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Contents 668I. 668II. 668III. 670IV. 671V. 672 672 References 672 Summary Soil salinity reduces crop yield. The extent and severity of salt‐affected agricultural land is predicted to worsen as a result of inadequate drainage of irrigated land, rising water tables and global warming. The growth and yield of most plant species are adversely affected by soil salinity, but varied adaptations can allow some crop cultivars to continue to grow and produce a harvestable yield under moderate soil salinity. Significant costs are associated with saline soils: the economic costs to the farming community and the energy costs of plant adaptations. We briefly consider mechanisms of adaptation and highlight recent research examples through a lens of their applicability to improving the energy efficiency of crops under saline field conditions.

Soil salinity affects large areas of cultivated land, causing significant reductions in crop yield globally. The Na + toxicity of many crop plants is correlated with overaccumulation of Na + in the shoot. We have previously suggested that the engineering of Na + exclusion from the shoot could be achieved through an alteration of plasma membrane Na + transport processes in the root, if these alterations were cell type specific. Here, it is shown that expression of the Na + transporter HKT1;1 in the mature root stele of Arabidopsis thaliana decreases Na + accumulation in the shoot by 37 to 64%. The expression of HKT1;1 specifically in the mature root stele is achieved using an enhancer trap expression system for specific and strong overexpression. The effect in the shoot is caused by the increased influx, mediated by HKT1;1, of Na + into stelar root cells, which is demonstrated in planta and leads to a reduction of root-to-shoot transfer of Na + . Plants with reduced shoot Na + also have increased salinity tolerance. By contrast, plants constitutively expressing HKT1;1 driven by the cauliflower mosaic virus 35S promoter accumulated high shoot Na + and grew poorly. Our results demonstrate that the modification of a specific Na + transport process in specific cell types can reduce shoot Na + accumulation, an important component of salinity tolerance of many higher plants. INTRODUCTIONSoil salinity affects large areas of cultivated land in more than 100 countries (Rengasamy, 2006). Increased soil salinity negatively affects the growth of many crop plants, and the continued salinization of arable land provides an increasing threat to global crop production, especially in irrigated systems (Munns and Tester, 2008). Increasing the salinity tolerance of crop plants will provide an important contribution to the maintenance of crop yields.The Na + toxicity of many crop plants is correlated with overaccumulation of Na + in the shoot (Munns, 1993(Munns, , 2002Tester and Davenport, 2003;Møller and Tester, 2007). Na + is taken up from the soil by the plant root system and transported to the shoot in the transpiration stream . Shoot Na + accumulation is the net result of distinct Na + transport processes occurring in different organs and cell types , and each of these processes contributes to the salinity tolerance of a plant.Such Na + transport processes include passive influx of Na + into the root system, which is likely to be mediated by nonselective cation channels (Davenport and Tester, 2000;, with cyclic nucleotidegated channels and Glu receptors being likely candidate gene families encoding these proteins Roy et al., 2008). The HKT family of ion transporters originally was named for the high-affinity potassium transporter properties of the first member of this family that was isolated, but it is more complex than originally realized, as the affinity and selectivity of many proteins encoded by members of this gene family are different to that indicated by the name. As has been shown for HKT2;1 and HKT2;2 in rice (Oryza sa...

We report physiological and anatomical characteristics of water transport across roots grown in soil of two cultivars of grapevine (Vitis vinifera) differing in response to water stress (Grenache, isohydric; Chardonnay, anisohydric). Both cultivars have similar root hydraulic conductances (L o ; normalized to root dry weight) that change diurnally. There is a positive correlation between L o and transpiration. Under water stress, both cultivars have reduced minimum daily L o (predawn) attributed to the development of apoplastic barriers. Water-stressed and well-watered Chardonnay had the same diurnal change in amplitude of L o , while water-stressed Grenache showed a reduction in daily amplitude compared with well-watered plants. Hydraulic conductivity of root cortex cells (L pcell ) doubles in Chardonnay but remains unchanged in Grenache. Of the two most highly expressed plasma membrane intrinsic protein (PIP) aquaporins in roots (VvPIP1;1 and VvPIP2;2), only VvPIP2;2 functions as a water channel in Xenopus laevis oocytes. VvPIP1;1 interacts with VvPIP2;2 to induce 3-fold higher water permeability. These two aquaporins are colocated in the root from in situ hybridization and immunolocalization of VvPIP1 and VvPIP2 subfamily members. They occur in root tip, exodermis, root cortex (detected up to 30 mm), and stele. VvPIP2;2 mRNA does not change diurnally or with water stress, in contrast to VvPIP1;1, in which expression reflects the differences in L o and L pcell between cultivars in their responses to water stress and rewatering. VvPIP1;1 may regulate water transport across roots such that transpirational demand is matched by root water transport capacity. This occurs on a diurnal basis and in response to water stress that corresponds to the difference in drought tolerance between the cultivars. Root hydraulic conductance is usually lowest within the liquid component of the soil-plant-air continuum. The hydraulic conductance of roots can be highly variable in both time and space, which will affect soilwater extraction and shoot water status (Steudle and Peterson, 1998;Steudle, 2000aSteudle, , 2000b. Steudle (2000aSteudle ( , 2000b explains variation in root hydraulic conductivity (L p ; hydraulic conductance normalized to root surface area) in terms of the composite transport model based on the composite anatomical structure of roots, where water can move radially toward the xylem along three pathways: the apoplastic, symplastic, and transcellular. The symplastic and transcellular pathways are difficult to separate experimentally and are collectively considered as the cell-to-cell pathway (Steudle, 2000b). The extent to which water flow predominates in either pathway varies according to the relative hydraulic conductances of the pathways and the relative magnitude of hydrostatic versus osmotic gradients (Steudle, 2000a; Bramley et al., 2007b). Apoplastic flow can be altered irreversibly by anatomical changes, including Casparian bands and suberin lamellae (Steudle and Peterson, 1998). The conductance of the cell...

Elevations in cytosolic free calcium concentration ([Ca(2+)](cyt)) constitute a fundamental signal transduction mechanism in eukaryotic cells, but the molecular identity of Ca(2+) channels initiating this signal in plants is still under debate. Here, we show by pharmacology and loss-of-function mutants that in tobacco and Arabidopsis, glutamate receptor-like channels (GLRs) facilitate Ca(2+) influx across the plasma membrane, modulate apical [Ca(2+)](cyt) gradient, and consequently affect pollen tube growth and morphogenesis. Additionally, wild-type pollen tubes grown in pistils of knock-out mutants for serine-racemase (SR1) displayed growth defects consistent with a decrease in GLR activity. Our findings reveal a novel plant signaling mechanism between male gametophyte and pistil tissue similar to amino acid-mediated communication commonly observed in animal nervous systems.

The non-protein amino acid, gamma-aminobutyric acid (GABA) rapidly accumulates in plant tissues in response to biotic and abiotic stress, and regulates plant growth. Until now it was not known whether GABA exerts its effects in plants through the regulation of carbon metabolism or via an unidentified signalling pathway. Here, we demonstrate that anion flux through plant aluminium-activated malate transporter (ALMT) proteins is activated by anions and negatively regulated by GABA. Site-directed mutagenesis of selected amino acids within ALMT proteins abolishes GABA efficacy but does not alter other transport properties. GABA modulation of ALMT activity results in altered root growth and altered root tolerance to alkaline pH, acid pH and aluminium ions. We propose that GABA exerts its multiple physiological effects in plants via ALMT, including the regulation of pollen tube and root growth, and that GABA can finally be considered a legitimate signalling molecule in both the plant and animal kingdoms.

The physiological role and mechanism of nutrient storage within vacuoles of specific cell types is poorly understood. Transcript profiles from Arabidopsis thaliana leaf cells differing in calcium concentration ([Ca], epidermis <10 mM versus mesophyll >60 mM) were compared using a microarray screen and single-cell quantitative PCR. Three tonoplast-localized Ca 2+ transporters, CAX1 (Ca 2+ /H + -antiporter), ACA4, and ACA11 (Ca 2+ -ATPases), were identified as preferentially expressed in Ca-rich mesophyll. Analysis of respective loss-of-function mutants demonstrated that only a mutant that lacked expression of both CAX1 and CAX3, a gene ectopically expressed in leaves upon knockout of CAX1, had reduced mesophyll [Ca]. Reduced capacity for mesophyll Ca accumulation resulted in reduced cell wall extensibility, stomatal aperture, transpiration, CO 2 assimilation, and leaf growth rate; increased transcript abundance of other Ca 2+ transporter genes; altered expression of cell wall-modifying proteins, including members of the pectinmethylesterase, expansin, cellulose synthase, and polygalacturonase families; and higher pectin concentrations and thicker cell walls. We demonstrate that these phenotypes result from altered apoplastic free [Ca 2+ ], which is threefold greater in cax1/cax3 than in wild-type plants. We establish CAX1 as a key regulator of apoplastic [Ca 2+ ] through compartmentation into mesophyll vacuoles, a mechanism essential for optimal plant function and productivity.

Element accumulation patterns are clearly defined by expression of key ion or solute transporters. Although the location of element accumulation is fairly robust, alterations in expression of certain solute transporters, through genetic modifications or by growth under stress, result in perturbations to these patterns. However, redundancy or induced pleiotropic expression effects may complicate attempts to characterize the pathways that lead to cell-specific elemental distribution. Accumulation of one element often has consequences on the accumulation of others, which seems to be driven largely to maintain vacuolar and cytoplasmic osmolarity and charge balance, and also serves as a detoxification mechanism. Altered cell-specific transcriptomics can be shown, in part, to explain some of this compensation.