Expression of the PIP aquaporin promoter-MipA from the common ice plant in tobacco.

The reasons for the presence of a multitude of plasma membrane-localized water channels (PIP aquaporins) in plants may be functional differences in water (or other solute) transport, or in developmental, environmental or tissue-specific regulation of expression. We compared tissue- and cell-specific expression of McMipA, an abundantly expressed PIP from the common ice plant (Mesembryanthemum crystallinum), with that of the previously characterized McMipB [Yamada et al. (1997) Plant Cell Physiol. 38: 1326]. The activity of a 2.2 kb DNA fragment containing the promoter region of McMipA in a fusion with the GUS coding region was studied in transgenic tobacco. The McMipA promoter was active in pericycle and cortex cells in roots and in phloem-associated cells and cells surrounding the pericycle in shoots. In green leaves, mesophyll cells and the minor veins showed GUS activity, but the major veins did not. In floral tissues, GUS activity was observed in the pistil and anthers of immature buds and the tip of the mature pistil and pollen. Neither the apical meristem nor root tip showed any GUS activity. The differences in tissue specificity between the McMipA and McMipB promoters indicated that the two PIPs, MC-MIPA and MC-MIPB, serve different functions in plants.