Characterization of the epidermis from barley primary leaves : II. The role of the epidermis in ion compartmentation

Energy costs of salt tolerance in crop plants

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺ -ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl^- concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.

Calcium modulates leaf cell-specific phosphorus allocation in Proteaceae from south-western Australia

Over 650 Proteaceae occur in south-western Australia, contributing to the region's exceptionally high biodiversity. Most Proteaceae occur exclusively on severely nutrient-impoverished, acidic soils (calcifuge), whilst only few also occur on young, calcareous soils (soil-indifferent), higher in calcium (Ca) and phosphorus (P). The calcifuge habit of Proteaceae is explained by Ca-enhanced P toxicity, putatively linked to the leaf cell-specific allocation of Ca and P. Separation of these elements is essential to avoid the deleterious precipitation of Ca-phosphate. We used quantitative X-ray microanalysis to determine leaf cell-specific nutrient concentrations of two calcifuge and two soil-indifferent Proteaceae grown in hydroponics at a range of Ca and P concentrations. Calcium enhanced the preferential allocation of P to palisade mesophyll (PM) cells under high P conditions, without a significant change in whole leaf [P]. Calcifuges showed a greater PM [P] compared with soil-indifferent species, corresponding to their greater sensitivity. This study advances our mechanistic understanding of Ca-enhanced P toxicity, supporting the proposed model, and demonstrating its role in the calcifuge distribution of Proteaceae. This furthers our understanding of nutrient interactions at the cellular level and highlights its importance to plant functioning.

Eudicots from severely phosphorus-impoverished environments preferentially allocate phosphorus to their mesophyll

Plants allocate nutrients to specific leaf cell types, with commelinoid monocots preferentially allocating phosphorus (P) to the mesophyll and calcium (Ca) to the epidermis, whereas the opposite is thought to occur in eudicots. However, Proteaceae from severely P-impoverished habitats present the same P-allocation pattern as monocots. This raises the question of whether preferential P allocation to mesophyll cells is a phylogenetically conserved trait, exclusive to commelinoid monocots and a few Proteaceae, or a trait that has evolved multiple times to allow plants to cope with very low soil P availability. We analysed the P-allocation patterns of 16 species from 10 genera, eight families and six orders within three major clades of eudicots across different P-impoverished environments in Australia and Brazil, using elemental X-ray mapping to quantitatively determine leaf cell-specific nutrient concentrations. Many of the analysed species showed P-allocation patterns that differed substantially from that expected for eudicots. Instead, P-allocation patterns were strongly associated with the P availability in the natural habitat of the species, suggesting a convergent evolution of P-allocation patterns at the cellular level, with P limitation as selective pressure and without a consistent P-allocation pattern within eudicots. Here, we show that most eudicots from severely P-impoverished environments preferentially allocated P to their mesophyll. We surmise that this preferential P allocation to photosynthetically active cells might contribute to the very high photosynthetic P-use efficiency of species adapted to P-impoverished habitats.

Chloride: not simply a 'cheap osmoticum', but a beneficial plant macronutrient

At macronutrient levels, chloride has positive effects on plant growth, which are distinct from its function in photosynthesis..

K+ retention in leaf mesophyll, an overlooked component of salinity tolerance mechanism: a case study for barley

Plant salinity tolerance is a physiologically complex trait, with numerous mechanisms contributing to it. In this work, we show that the ability of leaf mesophyll to retain K(+) represents an important and essentially overlooked component of a salinity tolerance mechanism. The strong positive correlation between mesophyll K(+) retention ability under saline conditions (quantified by the magnitude of NaCl-induced K(+) efflux from mesophyll) and the overall salinity tolerance (relative fresh weight and/or survival or damage under salinity stress) was found while screening 46 barley (Hordeum vulgare L.) genotypes contrasting in their salinity tolerance. Genotypes with intrinsically higher leaf K(+) content under control conditions were found to possess better K(+) retention ability under salinity and, hence, overall higher tolerance. Contrary to previous reports for barley roots, K(+) retention in mesophyll was not associated with an increased H(+) -pumping in tolerant varieties but instead correlated negatively with this trait. These findings are explained by the fact that increased H(+) extrusion may be needed to charge balance the activity and provide the driving force for the high affinity HAK/KUP K(+) transporters required to restore cytosolic K(+) homeostasis in salt-sensitive genotypes.

Cellular and tissue distribution of potassium: physiological relevance, mechanisms and regulation

Potassium (K(+)) is the most important cationic nutrient for all living organisms. Its cellular levels are significant (typically around 100mM) and are highly regulated. In plants K(+) affects multiple aspects such as growth, tolerance to biotic and abiotic stress and movement of plant organs. These processes occur at the cell, organ and whole plant level and not surprisingly, plants have evolved sophisticated mechanisms for the uptake, efflux and distribution of K(+) both within cells and between organs. Great progress has been made in the last decades regarding the molecular mechanisms of K(+) uptake and efflux, particularly at the cellular level. For long distance K(+) transport our knowledge is less complete but the principles behind the overall processes are largely understood. In this chapter we will discuss how both long distance transport between different organs and intracellular transport between organelles works in general and in particular for K(+). Where possible, we will provide examples of specific genes and proteins that are responsible for these phenomena.

Ion gradients in xylem exudate and guttation fluid related to tissue ion levels along primary leaves of barley

The concentration of ions in plant cells and tissues is an essential factor in determining physiological function. In the present study, we established that concentration gradients of mobile ions exist in both xylem exudates and tissues within a barley (Hordeum vulgare) primary leaf. For K(+) and NO3 (-) , ion concentrations generally decreased from the leaf base to the tip in both xylem exudates and tissues. Ion gradients were also found for Pi and Cl(-) in the xylem. The hydathode strongly absorbed Pi and re-translocated it to the rest of the plant, whereas Cl(-) was extruded. The ion concentration gradients developed early during leaf growth, increased as the tissue aged and remained under both high and low transpiration conditions. Measurement of the expression profiles of Pi, K(+) and NO3 (-) transporters along the longitudinal axis of the leaf revealed that some transporters are more expressed at the hydathode, but for most transporters, there was no significant variation along the leaf. The mechanisms by which longitudinal ion gradients develop in leaves and their physiological functions are discussed.

Single-Cell Sampling and Analysis (SiCSA)

Single-cell sampling and analysis allows the determination of solute concentrations in individual cells and tissues. This is particularly important when studying a stress such as salinity, where the cell- and tissue-specific distribution of sodium and chloride may decide a plant's fate. In this chapter, some selected SiCSA methods are described in detail, and their advantages and possible pitfalls discussed. These methods include pressure-driven extraction of cell contents (cell sap sampling) and the analysis of extracted cell sap through picolitre osmometry (osmolality), energy-dispersive X-ray analysis (concentrations of Na, K, P, S, Cl, Ca), and microfluorometry (concentrations of, for example, nitrate and total amino acids).

Plant single cell sampling

Plant single cell sampling and analysis allows the determination of solute concentrations in individual cells and tissues. This is particularly important when studying mineral nutrition, where the cell- and tissue-specific distribution of individual mineral nutrients increases a plant's options to store and mobilize these nutrients in response to a changing external availability. In this chapter, some selected single cell sampling and analysis methods are described in detail, and their advantages and possible pitfalls discussed. These methods include pressure-driven extraction of cell contents (cell sap sampling), and the analysis of extracted cell sap through picoliter osmometry (osmolality), energy-dispersive X-ray (EDX) analysis (concentrations of Na, K, P, S, Cl, Ca), and microfluorometry (concentrations of anions and amino acids). In most cases, the extracted cell sap is mainly vacuolar in origin.

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.

Cell-specific vacuolar calcium storage mediated by CAX1 regulates apoplastic calcium concentration, gas exchange, and plant productivity in Arabidopsis

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.

Comparative physiology of elemental distributions in plants

BACKGROUND

Plants contain relatively few cell types, each contributing a specialized role in shaping plant function. With respect to plant nutrition, different cell types accumulate certain elements in varying amounts within their storage vacuole. The role and mechanisms underlying cell-specific distribution of elements in plants is poorly understood.

SCOPE

The phenomenon of cell-specific elemental accumulation has been briefly reviewed previously, but recent technological advances with the potential to probe mechanisms underlying elemental compartmentation have warranted an updated evaluation. We have taken this opportunity to catalogue many of the studies, and techniques used for, recording cell-specific compartmentation of particular elements. More importantly, we use three case-study elements (Ca, Cd and Na) to highlight the basis of such phenomena in terms of their physiological implications and underpinning mechanisms; we also link such distributions to the expression of known ion or solute transporters.

CONCLUSIONS

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.

Quantitative X-ray microanalysis of hydrogen peroxide within plant cells

Using quantitative X-ray microanalysis in combination with CeCl3-based cytochemical staining of hydrogen peroxide (H2O2) we have developed a new solution for quantification of H2O2 at the subcellular level. Quantitative X-ray microanalysis of plastic-embedded leaves of Populus euphratica Oliv. showed that the obtained cerium precipitates by CeCl3 staining were the mixture of cerium perhydroxides and cerium phosphate, in which the fractions of CePO4 were: (1) 52-74% in cell walls of fresh leaf segments, and (2) 34-70% in the cytoplasm in 10 mM H2O2-treated leaf segments that were previously freeze-dried. Taking the concentration of cerium phosphate as staining background, we reached the cellular concentration of cerium perhydroxides and the corresponding concentration of H2O2. Results showed that H2O2 was present in the cytoplasm of rehydrated leaf segments (29-58 mM), but in fresh leaves, H2O2 was observed in the walls of all measured cell types (17-74 mM).

Photosynthetic capacity is related to the cellular and subcellular partitioning of Na+, K+ and Cl- in salt-affected barley and durum wheat

The capacity of plants to tolerate high levels of salinity depends on the ability to exclude salt from the shoot, or to tolerate high concentrations of salt in the leaf (tissue tolerance). It is widely held that a major component of tissue tolerance is the capacity to compartmentalize salt into safe storage places such as vacuoles. This mechanism would avoid toxic effects of salt on photosynthesis and other key metabolic processes. To test this, the relationship between photosynthetic capacity and the cellular and subcellular distribution of Na+, K+ and Cl- was studied in salt-sensitive durum wheat (cv. Wollaroi) and salt-tolerant barley (cv. Franklin) seedlings grown in a range of salinity treatments. Photosynthetic capacity parameters (Vcmax, Jmax) of salt-stressed Wollaroi decreased at a lower leaf Na+ concentration than in Franklin. Vacuolar concentrations of Na+, K+ and Cl- in mesophyll and epidermal cells were measured using cryo-scanning electron microscopy (SEM) X-ray microanalysis. In both species, the vacuolar Na+ concentration was similar in mesophyll and epidermal cells, whereas K+ was at higher concentrations in the mesophyll, and Cl- higher in the epidermis. The calculated cytoplasmic Na+ concentration increased to higher concentrations with increasing bulk leaf Na+ concentration in Wollaroi compared to Franklin. Vacuolar K+ concentration was lower in the epidermal cells of Franklin than Wollaroi, resulting in higher cytoplasmic K+ concentrations and a higher K+ : Na+ ratio. This study indicated that the maintenance of photosynthetic capacity (and the resulting greater salt tolerance) at higher leaf Na+ levels of barley compared to durum wheat was associated with the maintenance of higher K+, lower Na+ and the resulting higher K+ : Na+ in the cytoplasm of mesophyll cells of barley.

Salinity and the growth of non-halophytic grass leaves: the role of mineral nutrient distribution

Salinity is increasingly limiting the production of graminaceous crops constituting the main sources of staple food (rice, wheat, barley, maize and sorghum), primarily through reductions in the expansion and photosynthetic yield of the leaves. In the present review, we summarise current knowledge of the characteristics of the spatial distribution patterns of the mineral elements along the growing grass leaf and of the impact of salinity on these patterns. Although mineral nutrients have a wide range of functions in plant tissues, their functions may differ between growing and non-growing parts of the grass leaf. To identify the physiological processes by which salinity affects leaf elongation in non-halophytic grasses, patterns of mineral nutrient deposition related to developmental and anatomical gradients along the growing grass leaf are discussed. The hypothesis that a causal link exists between ion deficiency and / or toxicity and the inhibition of leaf growth of grasses in a saline environment is tested.

Effects of magnesium availability on the activity of plasma membrane ion transporters and light-induced responses from broad bean leaf mesophyll

Considering the physiological significance of Mg homeostasis in plants, surprisingly little is known about the molecular and ionic mechanisms mediating Mg transport across the plasma membrane and the impact of Mg availability on transport processes at the plasmalemma. In this study, a non-invasive ion-selective microelectrode technique (MIFE) was used to characterize the effects of Mg availability on the activity of plasma membrane H+, K+, Ca2+, and Mg2+ transporters in the mesophyll cells of broad bean (Vicia faba L.) plants. Based on the stoichiometry of ion-flux changes and results of pharmacological experiments, we suggest that at least two mechanisms are involved in Mg2+ uptake across the plasma membrane of bean mesophyll cells. One of them is a non-selective cation channel, also permeable to K+ and Ca2+. The other mechanism, operating at concentrations below 30 microM, was speculated to be an H+/Mg+ exchanger. Experiments performed on leaves grown at different levels of Mg availability (from deficient to excessive) showed that Mg availability has a significant impact on the activity of plasma-membrane transporters for Ca2+, K+, and H+. We discuss the physiological significance of Mg-induced changes in leaf electrophysiological responses to light and the ionic mechanisms underlying this process.

Processes modulating calcium distribution in citrus leaves. An investigation using x-ray microanalysis with strontium as a tracer

Citrus leaves accumulate large amounts of calcium that must be compartmented effectively to prevent stomatal closure by extracellular Ca2+ and interference with Ca(2+)-based cell signaling pathways. Using x-ray microanalysis, the distribution of calcium between vacuoles in different cell types of leaves of rough lemon (Citrus jambhiri Lush.) was investigated. Calcium was accumulated principally in palisade, spongy mesophyll, and crystal-containing idioblast cells. It was low in epidermal and bundle sheath cells. Potassium showed the reverse distribution. Rubidium and strontium were used as tracers to examine the pathways by which potassium and calcium reached these cells. Comparisons of strontium and calcium distribution indicated that strontium is a good tracer for calcium, but rubidium did not mirror the potassium distribution pattern. The amount of strontium accumulated was highest in palisade cells, lowest in bundle sheath and epidermal cells, and intermediate in the spongy mesophyll. Accumulation of strontium in palisade and spongy mesophyll was accompanied by loss of potassium from these cells and its accumulation in the bundle sheath. Strontium moved apoplastically from the xylem to all cell types, and manipulation of water loss from the adaxial leaf surface suggested that diffusion is responsible for strontium movement to this side of the leaf. The results highlight the importance of palisade and spongy mesophyll as repositories for calcium and suggest that calcium distribution between different cell types is the result of differential rates of uptake. This tracer technique can provide important information about the ion uptake and accumulation properties of cells in intact leaves.

Hyperaccumulation of cadmium and zinc in Thlaspi caerulescens and Arabidopsis halleri at the leaf cellular level

Vacuolar compartmentalization or cell wall binding in leaves could play a major role in hyperaccumulation of heavy metals. However, little is known about the physiology of intracellular cadmium (Cd) sequestration in plants. We investigated the role of the leaf cells in allocating metal in hyperaccumulating plants by measuring short-term (109)Cd and (65)Zn uptake in mesophyll protoplasts of Thlaspi caerulescens "Ganges" and Arabidopsis halleri, both hyperaccumulators of zinc (Zn) and Cd, and T. caerulescens "Prayon," accumulating Cd at a lower degree. The effects of low temperature, several divalent cations, and pre-exposure of the plants to metals were investigated. There was no significant difference between the Michaelis-Menten kinetic constants of the three plants. It indicates that differences in metal uptake cannot be explained by different constitutive transport capacities at the leaf protoplast level and that plasma and vacuole membranes of mesophyll cells are not responsible for the differences observed in heavy metal allocation. This suggests the existence of regulation mechanisms before the plasma membrane of leaf mesophyll protoplasts. However, pre-exposure of the plants to Cd induced an increase in Cd accumulation in protoplasts of "Ganges," whereas it decreased Cd accumulation in A. halleri protoplasts, indicating that Cd-permeable transport proteins are differentially regulated. The experiment with competitors has shown that probably more than one single transport system is carrying Cd in parallel into the cell and that in T. caerulescens "Prayon," Cd could be transported by a Zn and Ca pathway, whereas in "Ganges," Cd could be transported mainly by other pathways.

Where do all the ions go? The cellular basis of differential ion accumulation in leaf cells

Leaf cells accumulate solutes differently depending on their cell type. The accumulation profiles of inorganic ions have been well documented for the mesophyll and epidermis, particularly in cereals. These cell types accumulate ions such as phosphate and calcium to strikingly different extents. Understanding the processes that control ion accumulation could reveal how plants respond to either a limiting supply of important micro- and macronutrient ions or to potentially toxic loads of salts or heavy metal ions. Research has recently begun to reveal the processes that underlie this remarkable sorting of nutrient ions within the leaf.

Differential ion accumulation and ion fluxes in the mesophyll and epidermis of barley

In barley (Hordeum vulgare L.) leaves, differential ion accumulation commonly results in inorganic phosphate (Pi) being confined to the mesophyll and Ca(2+) to the epidermis, with preferential epidermal accumulation of Cl(-), Na(+), and some other ions. The pattern was confirmed in this study for major inorganic anions and cations by analysis of barley leaf protoplasts. The work focused on the extent to which differences in plasma membrane ion transport processes underlie these observations. Ion transport across the plasma membrane of barley epidermal and mesophyll protoplasts was investigated electrophysiologically (by microelectrode impalement and patch clamping) and radiometrically. Data from both approaches suggested that similar types of ion-selective channels and membrane transporters, which catalyze the transport of Ca(2+), K(+), Na(+), and Pi, exist in the plasma membrane of the two cell types. In general, the simple presence or absence of ion transporters could not explain cell-type-specific differences in ion accumulation. However, patch-clamp data suggested that differential regulation of instantaneously activating ion channels in the plasma membrane could explain the preferential accumulation of Na(+) in the epidermis.

Characterization of the epidermis from barley primary leaves : I. Isolation of epidermal protoplasts

A method is described for isolating epidermal protoplasts from the primary leaves of barley (Hordeum vulgare L.). Epidermal protoplasts are lighter than mesophyll protoplasts because of their smaller ratio of cytoplasm to vacuole, and can be separated from the latter by density-gradient centrifugation after complete digestion of the leaves. We have started a basic characterization of the epidermal protoplast fraction in comparison with mesophyll protoplasts. Epidermal protoplasts had a mean diameter of 63.5 μm, whereas that of mesophyll protoplasts was 35.7 μm. Their respiratory oxygen consumption was not influenced by light. They contained acid hydrolases and cytoplasmic enzymes in relative activities different from those of mesophyll protoplasts. Their polypeptide pattern as judged from two-dimensional separations was, in principle, similar to that of mesophyll cells after elimination of the plastids from the latter by the preparation of vacuoplasts. However, in addition, a considerable number of epidermis-specific polypeptides were observed. Isolated epidermal protoplasts were viable and efficiently incorporated [(35)S]methionine into newly synthesized proteins. The results show that epidermal protoplasts are suitable for the investigation of the physiological and molecular properties of epidermal cells in leaves.