The number of K(+) channels in the plasma membrane of guard cell protoplasts changes in parallel with the surface area

The activity of the two dominant K(+) channels in the plasma membrane of Vicia faba guard cell protoplasts was examined during pressure-driven swelling. For this purpose, the K(+) currents and the membrane capacitance (C(m)) of guard cell protoplasts were recorded in parallel. A rise in C(m), reflecting an increase of the membrane surface area, was coupled to a proportional rise in conductance of both the K(+) inward and K(+) outward rectifier. The activation kinetics of the K(+) channels were not affected during this process. The quantitative and temporal coupling of C(m) and K(+) conductance can hence be interpreted as the result of the addition of active inward and outward rectifier K(+) channels to the plasma membrane during an increase in surface area.

StGCPRP, a potato gene strongly expressed in stomatal guard cells, defines a novel type of repetitive proline-rich proteins

Guard cells represent a highly differentiated cell type within the epidermis of plant leaves and stems. They respond to many endogenous and environmental signals and thereby modify the size of the stomatal pore they surround. We identified a novel gene that is highly expressed in guard cells of potato (Solanum tuberosum). It encodes a repetitive proline (Pro)-rich protein of 54 kD (491 amino acids) and was named StGCPRP (S. tuberosum guard cell Pro-rich protein). StGCPRP has a bipartite structure. The C-terminal part of StGCPRP contains a high percentage (46%) of Pro residues organized in distinct repetitive sequence motifs, whereas its extended N terminus is essentially free of Pros. StGCPRP represents the first member of a novel class of hybrid Pro-rich proteins that we designated NHyPRPs. In young but not in mature leaves, StGCPRP transcripts were also present at high levels in mesophyll cells (in addition to guard cells), indicating developmental regulation of StGCPRP gene expression. In addition, StGCPRP expression is regulated by environmental factors, as shown by a decrease in StGCPRP transcript levels under drought stress. Two proteins similar to StGCPRP were found to be encoded by the Arabidopsis genome, indicating that NHyPRPs are more widely distributed in higher plants.

Ion distribution in cereal leaves: pathways and mechanisms

Measurement of ion concentrations in the vacuoles of different cell types in cereal leaves using a variety of techniques indicates that ions are differentially distributed between different cell types. Thus mesophyll cells are enriched in P but contain relatively little Ca 2+ or Cl - , whereas the reverse is true for epidermal cells. Solutes reach the leaf via the transpiration stream and we consider three possible pathways which they could follow from the xylem to leaf cells. The first is a fully apoplastic mesophyll pathway in which both water and solutes move together through the leaf apoplast passing bundle sheath, mesophyll and epidermis in turn. The second is a partly symplastic mesophyll pathway in which ions and water pass into the symplast at the mestome/bundle sheath cells. Water continues to sites of evaporation via either a transcellular or symplastic pathway, but ions may be secreted back to the mesophyll apoplast and move to the epidermis along an extracellular route. The third is a vein extension pathway which provides a diffusional pathway for ions to the epidermis. A testable hypothesis for the roles of the pathways in supplying solutes to the mesophyll and epidermis is proposed and the implications of each of these pathways for transport systems in individual cell types is discussed.

Abscisic acid signaling

The plant hormone abscisic acid (ABA) regulates the development and germination of seeds, as well as the adaptation of vegetative tissues to conditions of environmental stress. During the past year, considerable insights have been gained into the molecular nature of the complex signaling network that mediates the actions of ABA. Biophysical studies indicate that at least some of the effects of ABA in stomatal guard cells involve intracellular receptors. Also, increasing evidence supports the view that guard cells contain redundant ABA transduction pathways, and that cytoplasmic Ca2+ acts as a second messenger in at least one of these pathways. Finally, mutational analysis in Arabidopsis indicates that the multiple effects of ABA at the whole plant level are mediated by overlapping branches of a highly ramified signaling network. Two Arabidopsis loci that determine ABA sensitivity have already been cloned and found to encode a protein phosphatase and a transcriptional activator.

Current advances in abscisic acid action and signalling

Abscisic acid (ABA) participates in the control of diverse physiological processes. The characterization of deficient mutants has clarified the ABA biosynthetic pathway in higher plants. Deficient mutants also lead to a revaluation of the extent of ABA action during seed development and in the response of vegetative tissues to environmental stress. Although ABA receptor(s) have not yet been identified, considerable progress has been recently made in the characterization of more downstream elements of the ABA regulatory network. ABA controls stomatal aperture by rapidly regulating identified ion transporters in guard cells, and the details of the underlying signalling pathways start to emerge. ABA actions in other cell types involve modifications of gene expression. The promoter analysis of ABA-responsive genes has revealed a diversity of cis-acting elements and a few associated trans-acting factors have been isolated. Finally, characterization of mutants defective in ABA responsiveness, and molecular cloning of the corresponding loci, has proven to be a powerful approach to dissect the molecular nature of ABA signalling cascades.

Arabidopsis ABA response gene ABI1: features of a calcium-modulated protein phosphatase

The Arabidopsis ABI1 locus is essential for a wide spectrum of abscisic acid (ABA) responses throughout plant development. Here, ABI1 was shown to regulate stomatal aperture in leaves and mitotic activity in root meristems. The ABI1 gene was cloned and predicted to encode a signaling protein. Although its carboxyl-terminal domain is related to serine-threonine phosphatase 2C, the ABI1 protein has a unique amino-terminal extension containing an EF hand calcium-binding site. These results suggest that the ABI1 protein is a Ca(2+)-modulated phosphatase and functions to integrate ABA and Ca2+ signals with phosphorylation-dependent response pathways.