ANTISENSE INHIBITION OF THE SUCROSE TRANSPORTER IN POTATO - EFFECTS ON AMOUNT AND ACTIVITY

The sucrose proton-cotransporter gene from potato (StSUT1) is mainly e xpressed in the phloem of mature, exporting leaves. To study the in vi vo role of the protein, potato plants were transformed with antisense constructs of the sucrose transporter cDNA under control of the CaMV35 S and the rolC promoters, respectively. Both types of transgenic plant develop symptoms characteristic of an inhibition of phloem loading. T o determine the level of inhibition, immunological and transport studi es were performed. Purified antibodies directed against a peptide from the central loop of SUT1 recognized a transporter with an apparent mo lecular mass of 47 kDa in leaf plasma membrane vesicles. Antisense rep ression under control of the non-specific CaMV35S promoter led to a st rong reduction in SUT1 protein, whereas no such reduction could be det ected when the companion cell-specific rolC promoter was used. Similar ily, sucrose uptake in plasma membrane vesicles was reduced by 50-75% in CaMV35S but not in rolC plants. These data suggest that, unlike the rolC promoter, the sucrose transporter is expressed not only in the c ompanion cells but also in other leaf cells. However, inhibition of th e transporter by rolC-controlled antisense repression is sufficient to impair phloem loading.