Phytate utilization of maize mediated by different nitrogen forms in a plant–arbuscular mycorrhizal fungus–phosphate-solubilizing bacterium system

<div><p>A pot experiment was conducted to investigate the organic phosphorus (P) (phytate) utilization of <i>Zea mays</i> L. with different nitrogen (N) forms (NH₄⁺ and NO₃⁻) when both arbuscular mycorrhizal (AM) fungus (<i>Funelliformis mosseae</i>) and phosphate-solubilizing bacterium (PSB, <i>Pseudomonas alcaligenes</i>) are present. The soil was supplied with either KNO₃ or (NH₄)₂SO₄ (200 mg kg⁻¹ N) with or without phytin (75 mg P kg⁻¹). Results showed that the application of NH₄⁺ to the soil in a plant–AM fungus–PSB system decreased rhizosphere pH and increased phosphatase activity. It also enhanced the mineralization rate of phytin, which resulted in the release of more inorganic P. The application of NO₃⁻ promoted mycorrhizal colonization and hyphal length density in the soil. The inorganic P in the hyphosphere decreased, but more P was transferred to the plant through the mycorrhizal hyphae. Hence, in addition, the application of the two different N forms did not significantly alter the content of plant P. The plant supplied with different N fertilizers acquired P through different mechanisms associated with other microbes. NH₄⁺ application promoted phytin mineralization by decreasing soil pH, whereas NO₃⁻ application increased inorganic P uptake by strengthening the mycorrhizal pathway.</p></div

Shoot and root biomass, seed number, and root/shoot ratio of <i>C. arenarius</i> with or without AM fungi under field conditions and in the pot experiment.

<p>Different lowercase letters in each column indicate significant differences in colonization (<i>P</i><0.05) between mycorrhizal and non-mycorrhizal treatments.</p

PCR primers used in this study.

<p>PCR primers used in this study.</p

Mycorrhizal structures (×400) in roots of

<p><b><i>C. arenarius</i></b><b>.</b> A collected from Gurbantunggut Desert on 12 April 2009 in experiment 1, B from experiment 2, and C from experiment 3.</p

Neighbour-joining tree showing representatives of all sequence types identified in this work (in bold), and reference sequences from Genbank (in italics), using <i>Glomus drummondi</i> as the outgroup.

<p>The five topology has been tested by bootstrap analysis with 1000 replicates, and all bootstrap values >70% are shown. All new sequences have been submitted to the GenBank database (Accession nos JN805771–JN805847).</p

Dynamics of mycorrhizal colonization in the root system of <i>C. arenarius</i> from the field at different harvest times in Experiment 1.

<p>Different lowercase letters in each column represent significant difference (<i>P</i><0.05) among different times.</p

Shoot P concentration (A, B) and content (C, D) with (closed squares) or without (open squares) AM fungi in experiment 2 and 3.

<p>Asterisk indicates significant differences (<i>P</i><0.05) between mycorrhizal treatment and non-mycorrhizal treatment.</p

Mycorrhizal colonization in the root system of <i>C. arenarius</i> in experiments 2 and 3.

<p>Different lowercase letters in each column indicate significant differences in colonization (<i>P</i><0.05) between mycorrhizal and non-mycorrhizal treatments.</p

Reducing carbon: phosphorus ratio can enhance microbial phytin mineralization and lessen competition with maize for phosphorus

<div><p>We tested the hypothesis that reducing the carbon (C):Phosphorus (P) ratio in rhizosphere soil would reduce bacterial competition with the plant for P from phytin, which would then increase phytin use efficiency for the plant. A three-factor pot experiment was carried out to study the effect of inoculation with a phytin-mineralizing bacterium, <i>Pseudomonas alcaligenes</i> (PA), on maize P uptake from phytin. Two levels of organic P, two levels of inorganic P, and three different PA inoculation treatments were used. When maize plants were grown in low available P soil with phytin, PA transformed soil P into microbial biomass P, which caused competition for available P with plant and inhibited plant uptake. When 5 mg P kg⁻¹ as KH₂PO₄ was added, inoculation with PA increased soil acid phosphatase activity which enhanced the mineralization rate of phytin. PA mobilized more P than it immobilized in microbial pool and enhanced plant P uptake. We conclude that the decreased C:P ratio by adding small amount of inorganic P in the rhizosphere could drive phytin mineralization by the bacteria and improve plant P nutrition.</p></div

Presentation1.PDF

<p>Arbuscular mycorrhizal fungi (AMF) play a crucial role in enhancing the acquisition of immobile nutrients, particularly phosphorus. However, because nitrogen (N) is more mobile in the soil solution and easier to access by plants roots, the role of AMF in enhancing N acquisition is regarded as less important for host plants. Because AMF have a substantial N demand, competition for N between AMF and plants particularly under low N condition is possible. Thus, it is necessary to know whether or not AMF affect N uptake of plants and thereby affect plant growth under field conditions. We conducted a 2-year field trial and pot experiments in a greenhouse by using benomyl to suppress colonization of maize roots by indigenous AMF at both low and high N application rates. Benomyl reduced mycorrhizal colonization of maize plants in all experiments. Benomyl-treated maize had a higher shoot N concentration and content and produced more grain under field conditions. Greenhouse pot experiments showed that benomyl also enhanced maize growth and N concentration and N content when the soil was not sterilized, but had no effect on maize biomass and N content when the soil was sterilized but a microbial wash added, providing evidence that increased plant performance is at least partly caused by direct effects of benomyl on AMF. We conclude that AMF can reduce N acquisition and thereby reduce grain yield of maize in N-limiting soils.</p