The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil P for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive 32P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.

A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants

Publisher Summary This chapter elaborates the microbially mediated increases in plant-available phosphorus (P). The importance of microorganisms in soil nutrient cycling and their role in plant nutrition has been realized for a long time. Their active part in the decomposition and mineralization of organic matter and release of nutrients is crucial to sustaining the plant productivity. The concentration of total P in soils ranges from 0.02 to 0.5% and averages approximately 0.05%, the variation being largely because of differences in weathering intensity and parent material composition. The uptake of P from relatively insoluble sources can be affected by the type of plant growing in the soil. The effect of mycorrhizae on plant P uptake and the effect of soil P on mycorrhizae were among the first aspects of these symbioses studied. Under soil conditions, potential benefits of adding P-solubilizing (PS) organisms would depend on several factors, one of the most important being the activity of the PS microbial population already in the soil. In almost all cases, the major sources of PS isolates have been soils. The mechanism of action of PS microorganisms is also elaborated.

Microbially Mediated Increases in Plant-Available Phosphorus

summary
One of the most dramatic effects of infection by vesicular-arbuscular mycorrhizal fungi on the physiology of the host plant is an increase in phosphorus absorption. When phosphorus is limiting, the maximum extent to which mycorrhizal infection can improve plant performance is thus predicted to be a function of the phosphorus deficit of the plant, the difference between phosphorus demand and phosphorus supply. Phosphorus demand is defined as the rate of phosphorus absorption that would result in optimum performance of the plant as measured by growth rate, reproduction or fitness. The phosphorus supply is defined as the actual rate of phosphorus absorption under the prevailing conditions. Variation among plant taxa in morphological, physiological or phenological traits which affect either phosphorus demand or phosphorus supply (and thus phosphorus deficit) is predicted to lead to variation in potential response to mycorrhizal infection. The actual response to mycorrhizal infection is predicted to be a function of the increase in phosphorus uptake due to mycorrhizal infection and the phosphorus utilization efficiency of the plant. Demonstrated variability in responsiveness to mycorrhizal infection among plant taxa suggests that mycorrhizal fungi may play an important role in determining the structure of plant communities.

Mycorrhizal infection may alter the phosphorus deficit or phosphorus utilization efficiency independently from its direct effect on phosphorus uptake, making the prediction of response to mycorrhizal infection based on the traits of non-mycorrhizal plants quite difficult. For example, infection may at times increase the rate of phosphorus accumulation beyond that which can be currently utilized in growth, reducing the current phosphorus utilization efficiency. Such momentary ‘luxury consumption’ of phosphorus may, however, serve a storage function and be utilized subsequently, allowing mycorrhizal plants ultimately to outperform non-mycorrhizal plants.

Nutrient supply, nutrient demand and plant response to mycorrhizal infection.

The interaction of vesicular-arbuscular mycorrhizae (VAM) and phosphate-solubilizing bacteria (PSB) on plant growth, soil microbial activities, and the production of organic acids was studied in non-sterile soil containing hydroxyapatite and glucose. Glomus etunicatum (VAM), a fungus, and Enterobacter agglomerans, a bacterium able to solubilize insoluble phosphate, were used as inocula. Three treatments and a control were used: inoculation with E. agglomerans (treatment E), inoculation with G. etunicatum (treatment G), inoculation with E. agglomerans+G. etunicatum (treatment E+G) and the control (C). Inoculation with E, G, or E+G had increased plant growth by days 35, 55, and 75 compared with the control. Microbial biomass carbon (C) and alkaline phosphatase activity in the rhizosphere generally increased with time. Alkaline phosphatase activity was higher in treatments G and E+G compared with the control at 35 and 55 days. The highest acid phosphatase activity was observed in treatment E at 35 days; however, this markedly decreased with time. A significantly higher soluble phosphorus (P) concentration was observed in treatments E and E+G on day 55 compared with C. However, there was no significant difference in soluble P concentration in the rhizosphere between treatments with time. The P concentration was greatest in all treatments on day 55. The highest oxalic acid concentration was observed in the rhizosphere of the non-sterile soil in E+G on day 35. Total N and P uptake in plants from treatments E and G were higher compared with the control. However, the highest N and P uptake was observed in treatment E+G. This study suggests a synergistic interaction between E. agglomerans and G. etunicatum.

Effect of phosphate-solubilizing bacteria and vesicular-arbuscular mycorrhizae on tomato growth and soil microbial activity

I: Resource acquisition Photosynthesis Photosynthetic responses of tropical forest plants to contrasting light environments-- R. Chazdon, R. Pearcy, D. Lee, N. Fetcher CAM and C4 plants in the humid tropics-- E. Medina Diel patterns of CO2 exchange in rainforest canopy plants-- G. Zotz, K. Winter Scaling up from leaves to whole plants and canopies for photosynthetic gas exchange-- F. Meizner, G. Goldstein On the relationship of plant geometry and photosynthetic response-- T. Herbert Photosynthetic characteristics of tropical trees along successional gradients -- S. Strauss-Debenedetti, F. Bazzaz Water Relations: Influence of seasonal drought on the carbon balance of tropical forest plants-- S.S. Mulkey, S. Wright Hydraulic architecture of woody tropical plants-- M. Tyree, F. Ewers Evaluating aspects of water economy and photosynthetic performance with stable isotopes from water and organic matter-- G. Goldstein, F. Meinzer, L. Sternberg, P. Jackson, J. Cavelier, N. Holbrook Acquisition of soil resources Root growth and rhizosphere interactions in tropical forests-- R. Sanford Jr., E. Cuevas II: Ecophysiological aspects of species interactions Anti-herbivore defenses of young tropical leaves: physiological constraints and ecological trade-offs-- P. Coley, T. Kursar Causes and consequences of epiphyll colonization-- P. Coley, T. Kursar Physiology of tropical vines and hemiepiphytes: plants that climb up and plants that climb down-- N. Holbrook, F. Putz III: Ecophysiological patterns across tropical forest communities Environmental factors and ecophysiological processes along altitudinal gradients in wet tropical mountains-- J. Cavelier Phenological responses to seasonality in tropical forest plants-- S. Wright Comparative ecophysiology of Mangrove forest and tropical lowland moist rainforest -- M. Ball Ecotypic variation in the physiology of tropical plants-- K. Hogan IV: Ecophysiology of forestregeneration and succession Physiological ecology of seed dormancy and longevity-- C. Vazquez-Yanes, A. Orozco-Segovia Ecophysiology of tropical tree seedlings-- K. Kitajima Ecophysiological constraints on the distribution of Piper species-- A. Fredeen, C. Field Canopy structure and dynamics: integration of growth processes in tropical pioneer trees-- D. Ackerly.

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Tropical Forest Plant Ecophysiology

SUMMARY
Non-mycorrhizal and vesicular-arbuscular mycorrhizal Bouteloua gracilis infected with Glomus fasciculatus were grown in defined media containing different phosphate sources and compared with respect to phosphate content and form, biomass, chlorophyll concentration, and root phosphatase activity. The phosphate sources were sodium monobasic phosphate, a mixture of sodium monobasic phosphate and calcium phytate, and calcium phytate. Inositol and inositol plus calcium were added to the sodium phosphate medium as additional treatments. Mycorrhizal infection was highest in roots of plants grown in the presence of phytate (75%). Lower root infection levels were noted in plants from the sodium phosphate (19%) and mixed phosphate (22%) media. No penetration by fungi occurred in plants from the sodium phosphate plus inositol or inositol and calcium media. Dry wts of non-mycorrhizal plants were highest when grown in media containing phytate and sodium phosphate plus inositol and calcium followed in decreasing order by sodium phosphate plus inositol, mixed phosphates, and sodium phosphate. Mycorrhizal infection increased leaf dry wt in plants from the sodium phosphate medium and root dry wt from the phytate medium. Phosphate concentrations in the plants were highest when grown in mixed phosphate medium followed by sodium phosphate and phytate. Mycorrhizal infection always increased significantly leaf phosphate concentrations but increased root phosphate concentrations only in the phytate medium. Phosphates were found predominantly as organically-bound compounds in leaves of mycorrhizal plants whereas in leaves of non-mycorrhizal plants, most of the phosphate was inorganic. Chlorophyll concentrations increased significantly with mycorrhizal infection with no change in a/b ratios. Mycorrhizal plants grown in the phytate medium had substantially higher alkaline phosphatase activity than did non-mycorrhizal plants; acid phosphatase activity was not affected by mycorrhizal condition.

These results suggest that form of the phosphate in the root environment influences mycorrhizal establishment and effect of mycorrhizae on plant growth.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1981.tb01703.x

Influence of phosphate source on vesicular-arbuscular mycorrhizae of bouteloua gracilis

myo-Inositol is incorporated into phosphatidylinositol by an exchange reaction associated with the endoplasmic reticulum fraction isolated from post-germination castor bean endosperm. The reaction requires Mn2+, has a pH optimum of 8.0, an apparent Km for myo-inositol of 26 micromolar, and is stimulated about 15-fold by certain cytidine derivatives. The cytidine derivatives appear to be converted to CMP, which may be the only active stimulator. These optimal exchange reaction conditions, both with and without CMP, differ from those for cytidine-5′ -diphosphodiglyceride: myo-inositol transferase (EC 2.7.8), so the exchange does not appear to be a reversal of the transferase. This conclusion is augmented by the low rates of CDP-diglyceride formation from cytidine derivatives when compared to the high rate of myo-inositol incorporation into phosphatidylinositol in the presence of the same cytidine derivatives and identical reaction conditions.

/pdf/phosphatidylinositol-synthesis-by-a-mn2-dependent-exchange-3g36dav4gh.pdf

Phosphatidylinositol Synthesis by a Mn2+-Dependent Exchange Enzyme in Castor Bean Endosperm

CDP-diglyceride:inositol transferase in endoplasmic reticulum fractions from castor bean (Ricinus communis) endosperm was partially characterized. The enzyme had a pH optimum of 8.5 and required Mn2+ for activity. Maximal activity was at 1.5 millimolar MnCl2. A Km of 0.30 mM was calculated for myo-inositol and 1.35 millimolar was estimated for CDP-dipalmitoylglyceride. Concentrations of CDP-dipalmitoylglyceride above 1.2 millimolar inhibited the enzyme. A deoxycholate concentration of 0.1% (w/v) stimulated the reaction slightly while Triton X-100 inhibited at all concentrations tested. Some incorporation of myo-inositol into phosphatidylinositol occurred in the absence of CDP-diglyceride.

Phosphatidylinositol synthesis in castor bean endosperm: cytidine diphosphate diglyceride:inositol transferase.

/pdf/influence-of-phosphate-sourceon-vesicular-arbuscular-256oeitxt8.pdf

James C. Sexton

Papers

Influence of phosphate source on vesicular-arbuscular mycorrhizae of bouteloua gracilis

Phosphatidylinositol Synthesis by a Mn2+-Dependent Exchange Enzyme in Castor Bean Endosperm

Phosphatidylinositol synthesis in castor bean endosperm: cytidine diphosphate diglyceride:inositol transferase.

Influence of phosphate sourceon vesicular-arbuscular mycorrhizaeof bouteloua gracilis