A FAMILY OF TRANSCRIPTS ENCODING WATER CHANNEL PROTEINS - TISSUE-SPECIFIC EXPRESSION IN THE COMMON ICE PLANT

Seawater-strength salt stress of the ice plant (Mesembryanthemum cryst allinum) initially results in wilting, but full turgor is restored wit hin similar to 2 days. We are interested in a mechanistic explanation for this behavior and, as a requisite for in-depth biochemical studies , have begun to analyze gene expression changes in roots coincident wi th the onset of stress. cDNAs that suggested changes in mRNA amount un der stress were found; their deduced amino acid sequences share homolo gies with proteins of the Mip (major intrinsic protein) gene family an d potentially encode aquaporins. One transcript, MipB, was found only in root RNA, whereas two other transcripts, MipA and MipC, were detect ed in roots and leaves. Transcript levels of MipB were of low abundanc e. All transcripts declined initially during salt stress but later rec overed to at least prestress level. The most drastic decline was in Mi pA and MipC transcripts. MipA mRNA distribution in roots detected by i n situ hybridization indicated that the transcript was present in all cells in the root tip. In the expansion zone of the root where vascula r bundles differentiate, MipA transcript amounts were most abundant in the endodermis. In older roots, which had undergone secondary growth, MipA was highly expressed in cell layers surrounding individual xylem strands. MipA was also localized in leaf vascular tissue and, in lowe r amounts, in mesophyll cells. Transcripts for MipB seemed to be prese nt exclusively in the tip of the root, in a zone before and possibly c oincident with the development of a vascular system. MipA- and MipB-en coded proteins expressed in Xenopus oocytes led to increased water per meability. mRNA fluctuations of the most highly expressed MipA and Mip C coincided with turgor changes in leaves under stress. As the leaves regained turgor, transcript levels of these water channel proteins inc reased.