The occurrence of vesicular-arbuscular mycorrhiza in agro-ecosystems

In a long-lasting field experiment at the Research Station Hanninghof at Dülmen (Westphalia), laid out in 1971, the inoculum potential (IP) of vesicular-arbuscular mycorrhizal (VAM) fungi of soil from winter barley in a continuous monoculture (CM) and in a four-years cereal crop rotation (CR: winter wheat, winter barley, winter rye, oats) was estimated from 1980–1982. The influence of green manure and increased mineral N-fertilization (100 and 200 kg ha^–1) on VAM-IP was also assessed. VAM-IP was high at both levels of N-application, although the higher N-fertilization resulted in a reduction of VAM-IP in winter barley — in CR as well as in CM. No influence of N-fertilization on VAM-IP was observed with oats and winter rye, which were only cultivated in CR. Green manure had a negative effect on IP in CM barley, but not in CR barley. This may be due to the annual applications of green manure in CM, while green manure in CR was only applied prior to the cultivation of oats and winter wheat, but not to winter barley. Except for the effects of green manure remarkable differences in VAM-IP were found neither in CR nor in CM of winter barley. This is also reflected in the yields of barley in CM and indicates the relative self-tolerance of winter barley.     

The occurrence of vesicular-arbuscular mycorrhiza in agro-ecosystems

In a long-lasting field experiment at the Research Station Hanninghof at Dülmen (Westphalia), laid out in 1971, the inoculum potential (IP) of vesicular-arbuscular mycorrhizal (VAM) fungi of soil from winter wheat in a continuous monoculture (CM) and in a four-years cereal crop rotation (CR, winter wheat, winter barley, winter rye, oats) was investigated from 1980 to 1982. The influence of green manure and two levels of introgen fertilization (100 and 200 kg ha⁻¹) on VAM-IP was also assessed. It turned out that VAM is frequent also in intensively cultivated soils, but varies this with the crop rotation system and form and intensity of fertilization. Compared to CR the VAM-IP was significantly reduced in CM. A cumulation of this effect, with a particularly strong decrease of VAM occurred when other factors unfavourable for the symbiosis coincided in CM. Among these factors were the use of non-mycorrhizalRaphanus sativus andBrassica napus for green manure together with a N-fertilization of 200 kg ha⁻¹. On the other hand green manure and the higher N-fertilization resulted in less disturbance of the VAM-IP in the CR. A correlation between VAM-IP in the soil and of wheat yields could neither be observed in the CR nor in the CM.     

Response of tomato to inoculation with vesicular-arbuscular mycorrhiza

Growth of Lycopersicum esculentum transplants inoculated with Glomus mosseae in low phosphorus soil was much better than that of the non-mycorrhizal transplants. Mycorrhizal tomatoes had a greater dry weight and higher percentage of survival. Number of nodes, lateral branches and leaves per plant were almost doubled in mycorrhizal transplants.     

Development of vesicular-arbuscular mycorrhiza in groundnut and other hosts

Summary A vesicular-arbuscular mycorrhizal fungus, identified asGlomus mosseae Gerdemann and Trappe, was found to occur in groundnut and some other hosts. In groundnut roots under experimental conditions, the fungus showed three phases of development-a lag phase of 3–4 weeks by the end of which formation of vesicles was noticed, a phase of gradual development upto 12 weeks, by which time an average of 6 vesicles per centimeter of root developed and a constant phase where there was no further increase of the fungus. Pigeon-pea, black gram, green gram, angular gourd, onion, maize, sorghum and pearl millet also formed mycorrhizae with this fungus, but tomato and egg-plant did not. The lag phase was longer and the average number of vesicles developed per unit root length was less in the non-leguminous hosts.     

Stimulation of vesicular-arbuscular mycorrhiza by fungicides or rhizosphere bacteria

Stimulation of vesicular-arbuscular (VA) mycorrhizal fungi may secure the early establishment of symbiosis and benefit the host plant at an earlier stage of development. The application of Bacillus mycoides resulted in particular in the acceleration of early VA mycorrhiza formation. An increase in vigour of the symbiosis could be measured later in terms of increased sporulation of the mycorrhizal fungi after shoot removal. Natural sporulation during later mycorrhizal development was affected by combination of bacteria and just one mycorrhizal isolate. The stimulation of mycorrhizal development was shown to be non-specific with regard to host plant and the isolate of the VAM fungus. However, the effect could not be achieved in all combinations of soil types and host plants. Application of the systemic fungicides triadimefon and pyrazophos promoted VAM formation. Combinations of fungicide and bacterial treatments were not synergistic.     

Pseudomonas aeruginosa-plant root interactions. Pathogenicity, biofilm formation, and root exudation

Pseudomonas aeruginosa is an opportunistic human pathogen capable of forming a biofilm under physiological conditions that contributes to its persistence despite long-term treatment with antibiotics. Here, we report that pathogenic P. aeruginosa strains PAO1 and PA14 are capable of infecting the roots of Arabidopsis and sweet basil (Ocimum basilicum), in vitro and in the soil, and are capable of causing plant mortality 7 d postinoculation. Before plant mortality, PAO1 and PA14 colonize the roots of Arabidopsis and sweet basil and form a biofilm as observed by scanning electron microscopy, phase contrast microscopy, and confocal scanning laser microscopy. Upon P. aeruginosa infection, sweet basil roots secrete rosmarinic acid (RA), a multifunctional caffeic acid ester that exhibits in vitro antibacterial activity against planktonic cells of both P. aeruginosa strains with a minimum inhibitory concentration of 3 microg mL(-1). However, in our studies RA did not attain minimum inhibitory concentration levels in sweet basil's root exudates before P. aeruginosa formed a biofilm that resisted the microbicidal effects of RA and ultimately caused plant mortality. We further demonstrated that P. aeruginosa biofilms were resistant to RA treatment under in vivo and in vitro conditions. In contrast, induction of RA secretion by sweet basil roots and exogenous supplementation of Arabidopsis root exudates with RA before infection conferred resistance to P. aeruginosa. Under the latter conditions, confocal scanning laser microscopy revealed large clusters of dead P. aeruginosa on the root surface of Arabidopsis and sweet basil, and biofilm formation was not observed. Studies with quorum-sensing mutants PAO210 (DeltarhlI), PAO214 (DeltalasI), and PAO216 (DeltalasI DeltarhlI) demonstrated that all of the strains were pathogenic to Arabidopsis, which does not naturally secrete RA as a root exudate. However, PAO214 was the only pathogenic strain toward sweet basil, and PAO214 biofilm appeared comparable with biofilms formed by wild-type strains of P. aeruginosa. Our results collectively suggest that upon root colonization, P. aeruginosa forms a biofilm that confers resistance against root-secreted antibiotics.     

Effects of vesicular-arbuscular mycorrhiza on Tagetes erecta and Zinnia elegans

 Seeds of Tagetes erecta and Zinnia elegans were planted in soil inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus etunicatum. This procedure produced positive effects on both Tagetes and Zinnia compared with the control: faster flowering and an increased number of flowers. Shoot height, shoot and root dry weights and percentage infected root length were also measured. The reactions of these plants to mycorrhizal infection were shown to be independent of changes in the phosphorus, potassium and sodium contents of the plants. Accepted: 1 August 1995     

Interrelationships between vesicular-arbuscular mycorrhiza and rhizosphere microflora in apple replant disease

Phytotoxic micromycetes appear to be responsible for the apple replant disease (ARD). This was suppressed by the inoculation of apple-tree seedlings with some species of vesicular-arbuscular mycorrhizal (VAM) fungi—Glomus fasciculatum andG. macrocarpum. After the inoculation, growth of apple-tree seedlings improved in dependence on the type of soil, on VAM fungus species and on the ARD appearance. After 12-month cultivation, plant biomass (height, shoot and root dry masses) was markedly increased by inoculation withG. fasciculatum. Similarly, the numbers of colony forming units per unit soil (CFU) of phytotoxic micromycetes and of diazotroph bacteria (associative dinitrogen-fixing bacteria) in the rhizosphere was affected; CFU of phytotoxic micromycetes decreased, whereas CFU of the genusAzospirillum was higher. These bacteria could also serve as antagonists against phytotoxic micromycetes. It is also suggested that the ratio of CFU of diazotroph bacteria to CFU of phytotoxic micromycetes can be used as an indicator of the degree of ARD. It may be assumed that the use of some VAM fungi can replace the chemcial treatment of the soil with ARD.     

Quantitative and qualitative effects of phosphorus on extracts and exudates of sudangrass roots in relation to vesicular-arbuscular mycorrhiza formation

A comparison was made of water-soluble root exudates and extracts of Sorghum vulgare Pers. grown under two levels of P nutrition. An increase in P nutrition significantly decreased the concentration of carbohydrates, carboxylic acids, and amino acids in exudates, and decreased the concentration of carboxylic acids in extracts. Higher P did not affect the relative proportions of specific carboxylic acids and had little effect on proportions of specific amino acids in both extracts and exudates. Phosphorus amendment resulted in an increase in the relative proportion of arabinose and a decrease in the proportion of fructose in exudates, but did not have a large effect on the proportion of individual sugars in extracts. The proportions of specific carbohydrates, carboxylic acids, and amino acids varied between exudates and extracts. Therefore, the quantity and composition of root extracts may not be a reliable predictor of the availability of substrate for symbiotic vesicular-arbuscular mycorrhizal fungi. Comparisons of the rate of leakage of compounds from roots with the growth rate of vesicular-arbuscular mycorrhizal fungi suggest that the fungus must either be capable of using a variety of organic substrates for growth, or be capable of inducing a much higher rate of movement of specific organic compounds across root cell membranes than occurs through passive exudation as measured in this study.     

Influence of vesicular-arbuscular mycorrhiza and straw mulch on growth of barley

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) and of mulching on growth of barley were investigated in a factorial experiment. Plants were grown in cylinders buried in a field in soil with moderate amounts of available phosphate. VAM infection, dry weight and P uptake were determined at harvest after 10 and 161/2 weeks growth.VAM infection was reduced in the upper soil layer by straw mulch, possibly through a reduction in temperature. By the second harvest VAM increased growth by 56% in the non-mulched plots through increased P uptake but VAM did not increase growth in the mulched plots. Mulch increased growth by 85% in the non-mycorrhizal plots, and 28% in the mycorrhizal plots.     

Genetic control of root exudation

The literature on genetics of root exudation and on genotypic differences in qualitative and quantitative composition of root exudates in crop and native plant species was critically assessed. Differences in exudation have been reported for genotypes that differ in tolerance to nutrient deficiencies, ion toxicities, and pathogen attack. The exudation profile of a limited number of genotypes (frequently only two genotypes with the contrasting response to the environmental stress) have been reported to date. Little is known about the variability in larger samples of the germplasm or about actual genetics behind differential qualitative and quantitative composition of root exudates. Changing the exudation profile of a given genotype may be achieved by manipulating the biosynthetic capacity and by increasing the capacity of the plasma membrane to transport the specific compound out into the rhizosphere. Overexpression of the bacterial citrate synthase gene in the cytoplasm of tobacco plants resulted in exudation of large quantities of citrate into the rhizosphere and partial alleviation of the aluminium (Al) toxicity stress. A similar strategy of transforming plants with citrate synthase gene is being tried as a way of improving plant capacity to extract phosphorus (P) from soils with notoriously low P availability.More research into the genetic basis of qualitative and quantitative differences in root exudation is warranted. Understanding the genetic control of root exudation, followed by manipulation of qualitative and quantitative composition of root exudates, will result in better adaptation of plants to environmental conditions and a greater yield of crops.     

Effects of vesicular-arbuscular mycorrhiza on the availability of iron phosphates to plants

Summary The effect of inoculation with a mycorrhizal fungus on the growth of subterranean clover and of ryegrass was measured using three sources of phosphorus with different solubilities. These were (in order of decreasing solubility): potassium dihydrogen phosphate, colloidal iron phosphate and crystalline iron phosphate. Mycorrhizal infection increased growth more for subterranean clover than for ryegrass for all sources of phosphorus. For both species the greatest benefit from mycorrhizal inoculation was obtained with the least soluble source of iron phosphate. It is suggested that the mycorrhizas were able to explore the soil more thoroughly and hence were able to locate and use the point sources of phosphorus in the insoluble iron phosphates.     

The occurrence of vesicular-arbuscular mycorrhiza in isoetid-type submerged aquatic macrophytes under naturally varying conditions

The percentage infection of vesicular-arbuscular (VA) mycorrhiza was investigated for Isoetid-type species. Not all species lacking root hairs were found to possess VA-mycorrhiza. A range of habitats including marl and acid lakes, terrestrial and submerged plants and variations with depth showed little variation of VA-infection.     

Effect of vesicular-arbuscular mycorrhiza on survival,penetration and development of root-knot nematode in tomato

Summary Effect of mycorrhizal colonisation byGlomus fasciculatum on survival, penetration and development of the root knot nematodeMeloidogyne incognita in tomato was studied. The number of giant cells formed in mycorrhizal plants was significantly low. Mycorrhizal roots did not prevent the penetration by nematode larvae. Root extract from the mycorrhizal plants brought about 50% mortality of the nematode larvae in four days time.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.