals an increase in photosynthesis in green sectors. T-DNA knockouts of vacuolar invertase were crossed to create a double mutant lacking functional forms of both Arabidopsis genes, Atbetafruct3 and Atbetafruc4. The double mutant lacked soluble invertase activity and accumulated greater amounts of sucrose, but did not have altered hexose or starch accumulation. Mutant plants grown under continuous light accumulated more sucrose only after full leaf expansion, and only slightly more than wild type. Chlorophyll amounts were similar in plants grown under normal light combinations of and high light and ambient or high CO2 concentrations. The expression of RbcS was also unaltered in the mutants grown under the same conditions. The conclusion from this experiment was that vacuolar invertase has only a minimal effect on partitioning into sucrose or hexoses, and there is no effect on photosynthetic gene expression.