A MUTANT OF ARABIDOPSIS-THALIANA LACKING THE ABILITY TO TRANSPORT GLUCOSE ACROSS THE CHLOROPLAST ENVELOPE

At the end of a 12-h day leaves of the mutant of Arabidopsis thaliana L., TC265, contained 4-5 times more starch than those of the wild type. During a subsequent 12-h night the decline in the starch content of the leaves of the mutant was at least 50% of that of the wild-type leaves. Starch labelled in the light in a 30-min pulse in (CO2)-C-14 was rapidly broken down in a subsequent 12-h chase in the dark in air in the leaves of both mutant and wild type. Chloroplasts from leaves of the wild type took up [P-32]P-i and [U-C-14]glucose at 12 and 1.6 mu mol/h per mg of chlorophyll respectively; chloroplasts from the mutant showed a similar rate for [P-32]P-i but no uptake of [U-C-14]glucose. The glucose content of freshly isolated chloroplasts from the mutant was twice that of chloroplasts from the wild type; this difference was accentuated when the isolated chloroplasts were incubated in the dark. SDS/PAGE of preparations of chloroplast envelopes showed that those from the mutant were deficient in a protein band of approximate molecular mass 40 kDa. It is suggested that in mutant TC265 the primary lesion is in a hexose transporter in the chloroplast envelope, and that this transporter moves the products of starch breakdown that are destined for sucrose synthesis from the chloroplast to the cytosol.