Poster: Photosynthesis (carbon)

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Abs # 261: Phloem Transportation of Maltose in Maltose-excess Mutants

Presenter:	Lu, Yan , Contact Author
Authors	Lu, Yan  (A)   Gehan, Jackson P. (A)   Sharkey, Thomas D. (A)
Affiliations:	(A): University of Wisconsin

Glucose and maltose are two major forms of carbon exported from chloroplasts at night. Maltose is exported by the maltose transporter (MEX1) and is metabolized in the cytosol by several enzymes, including disporportionating enzyme (DPE2). Inhibition of maltose export or maltose metabolism results in a 20-90-fold increase in leaf maltose content and a dwarf phenotype. What happens to the extremely high amount of maltose in the mutants is still a mystery. Can maltose be transported from source tissues to sink tissues via phloem transportation? Arabidopsis is a type 1-2a species by Gamalei's definition, which has high plasmodesmatal frequencies at interfaces involving phloem parenchyma cells. The predominant transport sugar in Arabidopsis is sucrose, although some raffinose is also translocated. Evidence shows that exogenous maltose serves as a transported substrate for the Arabidopsis sucrose transporter (AtSUC2). We assayed the carbohydrates in the phloem saps from Arabidopsis leaves. We found large amounts of sucrose, glucose, and fructose. We also found a large amount of maltose in mex1-1 and dpe2-1 phloem saps. The amount of maltose in phloem sap was higher in dpe2-1 plants than in mex1-1 plants. The ratio between maltose and sucrose was higher in dpe2-1 plants than in mex1-1 plants. We confirmed that the maltose transporter is upstream of DPE2 in transitory starch degradation pathway. We hypothesize that excess maltose in maltose-excess mutants could be transported through phloem via sucrose transporters. We also hypothesize that the large amounts of fructose and glucose in phloem sap is possibly the product of apoplastic invertase.