Optimization of environmental and the other variables in the application of arbuscular mycorrhizal fungi as an ecotechnological tool for sustainable paddy cultivation: A critical review

Arbuscular Mycorrhizal Fungi (AMF) are effective natural alternatives to assist plants in improving crop productivity and immunity against pests and diseases. However, a comprehensive idea of the variables under which they show optimum activity, especially concerning particular soil, climate, geography, and crop characteristics, has yet to be adequately standardized. Since paddy is the staple food for half of the world's population, such standardization is highly significant globally. Research concerning determinants affecting AMF functioning in rice is limited. However, the identified variables include external variables such as abiotic, biotic, and anthropogenic factors and internal variables such as plant and AMF characteristics. Among the abiotic factors, edaphic factors like soil pH, phosphorus availability, and soil moisture significantly affect AMF functioning in rice. In addition, anthropogenic influences such as land use patterns, flooding, and fertilizer regimes also affect AMF communities in rice agroecosystems. The principal objective of the review was to analyse the existing literature on AMF concerning such variables generally and to assess the specific research requirements on variables affecting AMF in rice. The ultimate goal is to identify research gaps for applying AMF as a natural alternative in the sustainable agriculture of paddy with optimum AMF symbiosis enhancing rice productivity.

Heterologous expression of Aspen PTM3, a MADS box gene in cotton

The PTM3 gene of Aspen was ectopically expressed in cotton to explore the opportunity to introduce desirable agronomic traits with the potential to improve yield and modify the duration of the parent cotton variety. Sixty-seven transgenic cotton lines expressing Aspen PTM3 (MADS box) gene were developed. The transgenic cotton lines expressing PTM3 gene showed earliness of 4-15 days variations in flowering and maturity. The transgenic lines were confirmed by kanamycin leaf paint assay, GUS assay and PCR. Among 67 transgenic lines, the event-10 showed profuse branching, event-24 showed abnormal growth and the remaining events exhibited single erect phenotype. In addition, the event-24 produced no flower and this might be due to the positional effect of PTM3 gene integration. Southern blot analysis performed for event-10, 24 and 48 showed distinct single copy integrations of PTM3 gene cassette. GUS assay performed using various plant parts of event-10 showed constitutive expression of the transgene. In view of cotton breeding, among all the events, the event-10 was found to be phenotypically significant with earliness of 12 days in flowering and 15 days in maturity and yield enhancement of 27%. In addition, the event-10 showed no square dropping and allowed the plants to bear more number of bolls. Based on these results, event-10 was chosen to carry out the inheritance study of expressed characters in the progeny.

Metabolic inhibition activates a non-selective current through connexin hemichannels in isolated ventricular myocytes

Intracellular Na(+)accumulation and K(+)loss play important roles in the pathogenesis of arrhythmias and injury in the ischemic heart. We investigated the role of metabolically sensitive connexin hemichannels as a potential route for Na(+)influx and K(+)efflux during ischemia, using dye uptake and electrophysiological measurements to assay hemichannel activity in isolated rabbit ventricular myocytes. Consistent with the known size selectivity of connexin hemichannels,;50% of myocytes exposed to either low extracellular Ca(2+)(an established method for opening connexin hemichannels) or to metabolic inhibitors (a recently described method for opening hemichannels) accumulated fluorescent dyes with <1000 MW (propidium iodide and calcein), but excluded a larger dye with 1500-3000 MW (dextran-rhodamine). Using the whole cell patch clamp technique, we found that metabolic inhibitors activated a non-selective current permeant to both small and large cations, and blocked by La(3+), similar to the properties of connexin 43 when overexpressed in human embryonic kidney (HEK) cells. These findings indicate that isolated cardiac myocytes endogenously express metabolically-sensitive connexin hemichannels. If activated during ischemia, these hemichannels could contribute significantly to altered ionic fluxes promoting arrhythmias and myocardial injury.

Regulation of cloned ATP-sensitive K channels by phosphorylation, MgADP, and phosphatidylinositol bisphosphate (PIP(2)): a study of channel rundown and reactivation

Kir6.2 channels linked to the green fluorescent protein (GFP) (Kir6. 2-GFP) have been expressed alone or with the sulfonylurea receptor SUR1 in HEK293 cells to study the regulation of K(ATP) channels by adenine nucleotides, phosphatidylinositol bisphosphate (PIP(2)), and phosphorylation. Upon excision of inside-out patches into a Ca(2+)- and MgATP-free solution, the activity of Kir6.2-GFP+SUR1 channels spontaneously ran down, first quickly within a minute, and then more slowly over tens of minutes. In contrast, under the same conditions, the activity of Kir6.2-GFP alone exhibited only slow rundown. Thus, fast rundown is specific to Kir6.2-GFP+SUR1 and involves SUR1, while slow rundown is a property of both Kir6.2-GFP and Kir6.2-GFP+SUR1 channels and is due, at least in part, to Kir6.2 alone. Kir6. 2-GFP+SUR1 fast phase of rundown was of variable amplitude and led to increased ATP sensitivity. Excising patches into a solution containing MgADP prevented this phenomenon, suggesting that fast rundown involves loss of MgADP-dependent stimulation conferred by SUR1. With both Kir6.2-GFP and Kir6.2-GFP+SUR1, the slow phase of rundown led to further increase in ATP sensitivity. Ca(2+) accelerated this process, suggesting a role for PIP(2) hydrolysis mediated by a Ca(2+)-dependent phospholipase C. PIP(2) could reactivate channel activity after a brief exposure to Ca(2+), but not after prolonged exposure. However, in both cases, PIP(2) reversed the increase in ATP sensitivity, indicating that PIP(2) lowers the ATP sensitivity by increasing P(o) as well as by decreasing the channel affinity for ATP. With Kir6.2-GFP+SUR1, slow rundown also caused loss of MgADP stimulation and sulfonylurea inhibition, suggesting functional uncoupling of SUR1 from Kir6.2-GFP. Ca(2+) facilitated the loss of sensitivity to MgADP, and thus uncoupling of the two subunits. The nonselective protein kinase inhibitor H-7 and the selective PKC inhibitor peptide 19-36 evoked, within 5-15 min, increased ATP sensitivity and loss of reactivation by PIP(2) and MgADP. Phosphorylation of Kir6.2 may thus be required for the channel to remain PIP(2) responsive, while phosphorylation of Kir6.2 and/or SUR1 is required for functional coupling. In summary, short-term regulation of Kir6.2+SUR1 channels involves MgADP, while long-term regulation requires PIP(2) and phosphorylation.

Novel gating mechanism of polyamine block in the strong inward rectifier K channel Kir2.1

Inward rectifying K channels are essential for maintaining resting membrane potential and regulating excitability in many cell types. Previous studies have attributed the rectification properties of strong inward rectifiers such as Kir2.1 to voltage-dependent binding of intracellular polyamines or Mg to the pore (direct open channel block), thereby preventing outward passage of K ions. We have studied interactions between polyamines and the polyamine toxins philanthotoxin and argiotoxin on inward rectification in Kir2.1. We present evidence that high affinity polyamine block is not consistent with direct open channel block, but instead involves polyamines binding to another region of the channel (intrinsic gate) to form a blocking complex that occludes the pore. This interaction defines a novel mechanism of ion channel closure.

Connexin-43 hemichannels opened by metabolic inhibition

The cause of altered ionic homeostasis leading to cell death during ischemia and metabolic inhibition is unclear. Hemichannels, which are precursors to gap junctions, are nonselective ion channels that are permeable to molecules of less than Mr 1000. We show that hemichannels open upon exposure to calcium-free solutions when they are either heterologously overexpressed in HEK293 cells or endogenously expressed in cardiac ventricular myocytes. In the presence of normal extracellular calcium, hemichannels open during metabolic inhibition. During ischemia and other forms of metabolic inhibition, activation of relatively few hemichannels will seriously compromise the cell's ability to maintain ionic homeostasis, which is an essential step promoting cell death.

The sulphonylurea receptor SUR1 regulates ATP-sensitive mouse Kir6.2 K+ channels linked to the green fluorescent protein in human embryonic kidney cells (HEK 293)

1. Using a chimeric protein comprising the green fluorescent protein (GFP) linked to the C-terminus of the K+ channel protein mouse Kir6.2 (Kir6.2-C-GFP), the interactions between the sulphonylurea receptor SUR1 and Kir6.2 were investigated in transfected human embryonic kidney cells (HEK 293) by combined imaging and patch clamp techniques. 2. HEK 293 cells transfected with mouse Kir6.2-C-GFP and wild-type Kir6.2 exhibited functional K+ channels independently of SUR1. These channels were inhibited by ATP (IC50 = 150 microM), but were not responsive to stimulation by ADP or inhibition by sulphonylureas. Typically 15 +/- 7 active channels were found in an excised patch. 3. The distribution of Kir6.2-C-GFP protein was investigated by imaging of GFP fluorescence. There was a lamellar pattern of fluorescence labelling inside the cytoplasm (presumably associated with the endoplasmic reticulum and the Golgi apparatus) and intense punctate labelling near the cell membrane, but little fluorescence was associated with the plasma membrane. 4. In contrast, cells co-transfected with Kir6.2-C-GFP and SUR1 exhibited intense uniform plasma membrane labelling, and the lamellar and punctate labelling seen without SUR1 was no longer prominent. 5. In cells co-transfected with Kir6.2-C-GFP and SUR1, strong membrane labelling was associated with very high channel activity, with 484 +/- 311 active channels per excised patch. These K+ channels were sensitive to inhibition by ATP (IC50 = 17 microM), stimulated by ADP and inhibited by sulphonylureas. 6. We conclude that co-expression of SUR1 and Kir6.2 generates channels with the properties of native KATP channels. In addition, SUR1 promotes uniform insertion of Kir6.2-C-GFP into the plasma membrane and a 35-fold increase in channel activity, suggesting that SUR1 facilitates protein trafficking of Kir6.2 into the plasma membrane.

Mechanosensitivity of the cardiac muscarinic potassium channel. A novel property conferred by Kir3.4 subunit

Muscarinic potassium channels are heterotetramers of Kir3.1 and other Kir3 channel subunits and play major roles in regulating membrane excitability in cardiac atrial, neuronal, and neuroendocrine tissues. We report here that rabbit atrial muscarinic potassium channels are rapidly and reversibly inhibited by membrane stretch, possibly serving as a mechanoelectrical feedback pathway. To probe the molecular basis for this phenomenon, we heterologously expressed heteromeric Kir3.1/Kir3.4 channels in Xenopus oocytes and found that they possess similar mechanosensitivity in response to hypo-osmolar stress. This could be attributed in part, if not exclusively, to the Kir3.4 subunit, which reproduced the mechanosensitivity of the heteromeric channel when expressed as a homomeric channel in oocytes. Kir3.4 is the first stretch-inactivated potassium channel to be identified molecularly. Physiologically, this feature may be important in atrial volume-sensing and other responses to stretch.

Atomic force microscopy of arthropod gap junctions

Atomic force microscopy has been used to characterize gap junctions isolated from the hepatopancreas of Nephrops norvegicus. The major polypeptide of these gap junctions is ductin, a highly conserved 16- to 18-kDa protein. The hydrated gap junctions, imaged in phosphate-buffered saline, appeared as membrane plaques with a thickness of 14 nm, consistent with their being a pair of apposing membranes. The upper membrane was removed by force dissection using an increased imaging force. The thickness of the lower membrane was 6 nm, giving a separation or gap between the two membranes of 2 nm. High-resolution images show fine details of the force-dissected extracellular surfaces, as previously reported for vertebrate and heart gap junctions. In addition high-resolution AFM images show for the first time detailed substructure on the cytoplasmic face of hydrated gap junctions of either vertebrate or invertebrate. The plaques had particles on their exposed and force-dissected faces. These particles were packed in a hexagonal lattice (a = b = 8.9 nm on both faces) and had a diameter of approximately 6.5 nm, with a central, pore-like depression. Fourier maps calculated from the AFM data suggested that each particle was composed of six subunits. These images show a marked similarity to the widely accepted structure of the connexon channel of vertebrate gap junctions.

Inward rectification of the IRK1 channel expressed in Xenopus oocytes: effects of intracellular pH reveal an intrinsic gating mechanism

1. The effects of intracellular pH (pHi) were investigated in inside-out giant patches from Xenopus oocytes expressing the inward rectifier K+ channel IRK1. 2. After excising patches into Mg2(+)- and polyamine-free solution, a residual time-dependent inactivation of outward current during depolarizing voltage-clamp pulses persisted, reaching an apparent steady-state by 5 min. Raising pHi from 7.2 to 9.0 increased the inactivation rate of the outward current. 3. In the presence of intracellular Mg2+ or polyamines, however, pHi 9.0 either decreased or did not change the inactivation rate of outward current. 4. These results suggest that the inactivation of outward current remaining after > 5 min in Mg2(+)- and polyamine-free solution is not due to slow washout of these substances, but represents a third and probably intrinsic gating mechanism contributing to the inward rectifying property of IRK1. 5. The voltage dependence and kinetics of this gating mechanism were well described by a sequential two open- and one closed-state model in which the rate constants for transitions between the open states were voltage dependent, and those between the open and closed state were pH dependent. 6. In the absence of intracellular Mg2+ and polyamines, reduced pHi blocked inward and outward current through IRK1 channels in a voltage-independent manner without appreciably altering the kinetics. Half-maximal block occurred at pH 6.2-6.4 (Hill coefficient, 1.6). Block of IRK1 by intracellular protons may contribute to membrane depolarization in ischaemic tissue.

Heart gap junction preparations reveal hemiplaques by atomic force microscopy

Current structural models of gap junctions indicate two apposed plasma membranes with hexagonally packed hemichannels in each membrane aligning end to end. These channels connect the cytoplasms of contacting cells. Images of isolated rat heart gap junctions have been made with the atomic force microscope in aqueous media. We show that native cardiac gap junctions have a thickness of 25 +/- 0.6 nm. This decreases to 17 nm when they are treated with trypsin, which is known to remove some cytoplasmic components of connexin 43. Imaging shows subunits with a center to center spacing of approximately 9-10 nm and long range hexagonal packing, measurements in agreement with studies using freeze-fracture and negative-stain electron microscopy. In addition to gap junctions, we imaged structures that had all the characteristics of native gap junctions except their thickness was limited to 9-11 nm. They also show long range hexagonal packing and center to center spacing of 9-10 nm. These structures decrease in thickness, to 6-9 nm, when treated with trypsin. We have called these structures hemiplaques. They appear to be present endogenously in the preparation, as we have ruled out their being an artifact of imaging by AFM. However, it remains to be determined if they are a consequence of the procedure used in isolating gap junctions or a possible intermediary in gap junction formation.

Conserved cysteine residues in the shaker K+ channel are not linked by a disulfide bond

Many voltage-activated K+ channels contain two conserved cysteine residues in putative transmembrane segments S2 and S6. It has been proposed that these cysteines form an intrasubunit disulfide bond [Guy, H.R., & Conti, F. (1990) Trends Neurosci. 13, 201-206]. This proposal was tested using site-directed mutagenesis followed by electrophysiological and biochemical analysis of the Shaker B K+ channel. Each Shaker B subunit contains seven cysteine residues, including the conserved residues C286 and C462 and a less conserved cysteine, C245. Each cysteine in the Shaker B protein can be mutated individually without eliminating functional activity, indicating that the protein does not contain a disulfide bond that is essential for protein folding or the assembly of active channels. To determine whether there is a nonessential disulfide bond, Shaker B protein was subjected to limited proteolysis. Fragments were analyzed by electrophoresis under reducing and nonreducing conditions followed by immunoblotting. The results indicate that the two conserved residues C286 and C462 do not form a disulfide bond with each other or with C245. In addition, the subunits are not linked by disulfide bonds. In HEK293T cells, Shaker B protein is first made as an incompletely glycosylated precursor that is converted to the fully glycosylated mature protein. Glycosylation occurs at two positions in the S1-S2 loop.

Glycosylation of shaker potassium channel protein in insect cell culture and in Xenopus oocytes

We have studied the glycosylation of Shaker K+ channel protein made in two expression systems: an insect cell culture line and amphibian oocytes. In both systems, two potential sites for N-linked glycosylation were modified. The modified sites were located between the first and second putative transmembrane segments, S1 and S2. Although the same sites appeared to be glycosylated in both systems, the fraction of protein glycosylated and the size, structure, or composition of the oligosaccharide chains added were quite different. The results indicate that the S1-S2 loop is extracellular, consistent with a cytoplasmic location for the N-terminus and a transmembrane disposition for hydrophobic segment S1. We have also shown that glycosylation occurs in two stages in oocytes, generating an immature and a mature form of Shaker protein. However, glycosylation is not required either for the assembly of functional channels or for their transport to the cell surface.

Biological applications of atomic force microscopy

The newly developed atomic force microscope (AFM) provides a unique window to the microworld of cells, subcellular structures, and biomolecules. The AFM can image the three-dimensional structure of biological specimens in a physiological environment. This enables real-time biochemical and physiological processes to be monitored at a resolution similar to that obtained for the electron microscope. The process of image acquisition is such that the AFM can also measure forces at the molecular level. In addition, the AFM can interact with the sample, thereby manipulating the molecules in a defined manner--nanomanipulation! The AFM has been used to image living cells and the underlying cytoskeleton, chromatin and plasmids, ion channels, and a variety of membranes. Dynamic processes such as crystal growth and the polymerization of fibrinogen and physicochemical properties such as elasticity and viscosity in living cells have been studied. Nanomanipulations, including dissection of DNA, plasma membranes, and cells, and transfer of synthetic structures have been achieved. This review describes the operating principles, accomplishments, and the future promise of the AFM.

Aspects of gap junction structure and assembly

The development of ideas about gap junction structure is summarized, including some recent results obtained by use of atomic force microscopy. Particular attention is paid to novel aspects of the biosynthesis and assembly of connexons and to the formation of new junctions.

Atomic force microscopy and dissection of gap junctions

An atomic force microscope (AFM) was used to study the structure of isolated hepatic gap junctions in phosphate-buffered saline (PBS). The thickness of these gap junctions appears to be 14.4 nanometers, close to the dimensions reported by electron microscopy (EM). When an increasing force is applied to the microscope tip, the top membrane of the gap junction can be "dissected" away, leaving the extracellular domains of the bottom membrane exposed. When such "force dissection" is performed on samples both trypsinized and fixed with glutaraldehyde, the hexagonal array of gap junction hemichannels is revealed, with a center-to-center spacing of 9.1 nanometers.

Connexon integrity is maintained by non-covalent bonds: intramolecular disulfide bonds link the extracellular domains in rat connexin-43

Rat heart connexin-43 (RCx43) has been isolated using a modified procedure that is rapid and can be used with fresh or frozen hearts. When RCx43 is isolated in the presence of the alkylating reagent iodoacetamide, no intermolecular disulfide bonds are found. However, the alkylated RCx43 does have at least one intramolecular disulfide bond. By using site directed antibodies and proteolytic cleavage the location of the intramolecular disulfide bonding is shown between the two extracellular loops of RCx43.

Molecular cloning and characterization of a new member of the gap junction gene family, connexin-31

A new member of the connexin gene family has been identified and designated rat connexin-31 (Cx31) based on its predicted molecular mass of 30,960 daltons. Cx31 is 270 amino acids long and is coded for by a single copy gene. It is expressed as a 1.7-kilobase mRNA that is detected in placenta, Harderian gland, skin, and eye. Cx31 is highly conserved and can be detected in species as distantly related to rat as Xenopus laevis. It exhibits extensive sequence similarity to the previously identified connexins, 58, 50, and 40% amino acid identity to Cx26, Cx32, and Cx43, respectively. When conservation of predicted phosphorylation sites is used to adjust the alignment of Cx31 to other connexins, a unique alignment of three predicted protein kinase C phosphorylation sites near the carboxyl terminus of Cx31 with three sites at the carboxyl terminus of Cx43 is revealed.

The 43-kD polypeptide of heart gap junctions: immunolocalization, topology, and functional domains

Analysis by SDS-PAGE of gap junction fractions isolated from heart suggests that the junctions are comprised of a protein with an Mr 43,000. Antibodies against the electroeluted protein and a peptide representing the 20 amino terminal residues bind specifically on immunoblots to the 43-kD protein and to the major products arising from proteolysis during isolation. By immunocytochemistry, the protein is found in ventricle and atrium in patterns consistent with the known distribution of gap junctions. Both antibodies bind exclusively to gap junctions in fractions from heart examined by EM after gold labeling. Since only domains of the protein exposed at the cytoplasmic surface should be accessible to antibody, we conclude that the 43-kD protein is assembled in gap junctions with the amino terminus of the molecule exposed on the cytoplasmic side of the bilayer, that is, on the same side as the carboxy terminus as determined previously. By combining proteolysis experiments with data from immunoblotting, we can identify a third cytoplasmic region, a loop of some 4 kD between membrane protected domains. This loop carries an antibody binding site. The protein, if transmembrane, is therefore likely to cross the membrane four times. We have used the same antisera to ascertain if the 43-kD protein is involved in cell-cell communication. The antiserum against the amino terminus blocked dye coupling in 90% of cell pairs tested; the antiserum recognizing epitopes in the cytoplasmic loop and cytoplasmic tail blocked coupling in 75% of cell pairs tested. Preimmune serum and control antibodies (one against MIP and another binding to a cardiac G protein) had no or little effect on dye transfer. Our experimental evidence thus indicates that, in spite of the differences in amino acid sequence, the gap junction proteins in heart and liver share a general organizational plan and that there may be several domains (including the amino terminus) of the molecule that are involved in the control of junctional permeability.