Optimization of fermentation conditions for Metarhizium robertsii and its effect on rice growth

ABSTRACT Metarhizium robertsii is an important entomopathogenic fungus and plant endophyte, possessing dual functionality as both an insecticide and a plant growth promoter, which has led to its widespread application in agriculture. However, the effects and underlying mechanisms of M. robertsii (MAA)in promoting rice growth remain poorly understood. This study confirmed the growth-promoting effects of MAA and its fermentation supernatant on rice. Based on single-factor and response surface methodology experiments, the fermentation conditions of MAA were subsequently optimized using rice root length as a key indicator, and the potential mechanisms behind this growth promotion were preliminarily investigated. The results demonstrated that both MAA and its fermentation supernatant exerted significant growth-promoting effects on rice. Sucrose and peptone were identified as the optimal carbon and nitrogen sources, respectively. The most pronounced effect was observed when the fermentation supernatant, produced under the conditions of pH 6.0, 0.2% β-Ala, and 100 μmol/L Al 2 (SO 4 ) 3 , was applied, resulting in a 21.7% increase in rice root length compared to the control group. Furthermore, significant enhancements in chlorophyll content and denser root hairs were recorded in rice treated with the MAA fermentation supernatant, with the chlorophyll content increasing by 28.17% compared to the control. These findings provide a theoretical basis for the development and application of green and efficient bio-inoculants. IMPORTANCE Metarhizium robertsii is an endogenous plant fungus that can be effective for efficient insect control, but the plant-promoting effect of this fungus and its related studies are less reported. The colonization ability and growth promotion effect of Metarhizium robertsii on different crops vary. In this study, a sterile hydroponic system for rice was established. The research found that Metarhizium robertsii can colonize the root system of rice, significantly promoting rice growth, and its growtion-promoting effect is affected by the differences in its fermentation products. After the optimization of the response surface of the fermentation conditions of Metarhizium robertsii , its growth-promoting ability increased by 21.7%. Developing microbial biofertilizers/biopesticides as a basis for reducing chemical inputs in rice cultivation.