Phosphate solubilization and indole acetic acid production by rhizosphere yeast Torulaspora globosa: improvement of culture conditions for better performance in vitro

Abstract

The rhizosphere yeast Torulaspora globosa is known to produce indole acetic acid (IAA) and to solubilize minerals. Due to the prospective use of this yeast as a biostimulant for agricultural applications, this work aimed to optimize the cultural conditions for both IAA production and phosphate solubilization. For phosphate solubilization, the temperature (20, 25 and 30 °C), initial medium pH (3.0, 5.0, and 7.0), and shaker speed (without mixing, 100 rpm, 150 rpm, and 200 rpm) were considered using the one-factor-at-a-time (OFAT) design. Temperature of 25 °C, initial medium pH 7.0, and static cultures were the conditions of greatest phosphate solubilization, with 40% of the total phosphorus content solubilized from calcium phosphate (419.86 mg L^-1) after 48 h. By using the response surface methodology, the maximum IAA production (217.73 µg mL^-1) was obtained with the highest initial pH 7.0, the lowest nitrogen, and glucose concentrations (5 g L^-1 and 10 g L^-1, respectively) and the lowest agitator speed (100 rpm). Further tests indicated that nitrogen affected significantly IAA production and the absence of nitrogen in the medium promoted higher IAA production (457 µg mL^-1). The results obtained here may contribute to the scaling up for industrial and agricultural applications of a yeast-based product with T. globosa.

Keywords: Auxin; Biostimulant; Phosphate solubilization; Plant growth promotion; Yeast.

Fig. 1

Optimization of the cultural conditions (temperature, initial pH, and shaker speed) for T. globosa growth (optical density, diamond symbol) and phosphorus solubilization (in mg L⁻¹, gray bar) in NBRIP medium. For temperature: initial pH 7.0; shaker speed 100 rpm; for initial pH: temperature 25 °C, shaker speed 100 rpm; for shaker speed: temperature 25 °C, initial pH 7.0. Optical density values represent the difference between the final and initial values. Averages and standard deviations were calculated from three replicates

Fig. 2

Growth (optical density, diamond symbol in A), phosphorus solubilization (in mg L⁻¹, square symbol in A and B) and final pH (diamond symbol in B) of T. globosa cultivation in NBRIP medium under the following conditions: initial pH 7.0; without shaking; and temperature 25 °C. Averages and standard deviations were calculated from three replicates

Fig. 3

Pareto Chart of independent variables for IAA productivity (dashed red line indicates difference at 5% level of significance)

Fig. 4

Surface plots for IAA productivity versus initial ammonium nitrate, initial glucose, temperature, shaker speed, and pH

Fig. 5

IAA productivity profile without ammonium nitrate (□), add 5 g L⁻¹ (o), 10 g L⁻¹ (∆), and 15 g L⁻¹ (∇)

Fig. 6

Yield cell to IAA profiles (□) and cell concentration (o) without ammonium nitrate a add 5 g L⁻¹ b 10 g L⁻¹ c and 15 g L.⁻¹ (d)