COMPANION CELL-SPECIFIC INHIBITION OF THE POTATO SUCROSE TRANSPORTER SUT1

In many plants, translocation of sucrose from mesophyll to phloem for long-distance transport is carrier-mediated. The sucrose H+-symporter gene SUT1 from potato is expressed at high levels in the phloem of mat ure, exporting leaves and at lower levels in other organs. Inhibition of SUT1 by expression of an antisense gene in companion cells under co ntrol of the rolC promoter leads to accumulation of high amounts of so luble and insoluble carbohydrates in leaves and inhibition of photosyn thesis. The distribution of in situ localized starch does not correspo nd with areas of reduced photosynthesis as shown by fluorescence imagi ng. Dissection of antisense effects on sink and source organs by recip rocal grafts shows that inhibition of transporter gene expression in l eaves is sufficient to produce chlorosis in leaves and reduced tuber y ield. In contrast to the arrest of plasmodesmal development found in p lants that express yeast invertase in the apoplast, in mature leaves o f sucrose transporter antisense plants plasmodesmata are branched and have median cavities. These data strongly support an apoplastic mode o f phloem loading in potato, in which the sucrose transporter located a t the plasma membrane of the sieve element/companion cell complex repr esents the primary route for sugar uptake into the long-distance trans location pathway.