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β-Maltose Is the Metabolically Active Anomer of Maltose during Transitory Starch Degradation
Sean E. Weise, Kirsten S. Kim, Robert P. Stewart and Thomas D. Sharkey
Vol. 137, No. 2 (Feb., 2005), pp. 756-761
Published by: American Society of Plant Biologists (ASPB)
Stable URL: http://www.jstor.org/stable/4629716
Page Count: 6
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Maltose is the major form of carbon exported from the chloroplast at night as a result of transitory starch breakdown. Maltose exists as an α- or β-anomer. We developed an enzymatic technique for distinguishing between the two anomers of maltose and tested the accuracy and specificity of this technique using β-maltose liberated from maltoheptose by β-amylase. This technique was used to investigate which form of maltose is present during transitory starch degradation in bean (Phaseolus vulgaris), wild-type Arabidopsis (Arabidopsis thaliana), two starch deficient Arabidopsis lines, and one starch-excess mutant of Arabidopsis. In Phaseolus and wild-type Arabidopsis, β-maltose levels were low during the day but were much higher at night. In Arabidopsis plants unable to metabolize maltose due to a T-DNA insertion in the gene for the cytosolic amylomaltase, (Y. Lu, T. D. Sharkey  Planta 218: 466-473) levels of α- and β-maltose were high during both the day and night. In starchless mutants of Arabidopsis, total maltose levels were low and almost completely in the α-form. We also found that the subcellular concentration of β-maltose at night was greater in the chloroplast than in the cytosol by 278 μM. We conclude that β-maltose is the metabolically active anomer of maltose and that a sufficient gradient of β-maltose exists between the chloroplast and cytosol to allow for passive transport of maltose out of chloroplasts at night.
Plant Physiology © 2005 American Society of Plant Biologists (ASPB)