Plant Fe status affects the composition of siderophore-secreting microbes in the rhizosphere

Abstract

Background and aims: Soil microbes have been demonstrated to play an important role in favouring plant iron (Fe) uptake under Fe-limiting conditions. However, the mechanisms involved are still unclear. This present study reported the effects of plant Fe status on the composition of siderophore-secreting microbes in the rhizosphere, and their potential function in improving plant Fe nutrition.

Methods: An Fe-efficient plant, red clover (Trifolium pratense 'Kenland') was cultured in a calcareous soil to obtain rhizosphere soils with (Fe-sufficient) or without (Fe-stressed) foliar FeEDTA spraying. The siderophore-producing ability of rhizospheric microbes was measured. The bioavailability of the siderophore-solubilized Fe from iron oxides/hydroxides was tested in hydroponic culture.

Key results: In rhizosphere soil, the number of microbes that secreted siderophores quickly was more in the Fe-stressed treatment than in the Fe-sufficient one, while the number of microbes that did not secret siderophores was the opposite. A significantly higher concentration of phenolics was detected in the rhizosphere soil of Fe-stressed plants. Moreover, after the soil was incubated with phenolic root exudates, the composition of the siderophore-secreting microbial community was similar with that of the rhizosphere of Fe-stressed plant. Additionally, the siderophores produced by a rhizospheric microbe isolated from the Fe-stressed treatment can well solubilize iron oxides/hydroxides, and the utilization of the siderophore-solubilized Fe by plant was even more efficient than EDTA-Fe.

Conclusions: Iron-deficiency stress of red clover would alter the composition of siderophore-secreting microbes in the rhizosphere, which is probably due to the phenolics secretion of the root, and may in turn help to improve the solubility of Fe in soils and plant Fe nutrition via elevated microbial siderophore secretion.

Fig. 1.

(A) The siderophore-secreting ability of the microbes from the rhizosphere soil of Fe-stressed and Fe-sufficient red clover; (B) the percentage of soil microbes that secrete siderophores. The microbial suspension in serial soil dilutions was spread on agar plates with or without CAS indicator. The plates were incubated at 30 °C, and the siderophore-secreting ability of the soil microbes was conducted as described in Materials and methods. QSS, MSS and SSS are quicker-, middle- and slower-secreting siderophores, respectively. *, Significant differences (P < 0·05) between two treatments at each time point (n = 4).

Fig. 2.

Phenolics concentration in rhizosphere soil of Fe-stressed and Fe-sufficient red clover. The phenolics in the soil were extracted with deionized water on an orbital-action shaker for 18 h. The concentration of phenolics was mesured using Folin–Ciocalteu reagent. The concentration of all the phenolic compounds was expressed as an equivalent of gallic acid. *, Significant differences (P < 0·05) between two treatments at each time point (n = 4).

Fig. 3.

The siderophore-secreting ability of the microbes from the soil incubated with phenolic root exudates. Twenty-five grams of air-dried calcareous soil were mixed with 5 mL of distilled water containing 10 µmol (gallic acid equivalent) phenolics. After 4 d incubation at 30 °C, the siderophore-secreting ability of the soil microbes was conducted as described in Materials and methods (n = 4). QSS, MSS and SSS are quicker-, middle- and slower-secreting siderophores, respectively.

Fig. 4.

(A) Growth kinetics of Pseudomonas sp. yy in FeCl₃ (10 µm) or hydrous Fe(III)-oxides (200 mg L⁻¹) media, as measured by absorbance at 600 nm. (B) Solubilization of Fe(OH)₃ by concentrated siderophore compounds secreted by Pseudomonas sp. yy.

Fig. 5.

(A) Chlorophyll synthesis and (B) Fe concentration in red clover plants after 4 weeks culture in −Fe, +EDTA-Fe (20 µm) or +siderophore ‐ Fe complexes (20 µm). Data are means ± s.d. (n = 3).