Chloride accumulation by mung bean root tips: a low affinity active transport system at the plasmalemma

Plant Salinity Stress: Many Unanswered Questions Remain

Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na+ and Cl- uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na+ and Cl- uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K+ in plant salinity stress are discussed.

Cellular and whole-plant chloride dynamics in barley: insights into chloride-nitrogen interactions and salinity responses

The first analysis of chloride fluxes and compartmentation in a non-excised plant system is presented, examining ten ecologically pertinent conditions. The short-lived radiotracer couple (38)Cl/(39)Cl was used as a Cl(-) tracer in intact barley ( Hordeum vulgare L. cv. Klondike) seedlings, which were cultured and investigated under four external [Cl(-)], from abundant (0.1 mM) to potentially toxic (100 mM). Chloride-nitrogen interactions were investigated by varying N source (NO(3)(-) or NH(4)(+)) and strength (0.1 or 10 mM), in order to examine, at the subcellular compartmentation level, the antagonism, previously documented at the influx level, between Cl(-) and NO(3)(-), and the potential role of Cl(-) as a counterion for NH(4)(+) under conditions in which cytosolic [NH(4)(+)] is excessive. Cytosolic [Cl(-)] increased with external [Cl(-)] from 6 mM to 360 mM. Cl(-) influx, fluxes to vacuole and shoot, and, in particular, efflux to the external medium, also increased along this gradient. Efflux reached 90% of influx at the highest external [Cl(-)]. Half-times of cytosolic Cl(-) exchange decreased between high-affinity and low-affinity influx conditions. The relationship between cytosolic [Cl(-)] and shoot flux indicated the presence of a saturable low-affinity transport system ("SLATS") responsible for xylem loading of Cl(-). N source strongly influenced Cl(-) flux to the vacuole, and moderately influenced Cl(-) influx and shoot flux, whereas efflux and half-time were insensitive to N source. Cytosolic pool sizes were not strongly or consistently influenced by N source, indicating the low potential for Cl(-) to act as a counterion to hyperaccumulating NH(4)(+). We discuss our results in relation to salinity responses in cereals.

Generalized kinetic analysis of ion-driven cotransport systems: II. Random ligand binding as a simple explanation for non-michaelian kinetics

Solute uptake in many cells is characterized by a series of additive Michaelis-Menten functions. Several explanations for these kinetics have been advanced: unstirred layers, transport across more than one membrane, effects of solute concentration on membrane potential, numerous carrier systems. Although each of these explanations might suffice for individual cases, none provides a comprehensive basis for interpretation of the kinetics. The most common mechanism of solute absorption involves cotransport of solute with a driver ion. A model is developed in which solute and driver ion bind randomly to a membrane-bound carrier which provides a single transmembrane pathway for transport. The kinetic properties of the model are explored with particular reference to its capacity to generate additive Michaelian functions for initial rate measurements of isotopic solute influx. In accord with previous analysis of ordered binding models (Sanders, D., Hansen, U.-P., Gradmann, D., Slayman, C.L. (1984) J. Membrane Biol. 77:123), the conventional assumption that transmembrane transit rate-limits transport has not been applied. Random binding carriers can exhibit single or multiple Michaelian kinetics in response to changing substrate concentration. These kinetics include high affinity/low velocity and low affinity/high velocity phases (so-called "dual isotherms") which are commonly observed in plant cells. Other combinations of the Michaelis parameters can result in cis-(substrate) inhibition. Despite the generality of the random binding scheme and the complexity of the underlying rate equation, a number of predictive and testable features emerge. If external driver ion concentration is saturating, single Michaelian functions always result and increasing internal substrate concentration causes uncompetitive inhibition of transport. Numerical analysis of the model in conditions thought to resemble those in many experiments demonstrates that small relative differences in a few key component rate constants of the carrier reaction cycle are instrumental in generation of dual isotherms. The random binding model makes the important prediction that the contributions of the two isotherms show opposing dependence on external concentration of driver ion as this approaches saturation. In the one case in which this dependence has been examined experimentally, the model provides a good description of the data. Charge translocation characteristics of the carrier can be determined from steady-state kinetic data on the basis of the response of substrate flux to modulation of internal driver ion concentration.(ABSTRACT TRUNCATED AT 400 WORDS)

An attempt to use isolated vacuoles to determine the distribution of sodium and potassium in cells of storage roots of red beet (Beta vulgaris L.)

Vacuoles isolated from red beet (Beta vulgaris L.) storage roots contain Na(+) and K(+) but their analysis does not give reliable information about the size of vacuolar pools of these ions in vivo. Analyses of isolated vacuoles indicated that between 53% and 90% of the Na(+) was located in the vacuole and that the vacuolar concentrations of Na(+) ranged between 4 and 45 mol m(-3). Calculated concentrations of K(+) in the vacuoles varied between 32 and 72 mol m(-3) but, in contrast to Na(+), only about 50% of the K(+) was located in the vacuole. Considerations of the likely cytoplasmic concentrations of Na(+) and K(+) suggest that if these results indicate conditions in vivo a large proportion of these ions must be located in the extracellular space, where they would exert considerable osmotic pressure. To test this, the effect of washing on cell turgor (measured directly with a pressure probe) and on loss of Na(+) and K(+) was determined. Washing caused an increase in turgor of 5 bar but losses of Na(+) and K(+) were less than predicted by the experiments with isolated vacuoles. It is concluded that beet vacuoles leak Na(+) and K(+) when isolated resulting in an underestimation of the size of vacuolar pools of these cations in vivo. Nonetheless, the turgor measurements provide evidence for the presence of osmotically active solute in the extracellular space. The possible contribution of extracellular Na(+) and K(+) to the observed turgor reduction is calculated and the physiological importance of the accumulation of extracellular solutes is discussed.

Sucrose uptake by sugar beet tap root tissue

Sucrose uptake by discs of mature sugar beet root tissue incubated in [(14)C]-sucrose exhibited nonsaturating kinetics over the concentration range of 1 to 500 millimolar. Uptake was inhibited by dinitrophenol, sodium cyanide, low O(2), and penetrating sulfhydryl inhibitors. ATPase inhibitors, sodium fluoride, and oligomycin reduced uptake by 20 and 40%, respectively. Uptake as asymmetrically labeled sucrose ([(14)C]glucose) occurred with approximately 80% retention of asymmetry, indicating a nonhydrolytic pathway. Uptake was against a concentration gradient and required metabolic energy.Glucose and fructose uptake exhibited typical saturation kinetics but rates of uptake were lower than that of sucrose, particularly at high concentration. Glucose strongly inhibited the uptake of sucrose and fructose but sucrose and fructose had little effect on the rate of glucose uptake. It is proposed that a major protion of the sucrose movement between its free space and vacuole occurs via a nonsaturating carrier at sites where the plasmalemma and tonoplast are appressed.

Regulation of cytoplasmic and vacuolar volumes by plant cells in suspension culture

Quantitative microscopical measurements have been made of the proportion of cell volume occupied by cytoplasm in a cell suspension culture derived from cotyledons of bush bean (cv. Contender). On a 7-day culture cycle, the content of cytoplasm varies from 25% at the time of transfer to 45% at the start of the phase of rapid cell division. If the culture is continued beyond 7 days, the vacuole volume reaches 90% of cell volume by day 12.Attempts to measure relative cytoplasmic volumes by compartmental analysis of nonelectrolyte efflux were unsuccessful. The proportion of cell volume occupied by cytoplasm is roughly correlated with protein content, but shows no correlation with cell size or with intracellular concentrations of K or Na. The most striking observation is that the growth of cytoplasmic volume for the culture as a whole appears to be constant throughout the culture cycle, despite changes in the rate of cell division, cell size, rate of increase in fresh or dry weight, amount of cytoplasm per cell, the cellular concentration or fluxes of Na or K, and the rate of vacuolation. It is suggested that cytoplasmic volume is under the control of its own regulatory mechanism, which operates to give a constant exponential increase in cytoplasmic volume independently of most other observed cellular properties.