Vesicular-arbuscular mycorrhizal fungi can affect the water balance of both amply watered and droughted host plants. This review summarizes these effects and possible causal mechanisms. Also discussed are host drought resistance and the influence of soil drying on the fungi.

Water relations, drought and vesicular-arbuscular mycorrhizal symbiosis

Microbial communities play a pivotal role in the functioning of plants by influencing their physiology and development. While many members of the rhizosphere microbiome are beneficial to plant growth, also plant pathogenic microorganisms colonize the rhizosphere striving to break through the protective microbial shield and to overcome the innate plant defense mechanisms in order to cause disease. A third group of microorganisms that can be found in the rhizosphere are the true and opportunistic human pathogenic bacteria, which can be carried on or in plant tissue and may cause disease when introduced into debilitated humans. Although the importance of the rhizosphere microbiome for plant growth has been widely recognized, for the vast majority of rhizosphere microorganisms no knowledge exists. To enhance plant growth and health, it is essential to know which microorganism is present in the rhizosphere microbiome and what they are doing. Here, we review the main functions of rhizosphere microorganisms and how they impact on health and disease. We discuss the mechanisms involved in the multitrophic interactions and chemical dialogues that occur in the rhizosphere. Finally, we highlight several strategies to redirect or reshape the rhizosphere microbiome in favor of microorganisms that are beneficial to plant growth and health.

The rhizosphere microbiome: significance of plant beneficial, plant pathogenic, and human pathogenic microorganisms

Summary
The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of ‘promoter’ substances, such as ascorbate, β-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of ‘antinutrients’, such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

/pdf/biofortification-of-crops-with-seven-mineral-elements-often-3g2xc3trnk.pdf

Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine.

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable N isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input–output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization–N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors vary. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+ can become isotopically heavier than the organic N from which it is derived.Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and δ15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring δ15N in small and dynamic pools of N in the soil–plant system, and the complexity of the N cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer.Such attempts require, however, that the complex factors affecting δ15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOx, NH3, N2-fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N2-fixing species should differ more than 5‰ from N derived by N2-fixation, and that several non-N2-fixing references are used, when data on δ15N are used to estimate N2-fixation in poorly described ecosystems.As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces 15N-enriched NH4+ and 15N-depleted NO3−. As many forest plants prefer NH4+ they become enriched in 15N in such circumstances. This change in plant δ15N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in δ15N with depth in soils of N-limited forests.Generally, interpretation of 15N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the δ15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level 15N studies will be necessary to further develop the interpretation of variations in δ15N in soil–plant systems. Another promising approach is to study ratios of 15N[ratio ]14N together with other pairs of stable isotopes, e.g. 13C[ratio ]12C or 18O[ratio ]16O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied.

Tansley Review No. 95 15N natural abundance in soil-plant systems

http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

New information on N uptake and transport of inorganic and organic N in arbuscular mycorrhizal fungi is reviewed here. Hyphae of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) were shown to transport N supplied as 15N-Gly to wheat plants after a 48 h labelling period in semi-hydroponic (Perlite), non-sterile, compartmentalised pot cultures. Of the 15N supplied to hyphae in pot cultures over 48 h, 0.2 and 6% was transported to plants supplied with insufficient N or sufficient N, respectively. The increased 15N uptake at the higher N supply was related to the higher hyphal length density at the higher N supply. These findings were supported by results from in vitro and monoxenic studies. Excised hyphae from four Glomus isolates (BEG 84, 107, 108 and 110) acquired N from both inorganic (15NH4
 15NO3, 15NO3
 − or 15NH4
 +) and organic (15N-Gly and 15N-Glu, except in BEG 84 where amino acid uptake was not tested) sources in vitro during short-term experiments. Confirming these studies under sterile conditions where no bacterial mineralisation of organic N occurred, monoxenic cultures of Glomus intraradices Schenk and Smith were shown to transport N from organic sources (15N-Gly and 15N-Glu) to Ri T-DNA transformed, AM-colonised carrot roots in a long-term experiment. The higher N uptake (also from organic N) by isolates from nutrient poor sites (BEG 108 and 110) compared to that from a conventional agricultural field implied that ecotypic differences occur. Although the arbuscular mycorrhizal isolates used contributed to the acquisition of N from both inorganic and organic sources by the host plants/roots used, this was not enough to increase the N nutritional status of the mycorrhizal compared to non-mycorrhizal hosts.

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

Colonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomy...

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-μm nylon net. The root compartment received a uniform level of P (50 mg kg−1 soil) in combination with low or high levels of P (50 or 150 mg kg−1 soil) in the hyphal compartments. Plants were inoculated withGlomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected.

Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

summary
Maize (Zea mays L.) was grown in fertilized calcareous soil in pots which were separated by 30 μm nylon nets into three compartments, the central one for root growth and the two outer ones for hyphal growth. The size of each compartment was 40 × 25 × 3 cm. The treatments comprised of sterilized soil, either inoculated with rhizosphere microorganisms (other than VA mycorrhizal fungi), with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] or remained non-inoculated (sterile control). As inoculum for rhizosphere microorganisms the roots with adhering rhizosphere soil of non-mycorrhizal maize plants was used.

Compared to the non-inoculated (sterile) control, inoculation with rhizosphere microorganisms did not affect shoot dry weight and morphology, but increased total root length (17 %) and root length per unit root dry weight (35%). The additional inoculation with VA mycorrhizal fungi had no influence on the shoot dry weight but increased area and dry weight of the leaf blades by about 30% and the ratio leaf blade:leaf sheath + stem (w/w) by 41 %. The most profound effect of VA mycorrhizal fungi inoculation was on root growth and morphology. Compared to the non-inoculated control, root dry weight was decreased by 16%, root length by 31 % and root hair density and length by 41 and 43 %, respectively.

In mycorrhizal plants the transpiration rates per plant were about 30 % higher than in the other treatments and this is attributed to the larger leaf area. Water uptake rate per unit root length and per unit time was about twice as high in mycorrhizal plants. For several reasons a substantial hyphal water transport seems unlikely. The results stress the necessity of detailed studies on root morphology for interpretation of effects of mycorrhizal fungi on mineral nutrient uptake and water relations in plants.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb04718.x

Effect of va mycorrhizal fungi and rhizosphere microorganisms on root and shoot morphology, growth and water relations in maize

Relationships between root zone temperature, concentrations and uptake rates of NH
 4
 +
 and NO
 3
 −
 were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH
 4
 +
 and NO
 3
 −
 uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (Cmin) range of 100–150 μM uptake rates of NH
 4
 +
 were 3–4 times higher than those of NO
 3
 −
 The preference for NH
 4
 +
 uptake was also reflected in the minimum concentration (Cmin) values. Supplying NH4NO3, the rate of NO
 3
 −
 uptake was very low until the NH
 4
 +
 concentrations had fallen below about 100 μM. The shift from NH
 4
 +
 to NO
 3
 −
 uptake was correlated with a corresponding shift from net H+ production to net H+ consumption in the external solution. The uptake rates of NH
 4
 +
 were correlated with equimolar net production of H+. With NO
 3
 −
 nutrition net consumption of H+ was approximately twice as high as uptake rates of NO
 3
 −
 In the forest stand the NO
 3
 −
 concentration in the soil solution was more than 10 times higher than the NH
 4
 +
 concentration (<100 μM), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO
 3
 −
 uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.

Eckhard George

Papers

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

Effect of va mycorrhizal fungi and rhizosphere microorganisms on root and shoot morphology, growth and water relations in maize

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [ Picea abies (L.) Karst.]