Improved tolerance of Acacia nilotica to salt stress by Arbuscular mycorrhiza, Glomus fasciculatum may be partly related to elevated K/Na ratios in root and shoot tissues.

A pot experiment was conducted to examine the effect of arbuscular mycorrhizal fungus, Glomus fasciculatum, and salinity on the growth of Acacia nilotica. Plants were grown in soil under different salinity levels (1.2, 4.0, 6.5, and 9.5 dS m(-1)). In saline soil, mycorrhizal colonization was higher at 1.2, 4.0, and 6.5 dS m(-1) salinity levels in AM-inoculated plants, which decreased as salinity levels further increased (9.5 dS m(-1)). Mycorrhizal plants maintained greater root and shoot biomass at all salinity levels compared to nonmycorrhizal plants. AM-inoculated plants had higher P, Zn, and Cu concentrations than uninoculated plants. In mycorrhizal plants, nutrient concentrations decreased with the increasing levels of salinity, but were higher than those of the nonmycorrhizal plants. Mycorrhizal plants had greater Na concentration at low salinity levels (1.2, 4.0 dS m(-1)), which lowered as salinity levels increased (6.5, 9.5 dS m(-1)), whereas Na concentration increased in control plants. Mycorrhizal plants accumulated a higher concentration of K at all salinity levels. Unlike Na, the uptake of K increased in shoot tissues of mycorrhizal plants with the increasing levels of salinity. Our results indicate that mycorrhizal fungus alleviates deleterious effects of saline soils on plant growth that could be primarily related to improved P nutrition. The improved K/Na ratios in root and shoot tissues of mycorrhizal plants may help in protecting disruption of K-mediated enzymatic processes under salt stress conditions.

Mycorrhizal inoculant alleviates salt stress in Sesbania aegyptiaca and Sesbania grandiflora under field conditions: evidence for reduced sodium and improved magnesium uptake.

A field experiment was conducted to examine the effect of the arbuscular mycorrhizal fungus Glomus macrocarpum and salinity on growth of Sesbania aegyptiaca and S. grandiflora. In the salt-stressed soil, mycorrhizal root colonisation and sporulation was significantly higher in AM-inoculated than in uninoculated plants. Mycorrhizal seedlings had significantly higher root and shoot dry biomass production than non-mycorrhizal seedlings grown in saline soil. The content of chlorophyll was greater in the leaves of mycorrhiza-inoculated as compared to uninoculated seedlings. The number of nodules was significantly higher in mycorrhizal than non-mycorrhizal plants. Mycorrhizal seedling tissue had significantly increased concentrations of P, N and Mg but lower Na concentration than non-mycorrhizal seedlings. Under salinity stress conditions both Sesbania sp. showed a high degree of dependence on mycorrhizae, increasing with the age of the plants. The reduction in Na uptake together with a concomitant increase in P, N and Mg absorption and high chlorophyll content in mycorrhizal plants may be important salt-alleviating mechanisms for plants growing in saline soil.

Environmental effect on the reoccurrence of Alternaria alternata on Trianthema portulacastrum.

Trianthema portulacastrum is a noxious weed which spreads rapidly in the field. Pathogen Alternaria alternata was observed on this host for three years continuously i.e. 1997 to 1999. The incidence of the pathogen depended on rainfall and temperature.

Molecular aspects of pesticide degradation by microorganisms.

Microorganisms are able to degrade a large variety of compounds, including pesticides under laboratory conditions. However, methods have yet to be developed to decontaminate the environment from residues of pesticides. Pesticidal degradative genes in microbes have been found to be located on plasmids, transposons, and/or on chromosomes. Recent studies have provided clues to the evolution of degradative pathways and the organization of catabolic genes, thus making it much easier to develop genetically engineered microbes for the purpose of decontamination. Genetic manipulation offers a way of engineering microorganisms to deal with a pollutant, including pesticides that may be present in the contaminated sites. The simplest approach is to extend the degradative capabilities of existing metabolic pathways within an organism either by introducing additional enzymes from other organisms or by modifying the specificity of the catabolic genes already present. Continuous efforts are required in this direction, and at present several bacteria capable of degrading pesticides have been isolated from the natural environment. Catabolic genes responsible for the degradation of several xenobiotics, including pesticides, have been identified, isolated, and cloned into various other organisms such as Streptomyces, algae, fungi, etc. In addition, recombinant DNA studies have made it possible to develop DNA probes that are being used to identify microbes from diverse environmental communities with an unique ability to degrade pesticides.

Comparative kinetic studies on aflatoxin B1 binding to pulmonary and hepatic DNA of rat and hamster receiving the carcinogen intratracheally.

Several epidemiological studies have discussed the outcome of inhalation of airborne aflatoxins by humans. Metabolism of aflatoxin B1 (AFB1) by lung parenchyma leading to DNA binding is reported here. The tissue distribution pattern of [3H]AFB1 radioactivity revealed the lungs to be the second most important organ after the liver to retain a considerable amount of the radioactivity (66%). The lung indicated a selective activation of AFB1 as it showed only 7.7% binding of [3H]AFB1 to pulmonary DNA. Rats and hamsters were dosed with [3H]AFB1 (2 microCi containing 40 micrograms AFB1/100 g body wt.) intratracheally (i.t.) and sacrificed at different intervals after toxin treatment. Peak binding occurred at 0.5, 1, and 2 h in case of hamster lung, rat lung, and alveolar macrophages of both the species, respectively. At the end of 24 h, the relative AFB1-DNA binding (percentage of peak binding) in hamster lung was 72% while that in rat was 24%. The relative binding in rat lung alveolar macrophages (AMs) was generally higher than that of the hamster. AFB1 binding to hepatic DNA of both the species approached the peak at 1 h after the toxin administration i.t. Under these conditions, binding of AFB1 (or its metabolites translocated to liver) to hepatic DNA of both the species progressively diminished with time in contrast to lung, as revealed by the relative binding values at 12 h for rat and hamster lung, which were 48 and 67%, respectively, while for the rat and hamster liver they were 28 and 24%, respectively. Binding of i.t. administered [3H]AFB1 to rat liver DNA is only marginally higher than that observed with hamster liver, in contrast to the wide difference observed in animals receiving AFB1 intraperitoneally. These results highlight the persistence of AFB1 binding to pulmonary DNA, and the extent of translocated AFB1 binding to hepatic DNA presents an interesting difference from that observed when the toxin was administered through a gastrointestinal route. It is worth concluding that AMs unlike many other xenobiotics, possess specific mixed function oxidase activity to epoxidize AFB1.

A new species of Gigaspora from desert soils of Rajasthan, India.

A new species of the endogonaceous fungus Gigaspora, isolated from the Indian semi-arid region, is described. The fungus, named G. tuberculata, produces rusty-brown azygospores with septate subtending hypha. The azygospores bear warts all over the outer wall. The shape, size and general appearance of these spores resemble those of Scutellospora persica.

Epoxidation of aflatoxin B1 by Aspergillus flavus microsomes in vitro: interaction with DNA and formation of aflatoxin B1-glutathione conjugate.

Metabolism of aflatoxin B1 (AFB1) by subcellular preparations of Aspergillus flavus is least understood. The results reported here have demonstrated for the first time the epoxidation of AFB1 and subsequent conjugation with glutathione (GSH). Microsomes prepared from toxigenic mycelia catalysed [3H]AFB1 to calf thymus DNA to a greater extent (approximately 2-fold) as compared to that of non-toxigenic. The binding of [3H]AFB1 to exogenous and A. flavus nuclear DNA catalyzed by A. flavus microsomes was found to be comparable with that of mammalian extrahepatic tissue such as lung. Addition of phenobarbitone to the growing cultures resulted in 1.5-fold increase in [3H]AFB1-DNA binding mediated by microsomes prepared from either of the two strains. Tolnaftate, an inhibitor of aflatoxin synthesis enhanced the epoxidation rate in a dose-related manner. The binding of [3H]AFB1 to DNA catalyzed by A. flavus microsomes was significantly reduced (50% of control) upon addition of hamster liver cytosol, thereby substantiating the formation of the carcinogen adduct with DNA as reported in mammalian tissues. The metabolite formed by subcellular preparation of A. flavus was found to be AFB1-GSH having Rf value (6.5) similar to that obtained for mammalian liver preparations.

The distribution of vesicular-arbuscular mycorrhizal fungi in India.

Vesicular-arbuscular mycorrhizal fungi are widely distributed throughout the area studied including different altitudes ranging from sea level to 2500 ft above sea level. VAM fungi were recorded from 88% of the sites examined with Glomus fasciculatum and Glomus macrocarpum being the most commonly recorded. Mean species diversity was found to be maximum in the areas thickly vegetated and undisturbed.

Positive correlation exists between glutathione S-transferase activity and aflatoxin formation in Aspergillus flavus.

The presence of glutathione (GSH) S-transferase activity, using 1-chloro-2, 4-dinitrobenzene (CDNB) as a substrate, has been established in the cytosolic fraction of the toxigenic (aflatoxin producing) and nontoxigenic strains of Aspergillus flavus. Significant differences in the GSH S-transferase activity were observed between the toxigenic and non-toxigenic strains. A positive correlation has been demonstrated for the first time between aflatoxin formation and a biochemical parameter, namely GSH S-transferase activity. The evidence in support of A. flavus GSH S-transferase induction by endogenous aflatoxins is as follows: (i) the age-related production of aflatoxin follows the same pattern as the cytosolic GSH S-transferase activity profile; (ii) significantly higher enzyme activity was associated with mycelia of a toxigenic strain grown in medium supporting high aflatoxin production (sucrose-low-salts medium) while the enzyme activity was low in medium producing less aflatoxin (glucose-ammonium nitrate medium). The GSH S-transferase activity of the non-toxigenic strain was hardly affected by a change in the medium as it produces no aflatoxins; and (iii) the toxigenic strain demonstrated significantly higher apparent Vmax. with no change in Km as compared with the non-toxigenic strain. This indicates that the enzyme induction by endogenous aflatoxins is similar to the action of phenobarbitol and other inducing drugs (Kaplowitz et al., 1975).

Nitrogen metabolism in Aspergillus parasiticus NRRL 3240 and A. flavus NRRL 3537 in relation to aflatoxin production.

The relationship between nitrogen assimilation, metabolism and aflatoxin formation has been investigated in a toxigenic and a non-toxigenic strain of Aspergillus parasiticus. Ammonia from the medium is mainly assimilated via NADP-requiring glutamate dehydrogenase. During growth NAD-requiring glutamate dehydrogenase followed an inverse pattern of activity with respect to NADP glutamate dehydrogenase. Alpha-ketoglutarate, the product of NAD glutamate dehydrogenase, stimulated acetate incorporation into aflatoxins. Glutamine synthetase, ornithine transcarbamylase, both utilizing glutamate as substrate were assayed under different growth conditions. An important regulatory role for glutamine synthetase is suggested. The metabolic route of asparagine utilization was also investigated. Both the known pathways, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase are operative simultaneously.

Pyridine nucleotides and redox state regulation of aflatoxin biosynthesis in Aspergillus parasiticus NRRL 3240.

In vivo regulation of lipid and aflatoxin biosynthesis by pyridine nucleotides and their derived functions was studied in Aspergillus parasiticus NRRL 3240. Aflatoxins, total lipids and pyridine nucleotide content were estimated under different growth conditions. Aflatoxin formation was highest in cultures grown in sucrose-low salts medium followed by asparagine- and zinc-deficient media. The lipid content of the cultures followed an inverse pattern. The levels of oxidized nucleotides decreased with age under all culture conditions employed. Concentrations of NADPH peaked before the onset of aflatoxin biosynthesis. For each medium used, the estimated catabolite reduction charge was constant at all stages of growth whereas the anabolic reduction charge varied. A direct relationship between the level of extracellular ammonium ions and anabolic reduction charge was established. A high anabolic reduction charge was associated with increased lipid biosynthesis rather than aflatoxin biosynthesis.

Effect of aluminium and nickel on aflatoxin production by Aspergillus flavus.

Effect of nickel and aluminium was studied on aflatoxin and lipid production by two strains of Aspergillus flavus in a sucrose-asparagine-salts medium. Inclusion of aluminium in the medium established an inverse relationship between aflatoxin and lipid production. At lower concentrations aluminium stimulated aflatoxin production, whereas at higher concentrations it stimulated total lipid production. Nickel at higher concentrations resulted in an increase in total aflatoxin production. However, no definite correlation was observed between total aflatoxin and total lipid production when nickel was included in the medium.

Role of blastospores in protecting Aspergillus parasiticus NRRL 3240 from high levels of aflatoxins.

The role of blastospores in the protection of Aspergillus parasiticus from high levels of aflatoxins was studied. The strain protects itself from aflatoxicity by forming thick-walled blastospores. The formation of blastospores was not observed under conditions of reduced aflatoxin formation, e.g., under zinc and asparagine deficiencies. The germination of blastospores coincided with an increase in the specific activity of glutamate dehydrogenase (NADP) and a simultaneous decrease in the specific aflatoxin production.

Nigeran production in some Aspergillus and Penicillium species.

Thirteen additional members belonging to genera Aspergillus and Penicillium were found to be nigeran producers. Nigeran content was found to increase under carbon and nitrogen depletion and metal toxicity. Among the three metal ions--copper, iron and magnesium--copper was found to have the greatest effect in increasing nigeran content. Nigeran production varies from strain to strain and the amount is greatly influenced by environmental conditions.