A Family of Transcripts Encoding Water Channel Proteins: Tissue-Specific Expression in the Common Ice Plant

Abstract

Seawater-strength salt stress of the ice plant (Mesembryanthemum crystallinum) initially results in wilting, but full turgor is restored within approximately 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 with the onset of stress. cDNAs that suggested changes in mRNA amount under stress were found; their deduced amino acid sequences share homologies with proteins of the Mip (major intrinsic protein) gene family and potentially encode aquaporins. One transcript, MipB, was found only in root RNA, whereas two other transcripts, MipA and MipC, were detected in roots and leaves. Transcript levels of MipB were of low abundance. All transcripts declined initially during salt stress but later recovered to at least prestress level. The most drastic decline was in MipA and MipC transcripts. MipA mRNA distribution in roots detected by in situ hybridization indicated that the transcript was present in all cells in the root tip. In the expansion zone of the root where vascular 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 lower amounts, in mesophyll cells. Transcripts for MipB seemed to be present exclusively in the tip of the root, in a zone before and possibly coincident with the development of a vascular system. MipA- and MipB-encoded proteins expressed in Xenopus oocytes led to increased water permeability. mRNA fluctuations of the most highly expressed MipA and MipC coincided with turgor changes in leaves under stress. As the leaves regained turgor, transcript levels of these water channel proteins increased.

Two TIP‐like genes encoding aquaporins are expressed in sunflower guard cells

“…A weaker increase in permeability after expression in Xenopus oocytes was reported for the ice plant MIP related genes as compared with yElP (Yamada et aL, 1995).…”

“…Water channels are assumed to largely contribute to hydraulic conductivity in Chara cells (Henzler and Steudle, 1995), Arabidopsis leaf protoplasts (Kaldenhoff et al, 1995), the tomato root system (Maggio and Joly, 1995), and sunflower hypocotyls (Hejnowicz and Sievers, 1996). Indeed, expression of some aquaporin genes was affected by osmotic stress, strengthening the hypothesis that they play a significant role in the control of water movement (Fray et al, 1994;Guerrero et al, 1990;Yamada et aL, 1995;Yamaguchi-Shinozaki et aL, 1992).…”

“…The effective location in the membranes has only been demonstrated for a few MIP proteins. A water transport activity has been characterized for some members of both the PIP and TIP subfamilies (Daniels et aL, 1996;Kammerloher et aL, 1994;Maurel et aL, 1993;Yamada et aL, 1995). The expression of plant MIP genes can be regulated at both transcriptional (H6fte et al, 1992) and post-translational (Johansson et al, 1996;Maurel et aL, 1995) levels.…”

Genetic approaches towards overcoming water deficit in plants - special emphasis on LEAs

“…Aquaporins present in the plasma membrane and also in sthe tonoplast are regulated by environmental factors such as drought, salinity, hormones, and blue light (Guerrero et al, 1990;Yamaguchi-Shinozaki et al, 1992;Yamada et al, 1995). Antisense PIP2 transformed lines resulted in reduced hydraulic conductance (Javot et al, 2003) and alterations in plant morphology (Kaldenhoff et al, 1998) while, overexpression of a plasma membrane aquaporin PIP1b, in transgenic tobacco improved plant vigor under favorable growth conditions but not under drought or salt stress (Aharon et al, 2003).…”

Aquaporin structure–function relationships: Water flow through plant living cells

“…The expression of some genes that encode plasma membrane aquaporins, such as Arabidopsis RD28 and the NeMip2 and NeMip3 genes of Nicotiana excelsior, is stimulated under drought stress [111,112]. Conversely, expression of the M. crystallinum MIPA plasma membrane aquaporin gene is down-regulated under salt stress [112], whereas expression of the Arabidopsis PIP1a gene is not altered significantly by stress conditions [113].…”