Search

Biological nitrogen fixation involves the reduction of atmospheric N2 to ammonia by the bacterial enzyme nitrogenase. In legume-rhizobium symbioses, the nitrogenase-producing bacteria (bacteroids) are contained in the infected cells of root nodules within which they are enclosed by a plant membrane to form a structure known as the symbiosome. The plant provides reduced carbon to the bacteroids in exchange for fixed nitrogen, which is exported to the rest of the plant. This exchange is controlled by plant-synthesised transport proteins on the symbiosome membranes. This review summarises our current understanding of these transport processes, focusing on ammonia and amino acid transport.

Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halphytic angiosperm Zostera muelleri. In wholecell preparations, both outward and inward rectifying currents that activate in a time- and voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K+. The channels responsible for the outward currents have a unit conductance of approximately 70 pS and are five times more permeable to K+ than to Na+. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies with a conductance of 6 pS. The reversal potentials of these channels indicate a significant permeability to K+. In addition, the plasma membrane contains a much larger K+ channel with a conductance of 300 pS. Single channel recordings also indicate the existence of two Cl- channels, with conductances of 20 and 80 pS with distinct substates. The membrane potential difference of perfused protoplasts showed rapid action potentials of up to 50 mV from the resting level. The frequency of these action potentials increased as the external osmolarity was decreased. The action potentials disappeared with the addition of Gd3+, an effect that is reversible upon washout.

An electrogenic pump, a slowly activating K+ inward rectifier and an intermittent, "spiky," K+ inward rectifier, have been identified in the plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high external concentrations of K+ of 100 mM, the pump can maintain the membrane potential difference (PD) down to -180mV, more negative than the electrochemical equilibrium potentials of the various ions in the system. The slowly activating K+ inward rectifier, apparent in about 23% of protoplasts, allows inward current flow when the membrane PD becomes more negative than the electrochemical equilibrium potential for K+ by about 50 mV. The current usually consists of two exponentially rising components, the time constant of one about 10 times greater than the other. The longer time constant is voltage dependent, while the smaller time constant shows little voltage dependence. The rectifier deactivates, on return of the PD to less negative levels, with a single exponential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, allows intermittent current, of considerable magnitude, through the plasmalemma at PDs usually more negative than about -140mV. Patch clamp experiments on detached outside-out patches show that a possibly multi-state K+ channel, with maximum conductance greater than 400 pS, may constitute this rectifier. The paper also considers the role of the pump and the K+ inward rectifiers in physiological processes in the cell.

This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons of Amaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+ greater than Na+ much greater than Cl- (1:0.16:0.03): whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of subconductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (Vm) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2 mM) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO3- greater than Cl- greater than K+ greater than Aspartate (2.04:1:0.18 to 0.09:0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clamp Vm at a preferred value in the face of events which would otherwise perturb Vm.

Tradescantia virginiana L. plants were cultivated under contrasting conditions of temperature, humidity, light quality and intensity, and nutrient status in order to investigate the effect of growth conditions on the water relations parameters of the leaf epidermal cells. Turgor pressure (P), volumetric elastic modulus (ɛ), half-time of water potential equilibration (T 1/2 ), hydraulic conductivity (L p ) were measured with the miniaturized pressure probe in single cells of the upper and lower epidermis of leaves. Turgor differed (range: 0.1 bar to 7.2 bar) between treatments with lowest values under warm and humid conditions and additional supply of fertilizer, and highest values under conditions of low air humidity and low nutrient supply. The volumetric elastic modulus changed by 2 orders of magnitude (range: 3.0 bar to 350 bar, 158 cells), but ɛ was only affected by the treatments, in as much as it was dependent on turgor. The turgor dependence of ɛ, measured on intact leaves of T. virginiana, was similar to that for cells of the isolated (peeled) lower epidermis, where ɛ as a function of turgor was linear over the whole range of turgors. This result has implications for the discussion of pressure/volume curves as measured by the pressure bomb where changes in "bulk leaf ɛ" are frequently discussed as "adaptations" to certain treatments. The measurements of the hydraulic conductivity indicate that this parameter varies between treatments (range of means: 2.4×10 -6 cm s -1 bar -1 to 13.4×10 -6 cm s -1 bar -1 ). There was a negative correlation for L p in cells of intact leaves as a function of turgor which was altered by the growing conditions. However, a correlation with turgor could not be found for cells from isolated epidermis or cells from a uniform population of plants. The large variation in L p from cell to cell observed in the present and in previous studies was accounted for in a study of 100 cells from a uniform population of plants by the propagation of measurement errors in calculating L p . The results suggest that in T. virginiana cellular water relations are changed mainly by the turgor dependence of ɛ.