Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca 2+ , pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.

Plant cells contain proteins that are members of the major intrinsic protein (MIP) family, an ancient family of membrane channel proteins characterized by six membrane-spanning domains and two asparagine-proline-alanine (NPA) amino acid motifs i n the two halves of the protein. We recently demonstrated that y-TIP, one of the MIP homologs found i n the vacuolar membrane of plant cells, is an aquaporin or water channel protein (C. Maurel, J. Reizer, 1.1. Schroeder, M.J. Chrispeels [1993] EMBO J 12: 2241-2247). RD28, another MIP homolog in Arabidopsis thaliana, was first identified as being encoded by a turgor-responsive transcript. To find out if RD28 is a water channel protein, rd28 cRNA was injected into Xenopus laevis oocytes. Expression of RD28 caused a 10-to 15-fold increase i n the osmotic water permeability of the oocytes, indicating that the protein creates water channels in the plasma membrane of the oocytes and is an aquaporin just like its homolog ?-TIP. Although RD28 has several cysteine residues, its activity is not inhibited by mercury, and in this respect it differs from y-TIP and all but one of the mammalian water channels that have been described. Introduction of a cysteine residue next to the second conserved NPA motif creates a mercury-sensitive water channel, suggesting that this conserved loop is critical to the activity of the protein. Antibodies directed at the C terminus of RD28 were used in combination with a two-phase partitioning method to demonstrate that RD28 is located i n the plasma membrane. The protein is present in leaves and roots of well-watered plants, suggesting that its presence i n plants does not require a specific desiccation regime. These results demonstrate that plant cells contain constitutively expressed aquaporins i n their plasma membranes (RD28), as well as i n their tonoplasts (?-TIP).The vacuolar membranes of plant cells and the plasma membranes of mammalian cells contain aquaporins, proteins that form water-selective channels (see Chrispeels and Maurel, 1994, for review). These 27-kD integral membrane proteins belong to a family of proteins that has cognates in mammals, yeasts, and bacteria; they are part of the larger MIP family (see Reizer et al., 1993, for a recent review). In plants, more than half a dozen derived amino acid sequences and/or proteins have been recently identified. Some of them are expressed in a tissue-specific manner, whereas others are induced by specific physiological conditions. For example, a-TIP is a seed-specific protein found in the tonoplasts of protein storage vacuoles (Johnson et al., 1990), tobRB7 mRNA is found in the roots of tobacco (Nicotiana tabacum) (Yamamoto et al., 1991;Opperman et al., 1994), NOD26 is specific to the peribacteroid membranes of soybean (Glycine max) nodules (Sandal and Marcker, 1988), and trg-31 (Guerrero et al., 1990;Guerrero and Crossland, 1993) and rd28 (Yamaguchi-Shinozaki et al., 1992) transcripts are induced by desiccation of pea (Pisum sativum) and Arabidopsis thaliana, respectively. Dip is expres...

Plant cell signaling triggers the abscission of entire organs, such as fruit, leaves and flowers. Previously, we characterized an ADP-ribosylation factor GTPase-activating protein, NEVERSHED (NEV), that regulates membrane trafficking and is essential for floral organ shedding in Arabidopsis. Through a screen for mutations that restore organ separation in nev flowers, we have identified a leucine-rich repeat receptor-like kinase, EVERSHED (EVR), that functions as an inhibitor of abscission. Defects in the Golgi structure and location of the trans-Golgi network in nev abscission zone cells are rescued by a mutation in EVR, suggesting that EVR might regulate membrane trafficking during abscission. In addition to shedding their floral organs prematurely, nev evr flowers show enlarged abscission zones. A similar phenotype was reported for plants ectopically expressing INFLORESCENCE DEFICIENT IN ABSCISSION, a predicted signaling ligand for the HAESA/HAESA-LIKE2 receptor-like kinases, indicating that this signaling pathway may be constitutively active in nev evr flowers. We present a model in which EVR modulates the timing and region of abscission by promoting the internalization of other receptor-like kinases from the plasma membrane.

Receptor-like kinase-mediated cell signaling pathways play fundamental roles in many aspects of plant growth and development. A pair of Arabidopsis (Arabidopsis thaliana) leucine-rich repeat receptor-like kinases (LRR-RLKs), HAESA (HAE) and HAESA-LIKE2 (HSL2), have been shown to activate the cell separation process that leads to organ abscission. Another pair of LRR-RLKs, EVERSHED (EVR) and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1, act as inhibitors of abscission, potentially by modulating HAE/HSL2 activity. Cycling of these RLKs to and from the cell surface may be regulated by NEVERSHED (NEV), a membrane trafficking regulator that is essential for organ abscission. We report here the characterization of CAST AWAY (CST), a receptor-like cytoplasmic kinase that acts as a spatial inhibitor of cell separation. Disruption of CST suppresses the abscission defects of nev mutant flowers and restores the discrete identity of the trans-Golgi network in nev abscission zones. After organ shedding, enlarged abscission zones with obscured boundaries are found in nev cst flowers. We show that CST is a dual-specificity kinase in vitro and that myristoylation at its amino terminus promotes association with the plasma membrane. Using the bimolecular fluorescence complementation assay, we have detected interactions of CST with HAE and EVR at the plasma membrane of Arabidopsis protoplasts and hypothesize that CST negatively regulates cell separation signaling directly and indirectly. A model integrating the potential roles of receptor-like kinase signaling and membrane trafficking during organ separation is presented.

Highlights d Findings are shared for the first 17 participants in a phase 1/2 trial of VC-02 d This investigational device was implanted into type 1 diabetes patients d VC-02 contains pluripotent stem cell-derived pancreatic endoderm cells d C-peptide levels and insulin expression correlate with engraftment in 63% of subjects Authors