Summary The growth and metabolism of plants in response to different concentrations of tissue K is discussed in relation to current knowledge about the distribution and functions of this ion in plant cells. In the cytoplasm, K has an important role in providing the correct ionic environment for metabolic processes. The ionic requirements of protein synthesis seem to be particularly important in determining the composition of the cytoplasm. Potassium is not replaceable in its cytoplasmic functions and the plant probably needs to maintain the cytoplasmic concentration of K in the range of 100 to 200 mM. Potassium salts in the vacuole are involved in the generation of turgor but when unavailable they can be replaced by other solutes. Salts of other cations such as Na and Mg are often a readily available alternative to K but in their absence organic solutes must be accumulated. With these observations as a basis, a model is proposed in which, as the concentration of K in the tissue declines, the concentration in the cytoplasm is initially maintained constant, while that in the vacuole decreases. In order to maintain turgor, alternative solutes are accumulated in the vacuole as replacements for K. It is assumed that K in the vacuole can only drop to a certain minimum level and, once this is reached, any further decline of tissue K must be at the expense of that in the cytoplasm. This leads to a decrease in the rate of metabolic processes that depend on K and so to a decline in growth. The hypothesis explains the observed relationships between growth and concentrations of K in tissues, and their modification by Na and other cations.

-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2 -deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2 -deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed.

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.

Plant cells contain two major pools of K+, one in the vacuole and one in the cytosol. The behavior of K+ concentrations in these pools is fundamental to understanding the way this nutrient affects plantigrowth. Triple-barreled microelectrodes have been used to obtain the first fully quantitative measurements of the changes in K+ activity (ay) in the vacuole and cytosol of barley (Hordeum vulgare L.) root cells grown in different K+ concentrations. The electrodes incorporate a pH-selective barrel allowing each measurement to be assigned to either the cytosol or vacuole. The measurements revealed that vacuolar aK declined linearly with decreases in tissue K+ concentration, whereas cytosolic aK initially remained constant in both epidermal and cortical cells but then declined at different rates in each cell type. An unexpected finding was that cytoplasmic pH declined in parallel with cytosolic aK, but acidification of the cytosol with butyrate did not reveal any short-term link between these two parameters. These Potassium is the major ionic osmoticum in plant cells and occurs in two major pools, one in the vacuole and one in the cytosol. The vacuolar pool is the largest and K+ in this compartment has a purely biophysical function-the lowering of sap osmotic potential to generate turgor and drive cell expansion. In contrast, K+ in the cytosol has both osmotic and biochemical roles (1, 2). When the external K+ supply declines from sufficiency to deficiency the behavior of K+ concentrations in each of these compartments is thought to differ; that in the cytosol remains relatively constant to maintain the rate of K+-dependent processes, whereas that in the vacuole declines with other osmotica replacing it to maintain turgor (3). Cytosolic K+ concentration is thought to decline only when the vacuolar K+ concentration has been depleted to some minimum value below which it will not fall. The consequent changes in cytosolic K+ are hypothesized to cause a decrease in the rate of biochemical processes and, thus, to a decline in growth (3).The proposed behavior of K+ concentrations in the cytosol and vacuole is broadly accepted and is supported by a variety of studies (e.g., refs. 4-8). However, it has never been quantitatively tested because the techniques used in the above studies measured compartmental K+ concentrations either indirectly (e.g., ref. 4) or only semi-quantitatively (e.g., refs. 7 and 8). In this study we have used a new approach to measuring K+ compartmentation in plant cells and its response to K+ deficiency. Triple-barreled microelectrodes (9), able to measure K+ activity (aK), pH, and membrane potential (Em), have been employed to determine these parameters in root cells of barley plants grown with different K+ supplies. The incorporation of a pH-sensing barrel allows unequivocal assignment of aK values to the vacuole or the cytosol, based on the pH differences between these compartments (10). The results obtained provide the first fully quantitative study of K+ homeostasis in plants.MATERIA...

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SummaryThe high affinity potassium transporter, HKT1 from wheat was introduced into Florida wheat in sense and antisense orientation under control of a ubiquitin promoter. Ten transgenic lines expressing the transgene were identified and two of these showed strong down-regulation of the native HKT1 transcript. One line (271) was expressing the antisense construct and the other (223) was expressing a truncated sense construct. The two lines were examined further for phenotype relating to cation transport. Membrane depolarisations were measured in low (0.1 mM) K þ and high (100 mM) NaCl. Under these conditions there was no difference between line 271 and the control at low K þ , but at high Na þ there was a rapid depolarisation that was significantly larger in control plants. 22 Na uptake was measured in this line and there was a significant decrease in uptake at 100 mM NaCl in the transgenic line when compared with the control. The two transgenic lines were grown at high NaCl (200 mM) and analysed for growth and root sodium content. Lines 271 and 223 showed enhanced growth under salinity when compared with the control and had lower sodium in the root. Secondary ion mass spectrometry (SIMS) analysis of transverse sections of the root showed that Na þ and K þ were strongly localised to stelar regions when compared with other ions, and that the Na þ : K þ ratios were reduced in salt-stressed transgenic tissue when compared with the control.

Calcium (Ca) is a unique macronutrient with diverse but fundamental physiological roles in plant structure and signalling. In the majority of crops the largest proportion of long-distance calcium ion (Ca(2+)) transport through plant tissues has been demonstrated to follow apoplastic pathways, although this paradigm is being increasingly challenged. Similarly, under certain conditions, apoplastic pathways can dominate the proportion of water flow through plants. Therefore, tissue Ca supply is often found to be tightly linked to transpiration. Once Ca is deposited in vacuoles it is rarely redistributed, which results in highly transpiring organs amassing large concentrations of Ca ([Ca]). Meanwhile, the nutritional flow of Ca(2+) must be regulated so it does not interfere with signalling events. However, water flow through plants is itself regulated by Ca(2+), both in the apoplast via effects on cell wall structure and stomatal aperture, and within the symplast via Ca(2+)-mediated gating of aquaporins which regulates flow across membranes. In this review, an integrated model of water and Ca(2+) movement through plants is developed and how this affects [Ca] distribution and water flow within tissues is discussed, with particular emphasis on the role of aquaporins.

Triple-barrelled microelectrodes measuring K(+) activity (a(K)), pH and membrane potential were used to make quantitative measurements of vacuolar and cytosolic a(K) in epidermal and mesophyll cells of barley plants grown in nutrient solution with 0 or 200 mM added NaCl. Measurements of a(K) were assigned to the cytosol or vacuole based on the pH measured. In epidermal cells, the salt treatment decreased a(K) in the vacuole from 224 to 47 mM and in the cytosol from 68 to 15 mM. In contrast, the equivalent changes in the mesophyll were from 235 to 150 mM (vacuole) and 79 to 64 mM (cytosol). Thus mechanisms exist to ameliorate the effects of salt on a(K) in compartments of mesophyll cells, presumably to minimize any deleterious consequences for photosynthesis. Thermodynamic calculations showed that K(+) is actively transported into the vacuole of both epidermal and mesophyll cells of salinized and non- salinized plants. Comparison of the values of a(K) in K(+)-replete, non-salinized leaf cells with those previously measured in root cells of plants grown under comparable conditions indicates that cytosolic a(K) is similar in cells of both organs, but vacuolar a(K) in leaf cells is approximately twice that in roots. This suggests differences in the regulation of vacuolar a(K), but not cytosolic a(K), in leaf and root cells.