Maltose is the major form of carbon exported from the chloroplast at night as a result of transitory starch breakdown. Maltose exists as an a-or b-anomer. We developed an enzymatic technique for distinguishing between the two anomers of maltose and tested the accuracy and specificity of this technique using b-maltose liberated from maltoheptose by b-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, b-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 a-and b-maltose were high during both the day and night. In starchless mutants of Arabidopsis, total maltose levels were low and almost completely in the a-form. We also found that the subcellular concentration of b-maltose at night was greater in the chloroplast than in the cytosol by 278 mM. We conclude that b-maltose is the metabolically active anomer of maltose and that a sufficient gradient of b-maltose exists between the chloroplast and cytosol to allow for passive transport of maltose out of chloroplasts at night.In leaves, transitory starch is formed in the chloroplasts during the day and broken down at night. Transitory starch acts as an energy reserve, providing the plant with carbohydrate during the night when sugars cannot be made by photosynthesis, and as an overflow, allowing photosynthesis to go faster than Suc synthesis during the day (Ludewig et al., 1998).It has long been known that maltose is an intermediate in the breakdown of storage starch, for example during malting, but the importance of maltose in transitory starch degradation has only recently been proven. Early reports of significant maltose levels in leaves undergoing transitory starch breakdown (Levi and Gibbs, 1976;Peavey et al., 1977;Kruger and ap Rees, 1983;Neuhaus and Schulte, 1996;Servaites and Geiger, 2002) were counterbalanced by reports concluding that phosphorolytic starch breakdown was the predominant pathway in leaves (Heldt et al., 1977;Stitt et al., 1978; ap Rees, 1979, 1980;Stitt and Heldt, 1981).The form of carbon exported from the chloroplast at night was reexamined in light of accumulating genetic and biochemical evidence against phosphorolytic degradation for export. We found that at least two-thirds of the carbon exported from dark-adapted, isolated chloroplasts was in the form of maltose . The maltose is likely used by an amylomaltase enzyme (Chia et al., 2004;Lu and Sharkey, 2004) in metabolism that may be similar to that used by Escherichia coli (Boos and Shuman, 1998).Maltose transport across the chloroplast membrane has been known for some time (Rost et al., 1996) and recently the maltose transporter was ...

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Bennett, et al. than the expected van der Waals contact of 3.90 A using Pauling's radius of 1.95 az6 for bromine or the 3.70-8 sum using Bondi's 1.85-8 radius.z7 But in accord with views previously expressed," this is insufficient evidence that the short Br. . .Br contacts are the result of weak donor-acceptor interactions. Until empirical or theoretical evidence can be developed to support such an interpretation, a more plausible consideration for the short anionic contacts and the angles along the chain is crystal packing forces.in Figure 3. As in the MPNA salts, these ions make contact with six surrounding SbBr6-ions, two along the axis at the short distance of 3.607 (3) 8 and four approximately perpendicular to the axis at distances ranging from 3.661 (4) to 4.506 (5) 8. The shortest of these involve the terminal bromine atoms of the Br3-ion, in direct contrast with the MPNA salts in which the Sb-Br bonds are directed more toward the centers of the Br-Br bonds.There is no structural evidence in QNA that the 7~ electrons of the quinolinium ring participate in any donor-acceptor interactions of the type found in crystals of benzene with brominez8 and p-xylene with carbon tetrabr~mide.~' Figure 4 illustrates the arrangement of alternating layers of cations and the SbBr6-. . .Br3-chains, a packing sequence duplicated in both 2-MPNA and 4-MPNA. Perpendicular distances between the cation and neighboring bromine atoms of these chains, 3.61 and 3.62 A, are just slightly shorter than those in 2-MPNA (3.65 and 3.75 a) and 4-MPNA (3.72 8). All are equal to or greater than the expected van der Waals sum of 3.62 A using Bondi's radius of 1.85 8 for bromine and 1.77 8 for aromatic carbon atoms.?'The anionic environment of the tribromide ion is illustrated