Acquisition of rock phosphate by combined application of ammonium fertilizers and Bacillus amyloliquefaciens FZB42 in maize as affected by soil pH

AIMS

The use of plant growth-promoting micro-organisms (PGPMs) to improve plant-nutrient acquisition has a long history but reproducibility remains a challenge. Recent findings suggest an important role of suitable inoculant-fertilizer combinations for the expression of PGPM-effects, particularly with respect to nitrogen (N) supply. In face of the well-documented N form effects on rhizosphere pH, this study addressed the impact of ammonium-assisted PGPM-interactions on the acquisition of sparingly soluble calcium-phosphates as affected by soil pH.

METHODS AND RESULTS

The effects of stabilized ammonium fertilization combined with the PGPM inoculant Bacillus amyloliquefaciens FZB42 on the acquisition of rock phosphate in maize were examined on two soils (moderately acidic-pH 5·6 and alkaline-pH 7·8). On the two contrasting soils, FZB42 improved the P status and promoted plant growth by different mechanisms. On the acidic soil, a combination of ammonium-fertilization with FZB42 increased P-acquisition by Rock P solubilization via rhizosphere acidification but P-supply in the noninoculated control was already sufficient to meet the plant demands. By contrast, on the alkaline soil, plant growth-promotion was associated with FZB42-induced root growth stimulation.

CONCLUSION

The results suggest a significant impact of soil pH on performance and the mode of action of PGPM inoculants, to be considered for practical applications.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study advanced existing knowledge on PGPM-assisted P solubilization as affected by different soil properties. The results suggest perspectives for management options to be considered for efficient use of PGPMs in terms of selecting application strategies with compatible PGPM-fertilizer combinations, depending on soil pH conditions.

Role of AMT1;1 in NH4+ acquisition in Arabidopsis thaliana

AtAMT1;1 was the founding member of the family of AMT/Rh ammonium transporters and accounts for about one third of the total ammonium absorption in the roots of the model plant Arabidopsis. Recent evidence suggested that at least some AMT/Rh proteins are NH3 gas channels. In order to evaluate the transported form of ammonium in AtAMT1;1, the protein was functionally expressed in Xenopus oocytes. AtAMT1;1 elicited NH4+ and methylammonium (MeA+) inward currents that saturated in a voltage-dependent manner with a half maximal concentration of 2.7 +/- 1.6 microM for NH4+ and 5.0 +/- 0.7 microM for the transport analogue methylammonium. AtAMT1;1 was plasma membrane localized and expressed in the root cortex and epidermis, including root hairs. The AtAMT1;1-GFP fusion construct under control of its endogenous promoter revealed additional localization of the protein in the pericycle, in the leaf epidermis, and in mesophyll cells. The functional data and its localization suggest that AtAMT1;1 participates in concentrative NH4+ acquisition in roots, in long-distance transport to the shoots, and in re-uptake of apoplastic NH4+ that derives from photorespiration in shoots.

Ion transport versus gas conduction: function of AMT/Rh-type proteins

Although lipid membranes exhibit some permeability to the weak base NH3, organisms have developed specialized proteins that increase and regulate the NH3 fluxes across cellular membranes. In humans, the Rh glycoproteins, such as the erythrocyte-specific RhAG and the liver and kidney homologs RhBG and RhCG, are involved in the passage of NH3. Rh glycoproteins have distant relatives, called ammonium transporters (AMTs), in archae and bacteria. The crystal structures of AMTs show that the proteins are homo-trimers and that the center of each monomer forms a pore. AMT/Rh proteins have also been identified in plants. In contrast to the human Rh glycoproteins, these AMTs specifically transport NH4+ or co-transport NH3/H+. Hence, they can transport against NH3 gradients. The molecular determinants for the different transport mechanisms within proteins of the same family are currently unclear. The functional differences between AMT/Rh transporters are likely to be an evolutionary adaptation to different ammonium and nitrogen requirements in bacteria, plants and animals.

Secretory responses of intact glomus cells in thin slices of rat carotid body to hypoxia and tetraethylammonium

We have developed a thin-slice preparation of whole rat carotid body that allows us to perform patch-clamp recording of membrane ionic currents and to monitor catecholamine secretion by amperometry in single glomus cells under direct visual control. In normoxic conditions (P(O(2)) approximately 140 mmHg; 1 mmHg = 133 Pa), most glomus cells did not have measurable secretory activity, but exposure to hypoxia (P(O(2)) approximately 20 mmHg) elicited the appearance of a large number of spike-like exocytotic events. This neurosecretory response to hypoxia was fully reversible and required extracellular Ca(2+) influx. The average charge of single quantal events was 46 +/- 25 fC (n = 218), which yields an estimate of approximately 140,000 catecholamine molecules per vesicle. Addition of tetraethylammonium (TEA; 2-5 mM) to the extracellular solution induced in most (>95%) cells tested (n = 32) a secretory response similar to that elicited by low P(O(2)). Cells nonresponsive to hypoxia but activated by exposure to high external K(+) were also stimulated by TEA. A secretory response similar to the responses to hypoxia and TEA was also observed after treatment of the cells with iberiotoxin to block selectively Ca(2+)- and voltage-activated maxi-K(+) channels. Our data further show that membrane ion channels are critically involved in sensory transduction in the carotid body. We also show that in intact glomus cells inhibition of voltage-dependent K(+) channels can contribute to initiation of the secretory response to low P(O(2)).

Inward rectification in ClC-0 chloride channels caused by mutations in several protein regions

Several cloned ClC-type Cl- channels open and close in a voltage-dependent manner. The Torpedo electric organ Cl- channel, ClC-0, is the best studied member of this gene family. ClC-0 is gated by a fast and a slow gating mechanism of opposite voltage direction. Fast gating is dependent on voltage and on the external and internal Cl- concentration, and it has been proposed that the permeant anion serves as the gating charge in ClC-0 (Pusch, M., U. Ludewig, A. Rehfeldt, and T.J. Jentsch. 1995. Nature (Lond.). 373:527-531). The deactivation at negative voltages of the muscular ClC-1 channel is similar but not identical to ClC-0. Different from the extrinsic voltage dependence suggested for ClC-0, an intrinsic voltage sensor had been proposed to underlie the voltage dependence in ClC-1 (Fahlke, C., R. Rüdel, N. Mitrovic, M. Zhou, and A.L. George. 1995. Neuron. 15:463-472; Fahlke, C., A. Rosenbohm, N. Mitrovic, A.L. George, and R. Rüdel. 1996. Biophys. J. 71:695-706). The gating model for ClC-1 was partially based on the properties of a point-mutation found in recessice myotonia (D136G). Here we investigate the functional effects of mutating the corresponding residue in ClC-0 (D70). Both the corresponding charge neutralization (D70G) and a charge conserving mutation (D70E) led to an inwardly rectifying phenotype resembling that of ClC-1 (D136G). Several other mutations at very different positions in ClC-0 (K165R, H472K, S475T, E482D, T484S, T484Q), however, also led to a similar phenotype. In one of these mutants (T484S) the typical wild-type gating, characterized by a deactivation at negative voltages, can be partially restored by using external perchlorate (ClO4-) solutions. We conclude that gating in ClC-0 and ClC-1 is due to similar mechanisms. The negative charge at position 70 in ClC-0 does not specifically confer the voltage sensitivity in ClC-channels, and there is no need to postulate an intrinsic voltage sensor in ClC-channels.

Independent gating of single pores in CLC-0 chloride channels

The Cl- channel from the Torpedo electric organ, CLC-0, is the prototype of a large gene family of Cl- channels. At the single-channel level, CLC-0 shows a "double-barreled" behavior. Recently it was shown that CLC-0 is a dimer, and it was suggested that each subunit forms a single pore. The two protopores are gated individually by a fast voltage and anion-dependent gating mechanism. A slower common gating mechanism operates on both pores simultaneously. Previously, wild-type/mutant heteromeric channels had been constructed that display a large wild-type pore and small mutant pore. Here we use patch-clamp recording of single wild-type and mutant CLC-0 channels to investigate in detail the dependence of the gating of one protopore on the physically attached neighboring pore. No difference in rate constants of opening and closing of protopores could be found comparing homomeric wild-type and heteromeric wild-type/mutant channels. In addition, detailed kinetic analysis reveals that gating of single subunits is not correlated with the gating of the neighboring subunit. The results are consistent with the view that permeation and fast gating of individual pores are fully independent of the neighboring pore. Because the two subunits are associated in a common protein complex, opening and closing transitions of individual pores are probably due to only small conformational changes in each pore. In addition to the fast and slow gating mechanisms known previously for CLC-0, in the course of this study we occasionally observed an additional gating process that led to relatively long closures of single pores.

Analysis of a protein region involved in permeation and gating of the voltage-gated Torpedo chloride channel ClC-0

1. The chloride channel from the Torpedo electric organ, ClC-0, is controlled by two distinct ('fast' and 'slow') voltage-dependent gates. Here we investigate the effects of mutations in a region after putative transmembrane domain D12. A mutation in this region has previously been shown to change fast gating and permeation. 2. We used a combination of site-directed mutagenesis with two-electrode voltage-clamp and patch-clamp measurements. 3. Most conservative substitutions have minor effects, while more drastic mutations change kinetics and voltage dependence of fast gating, as well as ion selectivity and rectification. 4. While ClC-0 wild-type (WT) channels deactivate fully in two-electrode voltage clamp at negative voltages, channels do not close completely in patch-clamp experiments. Open probability is increased by intracellular chloride in a concentration- but not voltage-dependent manner. 5. In several mutants, including K519R, the minimal macroscopic open probability of fast gating is larger than in WT. Mutant channels fluctuate at negative potentials between open and closed conformations. Open probability is much more effectively increased by intracellular chloride than in WT. The observations support the idea that permeating ions inside the pore stabilize the open state. 6. Besides effects on permeation and gating of single protopores, some mutations affect 'slow' gating. In summary, the region after D12 participates in fast as well as in slow gating; mutations additionally influence permeation properties.

Temperature dependence of fast and slow gating relaxations of ClC-0 chloride channels

The chloride channel from the Torpedo electric organ, ClC-0, is the best studied member of a large gene-family (Jentsch, T.J. 1996. Curr. Opin. Neurobiol. 6:303-310.). We investigate the temperature dependence of both the voltage- and chloride-dependent fast gate and of the slow gate of the "double-barreled" ClC-0 expressed in Xenopus oocytes. Kinetics of the fast gate exhibit only a moderate temperature dependence with a Q10 of 2.2. Steady-state P(open) of the fast gate is relatively independent of temperature. The slow gate, in contrast, is highly temperature sensitive. Deactivation kinetics at positive voltages are associated with a Q10 of approximately 40. Steady-state open probability of the slow gate (P(open)slow(V)) can be described by a Boltzmann distribution with an apparent gating valence of approximately 2 and a variable "offset" at positive voltages. We note a positive correlation of this offset (i.e., the fraction of channels that are not closed by the slow gate) with the amount of expression. This offset is also highly temperature sensitive, being drastically decreased at high temperatures. Paradoxically, the maximum degree of activation of the slow gate also decreases at higher temperatures. The strong temperature dependence of the slow gate was also observed at the single channel level in inside-out patches. The results imply that within a Markovian-type description at least two open and two closed states are needed to describe slow gating. The strong temperature dependence of the slow gate explains the phenotype of several ClC-0 point-mutants described recently by Ludewig et al. (Ludewig, U., T.J. Jentsch, and M. Pusch. 1996. J. Physiol. (Lond.). In press). The large Q10 of slow gating kinetics points to a complex rearrangement. This, together with the correlation of the fraction of noninactivating channels with the amount of expression and the fact that the slow gate closes both protochannels simultaneously suggests that the slow gate is coupled to subunit interaction of the multimeric ClC-0 channel.

Two physically distinct pores in the dimeric ClC-0 chloride channel

The Torpedo chloride channel ClC-0 is the prototype of a large family of chloride channels that have roles in transepithelial transport and in regulating electrical excitability and cell volume. ClC-0 opens in bursts with two identical conductance levels of approximately 8pS. Hyperpolarization slowly increases the probability of bursts ('slow gating'), and depolarization increases channel opening within bursts ('fast gating'). Replacing serine 123 by threonine changes rectification, ion selectivity and gating, but retains the typical bursting behaviour with two identical independent albeit reduced, conductance states (approximately 1.5 pS). Coexpression with wild-type ClC-0, either as covalently linked concatamers or as independent proteins, leads to bursting channels with two different pores. Our experiments strongly suggest that conductance, ion selectivity and 'fast' gating are determined only by the single subunit forming a single pore, independent from the attached pore; in contrast, 'slow' gating is a function of both subunits. Thus ClC-0 is a homodimer with two largely independent pores.

Gating of the voltage-dependent chloride channel CIC-0 by the permeant anion

Chloride channels of the ClC family are important for the control of membrane excitability, cell volume regulation, and possibly transepithelial transport. Although lacking the typical voltage-sensor found in cation channels, gating of ClC channels is clearly voltage-dependent. For the prototype Torpedo channel ClC-0 (refs 11-15) we now show that channel opening is strongly facilitated by external chloride. Other less permeable anions can substitute for chloride with less efficiency. ClC-0 conductance shows an anomalous mole fraction behaviour with Cl-/NO3- mixtures, suggesting a multi-ion pore. Gating shows a similar anomalous behaviour, tightly linking permeation to gating. Eliminating a positive charge at the cytoplasmic end of domain D12 changes kinetics, concentration dependence and halide selectivity of gating, and alters pore properties such as ion selectivity, single-channel conductance and rectification. Taken together, our results strongly suggest that in these channels voltage-dependent gating is conferred by the permeating ion itself, acting as the gating charge.

A site accessible to extracellular TEA+ and K+ influences intracellular Mg2+ block of cloned potassium channels

The members of the RCK family of cloned voltage-dependent K+ channels are quite homologous in primary structure, but they are highly diverse in functional properties. RCK4 channels differ from RCK1 and RCK2 channels in inactivation and permeation properties, the sensitivity to external TEA, and to current modulation by external K+ ions. Here we show several other interesting differences: While RCK1 and RCK2 are blocked in a voltage and concentration dependent manner by internal Mg2+ ions, RCK4 is only weakly blocked at very high potentials. The single-channel current-voltage relations of RCK4 are rather linear while RCK2 exhibits an inwardly rectifying single-channel current in symmetrical K+ solutions. The deactivation of the channels, measured by tail current protocols, is faster in RCK4 by a factor of two compared with RCK2. In a search for the structural motif responsible for these differences, point mutants creating homology between RCK2 and RCK4 in the pore region were tested. The single-point mutant K533Y in the background of RCK4 conferred the properties of Mg2+ block, tail current kinetics, and inward ion permeation of RCK2 to RCK4. This mutant was previously shown to be responsible for the alterations in external TEA sensitivity and channel regulation by external K+ ions. Thus, this residue is expected to be located at the external side of the pore entrance. The data are consistent with the idea that the mutation alters the channel occupancy by K+ and thereby indirectly affects internal Mg2+ block and channel closing.

Extracellular K+ specifically modulates a rat brain K+ channel

Extracellular potassium concentration is actively maintained within narrow limits in all higher organisms. Slight variations in extracellular potassium levels can induce major alterations of essential physiological functions in excitable tissues. Here we describe that superfusion of cultured rat hippocampal neurones with potassium-free medium leads to a decrease of a specific outward potassium current, probably carried by RCK4-type channels (RCK4 are potassium channels found in rat brain). This is confirmed by heterologous expression of these channels in Xenopus oocytes. In this system, variations of extracellular potassium in the physiological concentration range induce significant differences in current amplitude. Moreover, the current is completely suppressed in the absence of extracellular potassium. The potassium dependence of macroscopic conductance in RCK4 channels was related by site-directed mutagenesis to that lysine residue in the extracellular loop between the transmembrane segments S5 and S6 of RCK4 protein that confers resistance to extracellular blockage by tetraethylammonium. It is shown that extracellular potassium affects the number of available RCK4 channels, but not the single-channel conductance, the mean open time, or the gating charge displacement upon depolarization.