Sergey Shabala

Extracellular Ca2+ ameliorates NaCl-induced K+ loss from Arabidopsis root and leaf cells by controlling plasma membrane K+ -permeable channels.

Calcium can ameliorate Na+ toxicity in plants by decreasing Na+ influx through nonselective cation channels. Here, we show that elevated external [Ca2+] also inhibits Na+-induced K+ efflux through outwardly directed, K+-permeable channels. Noninvasive ion flux measuring and patch-clamp techniques were used to characterize K+ fluxes from Arabidopsis (Arabidopsis thaliana) root mature epidermis and leaf mesophyll under various Ca2+ to Na+ ratios. NaCl-induced K+ efflux was not related to the osmotic component of the salt stress, was inhibited by the K+ channel blocker TEA+, was not mediated by inwardly directed K+ channels (tested in the akt1 mutant), and resulted in a significant decrease in cytosolic K+ content. NaCl-induced K+ efflux was partially inhibited by 1 mm Ca2+ and fully prevented by 10 mm Ca2+. This ameliorative effect was at least partially attributed to a less dramatic NaCl-induced membrane depolarization under high Ca2+ conditions. Patch-clamp experiments (whole-cell mode) have demonstrated that two populations of Ca2+-sensitive K+ efflux channels exist in protoplasts isolated from the mature epidermis of Arabidopsis root and leaf mesophyll cells. The instantaneously activating K+ efflux channels showed weak voltage dependence and insensitivity to external and internal Na+. Another population of K+ efflux channels was slowly activating, steeply rectifying, and highly sensitive to Na+. K+ efflux channels in roots and leaves showed different Ca2+ and Na+ sensitivities, suggesting that these organs may employ different strategies to withstand salinity. Our results suggest an additional mechanism of Ca2+ action on salt toxicity in plants: the amelioration of K+ loss from the cell by regulating (both directly and indirectly) K+ efflux channels.

Root Plasma Membrane Transporters Controlling K+/Na+ Homeostasis in Salt-Stressed Barley

Plant salinity tolerance is a polygenic trait with contributions from genetic, developmental, and physiological interactions, in addition to interactions between the plant and its environment. In this study, we show that in salt-tolerant genotypes of barley (Hordeum vulgare), multiple mechanisms are well combined to withstand saline conditions. These mechanisms include: (1) better control of membrane voltage so retaining a more negative membrane potential; (2) intrinsically higher H+ pump activity; (3) better ability of root cells to pump Na+ from the cytosol to the external medium; and (4) higher sensitivity to supplemental Ca2+. At the same time, no significant difference was found between contrasting cultivars in their unidirectional 22Na+ influx or in the density and voltage dependence of depolarization-activated outward-rectifying K+ channels. Overall, our results are consistent with the idea of the cytosolic K+-to-Na+ ratio being a key determinant of plant salinity tolerance, and suggest multiple pathways of controlling that important feature in salt-tolerant plants.

Free oxygen radicals regulate plasma membrane Ca2+- and K+-permeable channels in plant root cells

Free oxygen radicals are an irrefutable component of life, underlying important biochemical and physiological phenomena in animals. Here it is shown that free oxygen radicals activate plasma membrane Ca2+- and K+-permeable conductances in Arabidopsis root cell protoplasts, mediating Ca2+ influx and K+ efflux, respectively. Free oxygen radicals generate increases in cytosolic Ca2+ mediated by a novel population of nonselective cation channels that differ in selectivity and pharmacology from those involved in toxic Na+ influx. Analysis of the free oxygen radical-activated K+ conductance showed its similarity to the Arabidopsis root K+ outward rectifier. Significantly larger channel activation was found in cells responsible for perceiving environmental signals and undergoing elongation. Quenching root free oxygen radicals inhibited root elongation, confirming the role of radical-activated Ca2+ influx in cell growth. Net free oxygen radical-stimulated Ca2+ influx and K+ efflux were observed in root cells of monocots, dicots, C3 and C4 plants, suggesting conserved mechanisms and functions. In conclusion, two functions for free oxygen radical cation channel activation are proposed: initialization/amplification of stress signals and control of cell elongation in root growth.

Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Reactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K+ channel responsible for K+ efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca2+, tetraethylammonium, Ba2+, Cs+ and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K+ channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K+ efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K+-channel activation. In animals, activation of K+-efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K+-channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K+ channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K+ channels.

Arabidopsis protein kinase PKS5 inhibits the plasma membrane H +-ATPase by preventing interaction with 14-3-3 protein

Regulation of the trans-plasma membrane pH gradient is an important part of plant responses to several hormonal and environmental cues, including auxin, blue light, and fungal elicitors. However, little is known about the signaling components that mediate this regulation. Here, we report that an Arabidopsis thaliana Ser/Thr protein kinase, PKS5, is a negative regulator of the plasma membrane proton pump (PM H+-ATPase). Loss-of-function pks5 mutant plants are more tolerant of high external pH due to extrusion of protons to the extracellular space. PKS5 phosphorylates the PM H+-ATPase AHA2 at a novel site, Ser-931, in the C-terminal regulatory domain. Phosphorylation at this site inhibits interaction between the PM H+-ATPase and an activating 14-3-3 protein in a yeast expression system. We show that PKS5 interacts with the calcium binding protein SCaBP1 and that high external pH can trigger an increase in the concentration of cytosolic-free calcium. These results suggest that PKS5 is part of a calcium-signaling pathway mediating PM H+-ATPase regulation.

Learning from halophytes: physiological basis and strategies to improve abiotic stress tolerance in crops

BACKGROUND Global annual losses in agricultural production from salt-affected land are in excess of US

Potassium and sodium relations in salinised barley tissues as a basis of differential salt tolerance

12 billion and rising. At the same time, a significant amount of arable land is becoming lost to urban sprawl, forcing agricultural production into marginal areas. Consequently, there is a need for a major breakthrough in crop breeding for salinity tolerance. Given the limited range of genetic diversity in this trait within traditional crops, stress tolerance genes and mechanisms must be identified in extremophiles and then introduced into traditional crops. SCOPE AND CONCLUSIONS This review argues that learning from halophytes may be a promising way of achieving this goal. The paper is focused around two central questions: what are the key physiological mechanisms conferring salinity tolerance in halophytes that can be introduced into non-halophyte crop species to improve their performance under saline conditions and what specific genes need to be targeted to achieve this goal? The specific traits that are discussed and advocated include: manipulation of trichome shape, size and density to enable their use for external Na(+) sequestration; increasing the efficiency of internal Na(+) sequestration in vacuoles by the orchestrated regulation of tonoplast NHX exchangers and slow and fast vacuolar channels, combined with greater cytosolic K(+) retention; controlling stomata aperture and optimizing water use efficiency by reducing stomatal density; and efficient control of xylem ion loading, enabling rapid shoot osmotic adjustment while preventing prolonged Na(+) transport to the shoot.

A root's ability to retain K+ correlates with salt tolerance in wheat

A large-scale glasshouse trial, including nearly 70 barley cultivars (5300 plants in total), was conducted over 2 consecutive years to investigate plant physiological responses to salinity. In a parallel set of experiments, plant salt tolerance was assessed by non-invasive microelectrode measurements of net K+ flux from roots of 3-day-old seedlings of each cultivar after 1 h treatment in 80 mm NaCl as described in our previous publication (Chen et al. 2005). K+ flux from the root in response to NaCl treatment was highly (P < 0.001) inversely correlated with relative grain yield, shoot biomass, plant height, net CO2 assimilation, survival rate and thousand-seed weight measured in glasshouse experiments after 4-5 months of salinity treatment. No significant correlation with relative germination rate or tillering was found. In general, 62 out of 69 cultivars followed an inverse relationship between K+ efflux and salt tolerance. In a few cultivars, however, high salt tolerance (measured as grain yield at harvest) was observed for plants showing only modest ability to retain K+ in the root cells. Tissue elemental analysis showed that these plants had a much better ability to prevent Na+ accumulation in plant leaves and, thus, to maintain a higher K+/Na+ ratio. Taken together, our results show that a plants ability to maintain high K+/Na+ ratio (either retention of K+ or preventing Na+ from accumulating in leaves) is a key feature for salt tolerance in barley.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment.

Most work on wheat breeding for salt tolerance has focused mainly on excluding Na+ from uptake and transport to the shoot. However, some recent findings have reported no apparent correlation between leaf Na+ content and wheat salt tolerance. Thus, it appears that excluding Na+ by itself is not always sufficient to increase plant salt tolerance and other physiological traits should also be considered. In this work, it was investigated whether a roots ability to retain K+ may be such a trait, and whether our previous findings for barley can be extrapolated to species following a ‘salt exclusion’ strategy. NaCl-induced kinetics of K+ flux from roots of two bread and two durum wheat genotypes, contrasting in their salt tolerance, were measured under laboratory conditions using non-invasive ion flux measuring (the MIFE) technique. These measurements were compared with whole-plant physiological characteristics and yield responses from plants grown under greenhouse conditions. The results show that K+ flux from the root surface of 6-d-old wheat seedlings in response to salt treatment was highly correlated with major plant physiological characteristics and yield of greenhouse-grown plants. This emphasizes the critical role of K+ homeostasis in plant salt tolerance and suggests that using NaCl-induced K+ flux measurements as a physiological ‘marker’ for salt tolerance may benefit wheat-breeding programmes.

Salinity-induced ion flux patterns from the excised roots of Arabidopsis sos mutants

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plants life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.