Parasitic and mutualistic associations between a mycorrhizal fungus and soybean: The effect of phosphorus on host plant-endophyte interactions

Soybean [ Glycine max (L.) Merr. cv. Kent] plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe in pot cultures using an inert medium and a nutrient solution. Phosphorus was provided initially as 0, 25,50, 100 or 200 mg hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] per pot. Under the low (0 mg HAP) and high (100 and 200 mg HAP) P regimes, VAM plants showed 20, 25 and 38% growth retardation, respectively, relative to non-colonized controls. At 50 mg HAP, VAM plant growth was significantly enhanced (14%). Dry weight and P content of both VAM and control plants increased with increased P availability throughout the HAP gradient. Intraradical VAM fungal biomass increased linearly with increasing P availability. Extraradical VAM fungal biomass was smaller than the intraradical component of the fungus at the lowest and highest levels of P addition in the growth medium. The ratio of extra- to intraradical mycelium, a suggested index of VAM fungal effectiveness, was greatest for the 50 mg HAP treatment, coinciding with growth enhancement of the host plant. This enhanced growth of the host at an intermediate P level was apparently a result of increased P uptake by the endophyte.     

Resistance Responses of Potato to Vesicular-Arbuscular Mycorrhizal Fungi under Varying Abiotic Phosphorus Levels

In mycorrhizal symbioses, susceptibility of a host plant to infection by fungi is influenced by environmental factors, especially the availability of soil phosphorus. This study describes morphological and biochemical details of interactions between a vesicular-arbuscular mycorrhizal (VAM) fungus and potato (Solanum tuberosum L. cv Russet Burbank) plants, with a particular focus on the physiological basis for P-induced resistance of roots to infection. Root infection by the VAM fungus Glomus fasciculatum ([Thaxt. sensu Gerdemann] Gerdemann and Trappe) was extensive for plants grown with low abiotic P supply, and plant biomass accumulation was enhanced by the symbiosis. The capacity of excised roots from P-deficient plants to produce ethylene in the presence or absence of exogenous 1-amino cyclopropane-1-carboxylic acid (ACC) was markedly reduced by VAM infection. This apparent inhibition of ACC oxidase (ACC_ox) activity was localized to areas containing infected roots, as demonstrated in split-root studies. Furthermore, leachate from VAM roots contained a potent water-soluble inhibitor of ethylene generation from exogenous ACC by nonmycorrhizal (NM) roots. The leachate from VAM-infected roots had a higher concentration of phenolics, relative to that from NM roots. Moreover, the rates of ethylene formation and phenolic concentration in leachates from VAM roots were inversely correlated, suggesting that this inhibitor may be of a phenolic nature. The specific activity of extracellular peroxidase recovered in root leachates was not stimulated by VAM infection, although activity on a fresh weight basis was significantly enhanced, reflecting the fact that VAM roots had higher protein content than NM roots. Polyphenol oxidase activity of roots did not differ between NM and VAM roots. These results characterize the low resistance response of P-deficient plants to VAM infection. When plants were grown with higher abiotic P supply, the relative benefit of the VAM symbiosis to plant growth decreased and root infection was lower. The in vivo ACC_ox activity was also greater in roots of plants grown on high levels of P compared with those grown on low levels, although the influence of VAM infection was partially to counteract the nutritional effect of P on ACC_ox activity. Similar to ACC_ox activity, extracellular peroxidase activity of roots increased linearly with increasing abiotic P supply, thus indicating a greater potential for resistance to VAM infection. These findings suggest that VAM fungi may alter phenolic metabolism of roots so as to hinder ethylene production and the root's ability to invoke a defense response. Raising the abiotic P supply to plants at least partially restores the capacity of roots to produce ethylene and may, in this way, increase the root's resistance to VAM infection.     

Effect of the VAM fungus Glomus sp. on the growth and yield of soybean inoculated with Bradyrhizobium japonicum

The effect of two Bradyrhizobium japonicum strains (D344 and Urbana), on the frequency and intensity of infection by a VAM fungal Glomus sp. and the effect of VAM on biomass production by nodulating plants were tested in soybean growing in a soil containing low levels of accessible P and N. During the initial stage of vegetative growth, mycorrhiza frequency in roots inoculated with the two rhizobial strains did not differ. However, during flowering it was 178% higher in roots with the strain D344 than in the presence of the strain Ubrana. At final harvest (green pods) the VAM frequency did not differ in the presence of either strain. VAM positively affected biomass production, foliar concentrations of P, Zn and Cu, and number and dry matter yield of pods, but did not increase concentrations of total N and K. In nonmycorrhizal plants total nitrogenase activity (not nodule mass) and growth were higher with the rhizobial strain Urbana. The greatest nitrogenase activity, growth and yield occurred in the presence of the VAM fungus, and did not differ for plants with different strains of rhizobia.     

Defoliation effects on mycorrhizal colonization, nitrogen fixation and photosynthesis in the Glycine-Glomus-Rhizobium symbiosis

Soybean [ Glycine max (L.) Merr. cv. Wells] plants grown in a greenhouse were inoculated with Rhizobium japonicum strain 61A118 and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. & Trappe. Plants were defoliated (26, 48 and 66%) throughout the growth period and evaluated for VAM colonization, N₂, fixation and photosynthesis at harvest (six weeks). Photosynthate stress as a result of defoliation affected nodulation and nodule activity most severely. Colonization of the roots by the VAM fungus was little affected in comparison, and the intensity of colonization increased with increasing stress. The CO₂-exchange rate decreased less with defoliation than did leaf mass, and photosynthetic efficiency increased with the severity of defoliation. The increase in photosynthetic efficiency was significantly correlated with increases in leaf P (r = 0.91) and N (r = 0.97) concentrations. The results suggest that the VAM fungus should not be regarded as a simple P source and C sink in the tripartite legume association. Threeway source/sink relationships (VAM-P, Rhizobium-N, and host leaf-C) are discussed.     

Glycine-Glomus-Rhizobium Symbiosis : VI. Photosynthesis in Nodulated, Mycorrhizal, or N- and P-Fertilized Soybean Plants

Soybean (Glycine max [L.] Merr. cv Hobbit) plants were grown in a growth chamber for 56 days in a phosphorus- and nitrogen-deficient soil and were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd) Gerd. and Trappe and Rhizobium japonicum strain USDA 136, or by either organism alone, or by neither. Non-VAM plants received supplemental phosphorus and nonnodulated plants supplemental nitrogen to achieve the same rate of growth in all treatments. Plants of all four treatments had the same (P > 0.05) dry weights at harvest, but VAM plants had higher rates of CO₂ exchange (CER, P < 0.05) and lower leaf P concentrations (P < 0.01). Leaf nitrogen concentrations were lower in nodulated than in nitrogen-supplemented plants (P < 0.01) while starch concentrations were higher (P < 0.01). There was a significant negative relationship between nitrogen and starch (r = −0.989). Statistical evaluation of the data showed that some parameters (CER, leaf area and phosphorus content) were associated with phosphorus nutrition (or the presence of the VAM fungus), others (leaf fresh weight and root dry weight) with nitrogen nutrition (or the presence of Rhizobium), and some (leaf nitrogen and starch content) by both factors. The development of microsymbiont structures and nodule activity were significantly lower in the tripartite association than in plants colonized by one endophyte only. The findings suggest that endophyte effects go beyond those of simple nutrition and associated source-sink relationships.     

The Glycine-Glomus-Rhizobium Symbiosis : VII. Photosynthetic Nutrient-Use Efficiency in Nodulated, Mycorrhizal Soybeans

Four consecutive trifoliate leaves of 56-day-old symbiotic or nonsymbiotic soybean plants were evaluated individually for CO₂ exchange rates (CER), leaf area and dry weight, and leaf N, P, and starch concentrations. Plants had been inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae and Rhizobium japonicum, with either of the endophytes alone, or with neither at time of planting. Plants lacking one or both endophytes received N and/or P fertilizers to produce plants of equal total leaf dry weight in all four treatments. Photosynthetic P-use efficiency (CER per unit leaf P) was higher in the leaves of VAM plants than in P-fertilized plants regardless of the N source (N₂ fixation or combined N). Photosynthetic N-use efficiency was also higher in VAM than in non-VAM plants, but it was affected by the N source, with higher CER in the nodulated plants. The greatest differences in CER, starch accumulation and leaf area were found between the nonsymbiotic plants and those with both endophytes. Statistical evaluations of leaf parameters for treatment or nutrient concentration (N and P) effects between the tri-partite and the nonsymbiotic treatments showed significant changes in concentration of P, but not N, with decreasing leaf age. Both endophytes apparently enhance CO₂ fixation at N and/or P concentrations lower than those of the nonsymbiotic plants. The effects of the endophytes on CO₂ fixation were additive.     

Root Respiration and Growth in Plantago major as Affected by Vesicular-Arbuscular Mycorrhizal Infection

Effects of vesicular-arbuscular mycorrhizal (VAM) infection and P on root respiration and dry matter allocation were studied in Plantago major L. ssp. pleiosperma (Pilger). By applying P, the relative growth rate of non-VAM controls and plants colonized by Glomus fasciculatum (Thaxt. sensu Gerdemann) Gerdemann and Trappe was increased to a similar extent (55-67%). However, leaf area ratio was increased more and net assimilation rate per unit leaf area was increased less by VAM infection than by P addition. The lower net assimilation rate could be related to a 20 to 30% higher root respiration rate per unit leaf area of VAM plants. Root respiration per unit dry matter and specific net uptake rates of N and P were increased more by VAM infection than by P addition. Neither the contribution of the alternative respiratory path nor the relative growth rate could account for the differences in root respiration rate between VAM and non-VAM plants. It was estimated that increased fungal respiration (87%) and ion uptake rate (13%) contributed to the higher respiratory activity of VAM roots of P. major.     

Light effects in mycorrhizal soybeans

Soybean (Glycine max. L. Merr.) plants were grown in an experiment with a 3 × 3 factorial design using different levels of light (170, 350, and 700 μE·m⁻²·s⁻¹) and P as factors. Plants were grown in a greenhouse in pot cultures using a soil low in plant-available P under three P regimes: no additional P, P added as KH₂PO₄, or P uptake enhanced by colonization of the host plant with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe. Development of the VAM fungal endophyte and of plants under all three P regimes was depressed by limiting light. However, the growth response of VAM plants to increasing light relative to non-VAM plants in the absence of additional P increased while the response relative to non-VAM plants with additional P decreased slightly. The highly significant interaction between the factors (P < 0.001) of the experiment was due to differences in the magnitude and direction of simple effects of the factors. The implications of these differences in terms of source-sink relationships of the symbionts and the value of different non-VAM controls in interpreting VAM effects are discussed.     

Phosphorus effect on phosphatase activity in endomycorrhizal maize

Success of a mycorrhizal symbiosis is influenced by the availability of phosphorus (P) in the soil. Maize ( Zea mays L. cv. Great Lakes 586) plants were grown under five different levels of soil P, either in the presence or absence of formononetin or the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith. We detected physiological differences in mycorrhizal roots very early in the development of symbiosis, before the onset of nutrient‐dependent responses. Under low P levels, VAM roots accumulated a greater shoot dry weight (13%), root P concentration (15%) and protein concentration (30%) than non-VAM roots, although root growth was not statistically significantly different. At higher P levels, mycorrhizal roots weighed less than non-VAM roots (10%) without a concomitant host alteration of growth or root P concentration. Mycorrhizal colonization decreased as soil P increased. Formononetin-treatment enhanced colonization of the root by G. intraradices and partially overcame inhibition of VAM colonization by high soil P concentrations. This is the first report that formononetin improves root colonization under high levels of soil P. Acid phosphatase (ACP) and alkaline phosphatase (ALP) activities were closely related to the level of fungal colonization in corn roots. ACP activity in corn roots responded more to soil P availability than did ALP activity (38% more). These results suggest that ACP was involved in the increased uptake of P from the soil, while ALP may be linked to active phosphate assimilation or transport in mycorrhizal roots. Thus, soil P directly affected a number of enzymes essential in host-endophyte interplay, while formononetin enhanced fungal colonization.     

Growth of Arabidopsis seedlings on high fungal doses of Piriformospora indica has little effect on plant performance,stress, and defense gene expression in spite of elevated jasmonic acid and jasmonic acid-isoleucine levels in the roots

The endophytic fungus Piriformospora indica colonizes the roots of many plant species including Arabidopsis and promotes their performance, biomass, and seed production as well as resistance against biotic and abiotic stress. Imbalances in the symbiotic interaction such as uncontrolled fungal growth result in the loss of benefits for the plants and activation of defense responses against the microbe. We exposed Arabidopsis seedlings to a dense hyphal lawn of P. indica. The seedlings continue to grow, accumulate normal amounts of chlorophyll, and the photosynthetic parameters demonstrate that they perform well. In spite of high fungal doses around the roots, the fungal material inside the roots was not significantly higher when compared with roots that live in a beneficial symbiosis with P. indica. Fifteen defense- and stress-related genes including PR2, PR3, PAL2, and ERF1 are only moderately upregulated in the roots on the fungal lawn, and the seedlings did not accumulate H₂O₂/radical oxygen species. However, accumulation of anthocyanin in P. indica-exposed seedlings indicates stress symptoms. Furthermore, the jasmonic acid (JA) and jasmonic acid-isoleucine (JA-Ile) levels were increased in the roots, and consequently PDF1.2 and a newly characterized gene for a 2-oxoglurate and Fe²⁺-dependent oxygenase were upregulated more than 7-fold on the dense fungal lawn, in a JAR1- and EIN3-dependent manner. We conclude that growth of A. thaliana seedlings on high fungal doses of P. indica has little effect on the overall performance of the plants although elevated JA and JA-Ile levels in the roots induce a mild stress or defense response.     

Glycine-Glomus-Rhizobium Symbiosis: II. Antagonistic Effects between Mycorrhizal Colonization and Nodulation

Soybean (Glycine max [L.] Merr.) plants grown in pot cultures were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe and Rhizobium japonicum strain 61A118 at planting (G₁R₁) or at 20 days (G₂₀R₂₀), or with one of the endophytes after the other has colonized the host root (G₁R₂₀, G₂₀R₁). Nodulated (PR₁) and VAM (G₁N) dipartite associations, or nonsymbiotic plants (PN) using nutrient solutions with N, P, or N + P concentrations providing endophyte-equivalent nutrient inputs were used as controls. The delayed tripartite associations received the appropriate N, P, or N + P amendment while one or both endophytes were absent during the first 20 days of growth. Prior inoculation with one endophyte significantly inhibited development of the other. Root hexose sugar concentrations were negatively correlated with VAM colonization (r = −0.89), nodule activity (r = −0.91), and root P content (r = −0.93). Nodule (r = 0.97) and root (r = 0.96) P content correlated positively with VAM colonization. Nodule weight or VAM-fungal biomass were significantly greater in associations grown with only one endophyte. Dry weights of the PN, G₁N, PR₁, and G₂₀R₂₀ plants were significantly greater than those of tripartite plants inoculated at planting with either or both endophytes. Interendophyte inhibition is attributed to competition for root carbohydrates, and this effect apparently also affects overall plant productivity. The objective of the study was to determine if the timing of endophyte introduction and establishment affected the development of the other symbiotic partners.     

Interactive effects of elevated CO2, phosphorus deficiency, and soil drought on nodulation and nitrogenase activity in Alnus hirsuta and Alnus maximowiczii

We examined the interactive effects of elevated CO₂, soil phosphorus (P) availability, and soil drought on nodulation, nitrogenase activity, and biomass allocation in Alnus hirsuta and Alnus maximowiczii. Potted seedlings were grown in either ambient or elevated CO₂ (36 Pa and 72 Pa CO₂), with different levels of P (7.7 and 0.77 mgP pot^?1 week^?1 for high-P and low-P, respectively) and water supply in a natural daylight phytotron. Measurements of nitrogenase activity by an acetylene reduction assay failed to reveal significant effects of the treatments in any species. In high-P, nodule biomass increased under elevated CO₂ and decreased under drought. In low-P, nodule biomass decreased substantially compared to high-P, but the effect of elevated CO₂ on nodule biomass was unclear. Soil drought increased the partitioning of biomass into nodules, especially in A. hirsuta. These results suggest that with high P availability, elevated CO₂ could promote N₂ fixation by increasing nodule biomass even under drought. On the other hand, if soil P is limiting, elevated CO₂ may not enhance N₂ fixation because of the suppression of growth.     

Salinity and flooding stress effects on mycorrhizal and non-mycorrhizal citrus rootstock seedlings

Seedlings of the rootstocks Pineapple sweet orange (SwO), Carrizo citrange (CC), and sour orange (SO) were grown in low phosphorus (P) sandy soil and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus,Glomus intraradices, or were non-mycorrhizal (NM) and fertilized with P. VAM and NM seedings of similar shoot size and adequate P-status were selected for study of salinity and flooding stress. One-third of each of the VAM and NM plants were given 150 mM NaCl for a period of 24 days. One-third of the plants were placed into plastic bags and flooded for 21 days while the remaining third were non-stressed controls. In general, neither stress treatment affected mycorrhizal colonization. Salinity stress reduced the hydraulic conductivity of roots, leaf water potential, stomatal conductance and net assimilation of CO₂ (ACO₂) of mycorrhizal and non-mycorrhizal seedlings to a similar extent. VAM plants of CC and SO accumulated more Cl in leaves than NM plants. Cl was higher in non-mycorrhizal roots of SwO and CC than in mycorrhizal roots. Flooding the root zone for 3 weeks did not produce visible symptoms in the shoot but did influence plant water relations and reduce ACO₂ of all 3 rootstocks. VAM and NM plants of each rootstock were affected similarly by flooding. Comparable reduction in nitrogen and P content of both mycorrhizal and non-mycorrhizal plants suggested that flooding stress was primarily affecting root rather than hyphal nutrient uptake. Florida Agricultural Experimental Station Journal Series No. 7773.     

Effects of vesicular-arbuscular mycorrhizal inoculation at different soil P availabilities on growth and nutrient uptake of in vitro propagated coffee (Coffea arabica L.) plants

 In a pot experiment, the growth and the nutrient status of in vitro propagated coffee (Coffea arabica L.) microcuttings were investigated for 5 months following vesicular-arbuscular mycorrhizal (VAM) inoculation with either Acaulospora melleae or Glomus clarum at four soil P availabilities. Control plants remained P-deficient even at the highest soil P availability while mycorrhizal plants were P-sufficient at all soil P availabilities. Growth of control plants was only improved at the highest soil P availability. In P-deficient soil, neither of the two VAM species improved plant growth. Plant growth increased by 50% following inoculation with either A. melleae or G. clarum when P availability went from deficient to low. No further plant growth improvement was induced by either VAM species at intermediate and high soil P levels. Nevertheless, growth of plants inoculated with G. clarum was still significantly greater than that of non-mycorrhizal plants at the highest soil P availability. Root colonization by G. clarum increased with increasing soil P availability while root colonization by A. mellea decreased with soil P level increasing above low P availability. Soil P availability also affected Zn nutrition through its influence on VAM symbiosis. With increasing soil P availability, foliar Zn status increased with G. clarum or decreased with A. mellea in parallel to root colonization by VAM. This study demonstrates the beneficial effects of VAM inoculation on in vitro propagated Arabica coffee microcuttings, as shown previously for seedlings. This study also demonstrates differences in tolerance to soil P availability between VAM species, most likely resulting from their differing abilities to enhance coffee foliar P status. Accepted: 14 November 1996     

The Glycine-Glomus-Bradyhizobium symbiosis. VIII. Phosphorususe efficiency of CO2 and N2 fixation in mycorrhizal soybean

Soybean [ Glycine max (L.) Merr. cv. Hobbit] plants nodulated by Bradyhizobium japonicum strain USDA 110 were grown in pot cultures in severely P- and N-deficient soil and either colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or fertilized with a high (HP) or low (LP) level of KH₂PO₄ (0.6 or 0.3 m M , respectively), After 7 weeks of growth, nodule and chloroplast activities (C₂H₂ reduction and CO₂ exchange rate) were determined. Photosynthetic P-use efficiency of CO₂ fixation was significantly higher in VAM than in HP plants, while that of nitrogenase activity was lower. The LP plants were intermediate in both respects. The ratio of nodule to chloroplast activity [mol C₂H₂ reduced (mol CO₂ fixed)⁻¹] was highest in HP and lowest in VAM plants. Root colonization by the VAM fungus significantly increased nodule number and dry weight and reduced nodule specific mass and activity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N concentrations than the HP plants. The results suggest nonnutritional. VAM-elicited and host-mediated effects on the symbiotic functions of the legume association.     

Nutrient uptake in mycorrhizal symbiosis

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora. A capacity for the external hyphae to take up and deliver nutrients to the plant has been demonstrated for the following nutrients and mycorrhizas; P (VAM, EM, ECM), NH₄ ⁺ (VAM, EM, ECM), NO₃ ^- (ECM), K (VAM, ECM), Ca (VAM, EM), SO₄ ^2- (VAM), Cu (VAM), Zn (VAM) and Fe (EM). In experimental chambers, the external hyphae of VAM can deliver up to 80% of plant P, 25% of plant N, 10% of plant K, 25% of plant Zn and 60% of plant Cu. Knowledge of the role of mycorrhiza in the uptake of nutrients other than P and N is limited because definitive studies are few, especially for the ECM. Although further quantification is required, it is feasible that the external hyphae may provide a significant delivery system for N, K, Cu and Zn in addition to P in many soils. Proposals that ECM and VAM fungi contribute substantially to the Mg, B and Fe nutrition of the host plant have not been substantiated. ECM and EM fungi produce ectoenzymes which provide host plants with the potential to access organic N and P forms that are normally unavailable to VAM fungi or to non mycorrhizal roots. The relative contribution of these nutrient sources requires quantification in the field. Further basic research, including the quantification of nutrient uptake and transport by fungal hyphae in soil and regulation at the fungal-plant interface, is essential to support the selection and utilization of mycorrhizal fungi on a commercial scale.     

The Glycine-Glomus-Bradyrhizobiun symbiosis. XI. Nodule gas exchange and efficiency as a function of soil and root water status in mycorrhizal soybean

Soybean [ Glycine max (L.) Merr. cv. Hobbit] plants were inoculated with a HUP− strain of Bradyrhizobium japonicum (Nitragin 61A118) and either colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol & Gerd.) Gerd. and Trappe or fertilized with KH₂PO₄ (nonVAM). They were grown for 50 days in a growth chamber and harvested over a 4-day drought period during which available soil water decreased to 0. Nodule P concentrations and P-use efficiency declined linearly with soil and root water content during the harvest period in both VAM and nonVAM plants. Nitrogenase activity, estimated from H₂ evolution and C₂H₂ reduction data, was also a linear function of declining nodule P concentrations and CO₂-exchange rates and showed simular patterns in both treatments. Hydrogen evolution and the relative efficiency of N₂ fixation, on the other hand, reacted differently to increasing drought in VAM and nonVAM plants. Differences in the responses of nodule activity in VAM and nonVAM plants to drought are interpreted in terms of demand for nodule P and carbohydrates and of the effects of dehydration on O₂ diffusion through nodule tissue.     

Effect of liming on spore germination,germ tube growth and root colonization by vesicular-arbuscular mycorrhizal fungi

Summary The effect of soil acidity on spore germination, germ tube growth and root colonization of vesicular-arbuscular mycorrhizal (VAM) fungi was examined using a Florida Ultisol. Soil samples were treated with 0, 4, 8 and 12 meq Ca/MgCO₃/100 g soil and each lime level received 0, 240, and 720 ppm P as superphosphate. Corn (Zea mays L.) was planted in the soil treatments, inoculated with eitherGlomus mosseae orGigaspora margarita spores and grown for 31 days. Acid soil inhibits mycorrhizal formation byG. mosseae through its strong fungistatic effect against the spores. The dolomitic lime increased mycorrhizal formation by both fungal species.G. margarita is much less sensitive to acidic conditions thanG. mosseae. Al ions are a very important component of the fungistatic property against the VAM symbiosis. VAM fungus adaptation may be important for plants growing on infertile acid soils if soil inoculation with these fungi is to contribute significantly to low-input technology for tropical agricultural systems.     

Influence of phosphorus and formononetin on isozyme expression in the Zea mays‐Glomus intraradices symbiosis

Maize (Zea mays L. cv. Great Lakes 586) plants were either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith, or grown in the presence of the isoflavone formononetin or were provided with both G. intraradices and formononetin. All plants were grown in soil containing one of five levels of inorganic P (between 8 and 110 µg g^?1 soil). By 3 weeks there were significant differences in a number of enzyme activities and in the pattern of isoenzymes in roots colonized by the VAM fungus or treated with formononetin. One NAD-malate dehydrogenase (MDH) isozyme was expressed only in mycorrhizal roots, whether treated or not with formononetin. Despite differences in the soil P level, the expression of this isozyme was not observed in non-mycorrhizal roots, indicating specific expression in the mycorrhizae. We suggest that MDH isozyme could serve as a specific, early indicator of the Zea-Glomus symbiosis. Differences in the esterase (EST) isozyme pattern were not detectable between VAM and non-VAM roots, suggesting that this enzyme system is not a good parameter for the evaluation of mycorrhizal colonization. As available P in the soil increased, total EST activity appeared to increase as well. Interestingly, total peroxidase (POX) activity increased along with P suggesting that as plant P nutrition improved, both cell wall ramification and the quantity of defense peroxidases increased as well. Total POX activity from mycorrhizal roots was inversely correlated with root colonization, indicating that there was suppression of POX activity by the host under low soil P. Most interestingly, formononetin further decreased POX activity regardless of the level of P or mycorrhizal status. This may suggest one mechanism by which formononetin enhances root VAM colonization. The presence of this isoflavone suppressed POX activity in mycorrhizal roots allowing a rapid penetration and spread of the fungus in the root cortex. The interplay between host root, soil P levels, secondary metabolites and endogenous host enzyme activities and a particular VAM fungus has a profound effect on the efficiency, duration and functioning of an endomycorrhizal symbiosis.     

Influence of vesicular arbuscular mycorrhizae and leaf age on net gas exchange of citrus leaves

The purpose of this study was to test the hypothesis that vesicular arbuscular mycorrhizal (VAM) fungi affect net assimilation of CO₂ (A) of different-aged citrus leaves independent of mineral nutrition effects of mycorrhizae. Citrus aurantium L., sour orange plants were grown for 6 months in a sandy soil low in phosphorus that was either infested with the VAM fungus, Glomus intraradices Schenck & Smith, or fertilized with additional phosphorus and left nonmycorrhizal (NM). Net CO₂ assimilation, stomatal conductance, water use efficiency, and mineral nutrient status for expanding, recently expanded, and mature leaves were evaluated as well as plant size and relative growth rate of leaves. Nutrient status and net gas exchange varied with leaf age. G. intraradices-inoculated plants had well-established colonization (79% of root length) and were comparable in relative growth rate and size at final harvest with NM plants. Leaf mineral concentrations were generally the same for VAM and NM plants except for nitrogen. Although leaf nitrogen was apparently sufficient for high rates of A, VAM plants did have higher nitrogen concentrations than NM at the time of gas exchange measurements. G. intraradices had no effect on A, stomatal conductance, or water use efficiency, irrespective of leaf age. These results show that well-established VAM colonization does not affect net gas exchange of citrus plants that are comparable in size, growth rate, and nutritional status with NM plants.