Ion Transport In Roots Research Articles

Comprehensive Analysis of Transcriptome and Metabolome Elucidates the Molecular Regulatory Mechanism of Salt Resistance in Roots of Achnatherum inebrians Mediated by Epichloë gansuensis

Salinization of soil is a major environmental risk factor to plant functions, leading to a reduction of productivity of crops and forage. Epichloë gansuensis, seed-borne endophytic fungi, establishes a mutualistic symbiotic relationship with Achnatherum inebrians and confers salt tolerance in the host plants. In this study, analysis of transcriptome and metabolome was used to explore the potential molecular mechanism underlying the salt-adaptation of A. inebrians roots mediated by E. gansuensis. We found that E. gansuensis played an important role in the gene expression of the host’s roots and regulated multiple pathways involved in amino acid metabolism, carbohydrate metabolism, TCA cycle, secondary metabolism, and lipid metabolism in the roots of A. inebrians. Importantly, E. gansuensis significantly induced the biological processes, including exocytosis, glycolytic process, fructose metabolic process, and potassium ion transport in roots of host plants at transcriptional levels, and altered the pathways, including inositol phosphate metabolism, galactose metabolism, starch, and sucrose metabolism at metabolite levels under NaCl stress. These findings provided insight into the molecular mechanism of salt resistance in roots of A. inebrians mediated by E. gansuensis and could drive progress in the cultivation of new salt-resistance breeds with endophytes.

Integrative analysis of transcriptome and metabolome reveal mechanism of tolerance to salt stress in oat (Avena sativa L.)

Soil salinity is among the crucial factors that impact on crop productivity, including oat (Avena sativa L.). Herein, we used two distinct oat cultivars with varied salt tolerance levels to unravel adaptive responses to salt stress by metabolomic and transcriptomic characterization. Metabolomic profiling revealed 201 metabolites, including saccharides, amino acids, organic acids, and secondary metabolites. The levels of most saccharides and amino acids were elevated in Baiyan 2 (BY2) as well as in Baiyan 5 (BY5) exposed to salt stress. In the tolerant cultivar BY2 exposed to 150 mM NaCl, concentrations of most of the metabolites increased significantly, with sucrose increased by 38.34-fold, Sophorose increased by 314.15-fold and Isomaltose 2 increased by 25.76-fold. In the sensitive cultivar BY5, the concentrations of most metabolites increased after the plant was exposed to 150 mM NaCl but decreased after the plant was exposed to 300 mM NaCl. Transcriptomic analysis revealed that gene expressions in BY5 were significantly affected under exposure to 300 mM NaCl (34040 genes up-regulated and 14757 genes down-regulated). Assessment of metabolic pathways as well as KEGG enrichment revealed that salt stress interferes with the biosynthesis of two oat cultivars, including capacity expenditure and sugar metabolism. Most of the BY2 genes enhanced energy consumption (for example, glycolysis) and biosynthesis (for instance, starch and sugar metabolism) under salt stress. In contrast, genes in BY5 were found to be down-regulated, leading to the inhibition of energy consumption and biosynthesis, which may also be attributed to salt sensitivity in BY5. In addition, the modified Na+/K+ transporter genes expression is associated with the predominant ionic responses in BY2, which leads low concentration of Na+ and high K+ when exposed to high salt situations. These findings suggest that the varied defensive capacities of these two oat cultivars in response to salt stress are due to their variations in energy-expenditure strategy, synthesis of energy substances and ion transport in roots. Our present study offers a crucial reference for oat cultivation under saline soil.

A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. III. Quantifying the Energy Costs of Ion Transport in Salt-Stressed Roots of Arabidopsis.

Salt stress defense mechanisms in plant roots, such as active Na+ efflux and storage, require energy in the form of ATP. Understanding the energy required for these transport mechanisms is an important step toward achieving an understanding of salt tolerance. However, accurate measurements of the fluxes required to estimate these energy costs are difficult to achieve by experimental means. As a result, the magnitude of the energy costs of ion transport in salt-stressed roots relative to the available energy is unclear, as are the relative contributions of different defense mechanisms to the total cost. We used mathematical modeling to address three key questions about the energy costs of ion transport in salt-stressed Arabidopsis roots: are the energy requirements calculated on the basis of flux data feasible; which transport steps are the main contributors to the total energy costs; and which transport processes could be altered to minimize the total energy costs? Using our biophysical model of ion and water transport we calculated the energy expended in the trans-plasma membrane and trans-tonoplast transport of Na+, K+, Cl−, and H+ in different regions of a salt-stressed model Arabidopsis root. Our calculated energy costs exceeded experimental estimates of the energy supplied by root respiration for high external NaCl concentrations. We found that Na+ exclusion from, and Cl− uptake into, the outer root were the major contributors to the total energy expended. Reducing the leakage of Na+ and the active uptake of Cl− across outer root plasma membranes would lower energy costs while enhancing exclusion of these ions. The high energy cost of ion transport in roots demonstrates that the energetic consequences of altering ion transport processes should be considered when attempting to improve salt tolerance.

Multi-Element Bioimaging of Arabidopsis thaliana Roots.

Better understanding of root function is central for the development of plants with more efficient nutrient uptake and translocation. We here present a method for multielement bioimaging at the cellular level in roots of the genetic model system Arabidopsis (Arabidopsis thaliana). Using conventional protocols for microscopy, we observed that diffusible ions such as potassium and sodium were lost during sample dehydration. Thus, we developed a protocol that preserves ions in their native, cellular environment. Briefly, fresh roots are encapsulated in paraffin, cryo-sectioned, and freeze dried. Samples are finally analyzed by laser ablation-inductively coupled plasma-mass spectrometry, utilizing a specially designed internal standard procedure. The method can be further developed to maintain the native composition of proteins, enzymes, RNA, and DNA, making it attractive in combination with other omics techniques. To demonstrate the potential of the method, we analyzed a mutant of Arabidopsis unable to synthesize the metal chelator nicotianamine. The mutant accumulated substantially more zinc and manganese than the wild type in the tissues surrounding the vascular cylinder. For iron, the images looked completely different, with iron bound mainly in the epidermis of the wild-type plants but confined to the cortical cell walls of the mutant. The method offers the power of inductively coupled plasma-mass spectrometry to be fully employed, thereby providing a basis for detailed studies of ion transport in roots. Being applicable to Arabidopsis, the molecular and genetic approaches available in this system can now be fully exploited in order to gain a better mechanistic understanding of these processes.

Salt-imposed restrictions on the uptake of macroelements by roots of Arabidopsis thaliana

The aim of this investigation was to identify the growth limiting factors in Arabidopsis thaliana subjected to a mild salt stress. Two natural accessions (Col and N1438) were compared. In spite of their morphological and developmental similarity, they have been previously shown to differ in the response of their superoxide dismutase genes to salt stress (Physiol Plant 132:293–305, 2008). Thirty-day-old seedlings were grown for 15 days using a split-root configuration in which the root system was divided into two equal parts: the first was immersed in a complete nutrient solution with 50 mM NaCl added, while the second part was immersed in either complete or incomplete K-, Ca-, or N-free medium. Using this approach, we demonstrated that K+ and Ca2+ uptake was impaired in the roots subjected to NaCl. There was no indication of the salt-induced inhibition of N uptake. If K+ or Ca2+ were available from salt-free medium, plants were able to grow at normal rate and accumulate large amounts of Na+ in the shoots. These results indicate that the sensitivity of Arabidopsis growth to mild salinity was probably due to an inhibition of K+ or Ca2+ root transport by salt rather than due to salt accumulation in shoots. Furthermore, the salt sensitivity of ion transport in roots seemed to depend on the genotype, since K+ was limiting for Col growth, in contrast to N1438, the growth of which was limited by Ca2+.

Ion transport in roots: measurement of fluxes using ion-selective microelectrodes to characterize transporter function.

The transport of mineral ions into and out of tissues and cells is central to the life of plants. Ion transport and the plasma membrane transporters themselves have been studied using a variety of techniques. In the last 15 years, measurement of specific ion fluxes has contributed to the characterization of transport systems. Progress in molecular genetics is allowing gene identification and controlled expression of transporter molecules. However the molecular expression of transporter gene products must be characterized at the functional level. The ion-selective microelectrode technique to measure specific ion fluxes non-invasively is ideally suited to this purpose. This technique, its theory, its links with others and its application and prospects in plant science, are discussed. Ions studied include hydrogen, potassium, sodium, ammonium, calcium, chloride and nitrate. Applications discussed include: solute ion uptake by roots; gravitropism and other processes in the root cap, meristematic and elongation zones; Nod factor effect on root hairs; osmotic and salt stresses; oscillations; the effects of light and temperature. Studies have included intact roots, leaf mesophyll and other tissues, protoplasts and bacterial biofilms. A multi-ion capability of the technique will greatly assist functional genomics, particularly when coupled with imaging techniques, patch clamping and the use of suitable mutants.

Membrane ATPase Activity of Barley Roots and Potassium Uptake by Isolated Stele and Cortex

AbstractUptake of potassium ions by isolated stelar tissues of barley from 0.5 and 10 mM K+ was respectively 13 and 3.6% of that of the cortical tissues. 0.1 mM H2PO4, LO mM ATP and 10 mM Ca(NO3)2 did not increase the potassium uptake of either stele or cortex during 5 h of uptake period.A time‐course incubation for histological demonstration of the ATPase activity of the plasmalemma and tonoplast of the matured sections of the roots demonstrated a greater activity for the cortical than the stelar tissue. In the stelar parenchyma cells, the plasma lemma showed a higher activity than the tonoplast. These results, which support the “leakiness hypothesis” of the stele, are discussed in relation to the proposed mechanisms of radial ion transport in roots.

The Transport of Potassium to the Xylem Exudate of Ryegrass

The membrane potentials of ryegrass root cells (Evo) were found to be linearly related to the logarithm of the external KC1 concentration ([KC1]0), over the range 0-1 to 20-0 mM. Exuda tion was studied over the same concentration range. The concentration of potassium in the exudate did not vary significantly with [KC1]0 but the rates of movement of water and potassium to the exudate (/h2o and /K respectively) and the electrical potential and electro chemical potential for potassium in the exudate (Exo and ApLx0iK respectively) all tended to de crease as [KC1]0 increased. There was a very highly significant correlation between/K and /Hi0. By rapidly increasing [KC1]0 and following the depolarization, two components of Exo were observed. The first of these was instantaneous and was attributed to Evo of the epidermal cells. The second component, a gradual repolarisation which commenced about 9 min later, was attributed to Evo of the stelar cells. With an additional contribution from electro-osmosis, these two components quantitatively account for Exo. The implications of these data for the mechanism of radial ion transport in roots are discussed and it is concluded that the stelar cells are not exclusively specialized for transport ing potassium into the xylem vessels. INTRODUCTION In the preceding paper (Dunlop, 1973) it was reported that data on the transport properties of cells of ryegrass roots did not support the stelar pump proposed by Lauchli, Spurr, and Epstein (1971) and Pitman (1972). They were consistent with results for maize (Dunlop and Bowling, 1971a) and sunflower (Bowling, 1972) and indicated that there was no specialized mechanism for the radial transport of potassium operating exclusively in the stelar cells. Using their data for membrane potentials, Evo, and potassium concentrations in the xylem exudate, [K]x, of maize roots, Dunlop and Bowling (1971c) were able to calculate values for the electrical potential, Exo, and the electrochemical potentials for potassium, A/ZxoK, and chloride, A/ZxoC1, in the xylem exudate which agreed well with the measured values. They were also able to explain quantitatively the time course of the depolarization of Ex0 caused by an increase in the external KC1 concentration [KC1]0. This supported their model of radial ion transport, which is based upon the uniformity of the ion transport properties of the cells involved. The application of this approach to ryegrass roots is reported in this paper. 5160.1 B This content downloaded from 157.55.39.95 on Sat, 11 Jun 2016 06:38:31 UTC All use subject to http://about.jstor.org/terms 2 Dunlop—Transport of Potassium to the Xylem Exudate of Ryegrass. II MATERIALS AND METHODS Ryegrass seeds (Lolium multiflorum Lam. var. 'Grasslands Paroa') were germinated in distilled water, grown for 4 d in 0-1 mM CaCl2, and then for a further 24 h on the experimental solution (KC1 plus 0-1 mM CaCl2) before being used. Illumination of 4300 lx was provided by fluorescent tubes for 16 h each day. The apical 4-0 cm of roots were excised and sealed into 5 p Microcap microcapillaries with a lanolin-paraffin wax mixture. The Microcaps were adjusted in holders, which contained five roots each, so that the roots were almost completely immersed in the experimental solution. There were 450 ml of solution for each holder and this was maintained at 25 °C and continuously aerated. After 24 h Exo was measured with 3 M KCl-agar micro salt bridges connected to a Keithly model 602 Electrometer by Ag/AgCl wires and coaxial cable. The volume of the exudate was determined by measuring the length of microcapillary it occupied and then diluted to 1-0 ml for potassium analysis by flame photometry. There were between 10 and 20 replicates for each concentration. The membrane potentials of cells between 7 and 15 mm from the root apex were determined with microcapillary electrodes as described in the preceding paper (Dunlop, 1973).

Gramicidin-D-stimulated influx of monovalent cations into plant roots

Gramicidin D and nigericin were found to stimulate K(+) influx into oat roots. Valinomycin and nonactin had little effect on K(+) influx. The region of the root most sensitive to gramicidin was the elongation zone. Monocot roots were more sensitive to gramicidin than dicot roots. At 0.2 mM KCl, gramicidin stimulated K(+) influx by 4- to 8fold over a 30-min absorption period. Although a gramicidin response is detectable within one minute, maximum stimulation occurred after a slight (approximately 2-min) lag period. The gramicidin effect was much greater at 0.2 mM KCl than at 20 mM KCl. Respiratory inhibitors reduced the gramicidin-stimulated K(+) influx by 50-80%. The results are discussed in terms of possible mechanisms of action of the various ionophores on ion transport in roots.

Correlation between ion fluxes and ion-stimulated adenosine triphosphatase activity of plant roots.

The energy-dependent influx of Rb(+) into excised roots of corn, wheat, and barley has been determined and compared to the Rb(+)-stimulated ATPase activity of membrane fractions obtained from root homogenates of these species. The external Rb(+) concentrations studied were in the range of 1 to 50 mm. The ratio of Rb(+) influx/Rb(+)-stimulated ATPase was approximately 0.85 and was nearly constant for all the species and Rb(+) concentrations studied. The correlation coefficient for Rb(+) influx versus Rb(+)-activated ATPase was 0.94. The results support the concept that ATP is the energy source for ion transport in roots and that an ATPase participates in the energy transduction process involved in energy-dependent ion transport.

Speculation on the Mechanism of Ion Transport in Roots Based Upon Indirect Evidence from Histochemical Studies

Peroxidase distribution in the roots of many plants is characterized by an all-or-none disposition of the enzyme in cells of tangentially adjacent layers of tissue. Concentric bands of cells in the zone of greatest ion-exchange capacity in onion roots exhibit an alternating pattern of peroxidase activity, that is, root cap (erratic, but predominately +); epidermis (- except for weak reaction in outer part); hypodermis (+); cortex (-); endodermis (+); pericycle (-); phloem (+); xylem (-). Intracellular localization of peroxidase traced from root apex to base shifts from the cytoplasm into the walls and outer membranes. Instances of intracellular polarization of peroxidase sites within root apices are of two types: (a) In some plants, Sorghum, for example, the horizontally polar extremes of epidermal cells show a strong peroxidase reaction leaving a negative strip through the middle of the cells. (b) In other plants, including Allium, the peroxidase reaction is typically one-sided. The asymmetry of enzyme activity occurs in epidermal cells at the root apex, the outer portion of each cell being weakly positive for peroxidase. In the case of onion the two tissues that display greatest peroxidase activity in the apical part of the root (i.e., hypodermis and endodermis) subsequently synthesize cell-wall lipids with similar Sudanophilic properties. The role of peroxidase and lipids in the root could relate to absorption processes in terms of the symplast concept of solute translocation.