The expression of an aquaporin promoter from Mesembryanthemum crystallinum in tobacco.

The promoter region of the MipB gene encoding an aquaporin from Mesembryanthemum crystallinum was isolated and used in a transcriptional fusion to control uidA expression in tobacco. The sequence of the promoter was determined for 2 kb upstream of the translation initiation site. Three start sites were utilized with approximately equal frequency, located 176, 170, and 161 bases, respectively, upstream of the translation initiation site. As judged by analysis of GUS expression, promoter MipB retains its specificity in transgenic tobacco. In germinating seedlings, all cells showed GUS expression of different intensities with the strongest signals in root meristems. In older seedlings, GUS staining was observed in rapidly expanding cells--root and apical meristem, and lateral root primordia. In mature plants, strong GUS activity was located to glandular trichomes, subepidermal cells of the stem and petioles, to cells surrounding vascular tissues as well as in xylem parenchyma cells. In immature floral organs, GUS expression was strong in sepals, petals, stamen, and pistil. The intensity declined as they matured. In general, this promoter was active in rapidly expanding cells and cells with high water flux capacity, especially in the xylem parenchyma.