THE ROLE OF THE HEXOSE TRANSPORTER IN THE CHLOROPLASTS OF ARABIDOPSIS-THALIANA L

The metabolism of wild-type Arabidopsis thaliana L. and its mutant TC2 65 were compared in order to reveal the role of the chloroplast glucos e transporter. Plants were grown in a 12-h photoperiod. From 20 to 40 days after germination, starch per gram fresh weight of shoot in the m utant was four times that in the wild type. The extent of this differe nce did not alter during this period. Stereological analysis showed th at the chloroplasts in the mutant were larger than those in the wild t ype; the thylakoids appeared to be distorted by the high starch conten t. [U-C-14]Glucose and [U-C-14]glycerol were supplied, separately, to excised leaves in the dark. [U-C-14]Glucose was a good precursor of su crose in the wild type and mutant; [U-C-14]glycerol was a poor precurs or of sucrose in both. The distribution of C-14 in the wild type was u sed to calculate that the net flux was from hexose monophosphates to t riose phosphates, not vice versa. During the first 4 h of the night th e sugar content (75% sucrose, 20% glucose) of the leaves of the mutant dropped sharply, and at all times during the night it was less than t hat of the wild-type leaves. This drop in sugar coincided with a decre ase in the rate of respiration. The growth rate of the mutant was less than that of the wild type. Addition of sucrose restored the rate of respiration at night and increased the rate of growth. It is argued th at a major function of the glucose transporter in Arabidopsis chloropl asts is export of the products of starch breakdown that are destined f or sucrose synthesis at night.