Reduced amino acid content in transgenic potato tubers due to antisense inhibition of the leaf H+/amino acid symporter StAAP1

Transport processes across the plasma membrane of leaf vascular tissue are essential for transport and distribution of assimilates. In potato, leaves are the predominant sites for nitrate reduction and amino acid biosynthesis. From there, assimilated amino acids are exported through the phloem to supply tubers with organic nitrogen. To study the role of amino acid transporters in long-distance transport and allocation of organic nitrogen in potato plants, a gene encoding a functional, leaf-expressed amino acid permease StAAP1 was isolated. Similar to the sucrose transporter SUT1, StAAP1 expression was induced during the sink-to-source transition, indicating a role in phloem loading. To test the role of StAAP1, expression was inhibited by an antisense approach. Transgenic plants with reduced StAAP1 expression were phenotypically indistinguishable from wild type, as were photosynthetic capacity and tuber yield. However, tubers from antisense StAAP1 plants showed up to 50% reduction in free amino acid contents. In comparison, starch content was not affected or tended to increase relative to wild type. The reduction in all amino acids except aspartate in the antisense plants is consistent with the properties of amino acid permeases (AAPs) found in heterologous systems. The results demonstrate an important role for StAAP1 in long-distance transport of amino acids and highlight the importance of plasma membrane transport for nutrient distribution in plants.