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BIOENERGETICS AND PHOTOSYNTHESIS

-Maltose Is the Metabolically Active Anomer of Maltose during Transitory Starch Degradation^1,[w]

Department of Botany, University of Wisconsin, Madison, Wisconsin 53706

Maltose is the major form of carbon exported from the chloroplast at night as a result of transitory starch breakdown. Maltose exists as an - or -anomer. We developed an enzymatic technique for distinguishing between the two anomers of maltose and tested the accuracy and specificity of this technique using -maltose liberated from maltoheptose by -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, -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 [2004] Planta 218: 466–473) levels of - and -maltose were high during both the day and night. In starchless mutants of Arabidopsis, total maltose levels were low and almost completely in the -form. We also found that the subcellular concentration of -maltose at night was greater in the chloroplast than in the cytosol by 278 µM. We conclude that -maltose is the metabolically active anomer of maltose and that a sufficient gradient of -maltose exists between the chloroplast and cytosol to allow for passive transport of maltose out of chloroplasts at night.

^[w] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.055996.

^* Corresponding author; e-mail tsharkey{at}wisc.edu; fax 608–262–7509.

Received November 3, 2004; returned for revision December 9, 2004; accepted December 15, 2004.

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