Molecular cloning and identification of a putative tomato cationic amino acid transporter-2 gene that is highly expressed in stamens

Abstract

Amino acid transporters are critical components of amino acid allocation processes in plants. However, they are not well known in tomato, a model for studying biological processes of fleshy fruits. In this study, a putative cationic amino acid transporter gene-2 (SlCAT2) from tomato was isolated and identified. The SlCAT2 gene encodes a deduced 650-amino-acid protein that shares high similarity to Arabidopsis AtCAT2 and poplar PtCAT2. The SlCAT2 protein contains 14 putative transmembrane domains and is targeted to the tonoplast. Transcripts of SlCAT2 are present in major reproductive tissues (floral buds, flowers, fruits) and vegetative tissues (roots, stems, leaves), but accumulate at high levels in flowers. Within the flowers, SlCAT2 transcript levels in stamens are especially high and increase dramatically during bud-to-anthesis transition, when SlCAT2 expression elevates slightly in ovaries. The abundance of SlCAT2 transcripts in ovaries declines substantially from anthesis to postanthesis, when fruit set is expected to occur. Subsequently, SlCAT2 expression in fruits increases moderately during very early fruit development and keeps steady levels until mature green stage, and then gradually increases as fruit ripening. Expression of SlCAT2 is negatively regulated by ethylene and auxin. The dynamic transitions in the expression of SlCAT2 suggest its roles in the crucial stages of flower and fruit development, especially in the stamen. These findings should be helpful in guiding further investigation of the physiological role of SlCAT2 in tomato.