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

ABSTRACT Metarhizium robertsii ARSEF 23 is a significant insect-pathogenic fungus and plant-associated endophyte, with dual roles as an insect-control agent and plant growth-enhancing agent that have fueled its extensive use in agricultural practices. However, the growth-promoting impacts of M. robertsii (referred to as MAA) on rice, and their underlying mechanisms are still not fully clarified. This research verified the growth-enhancing effects of both MAA itself and its fermented supernatant on rice plants. By means of single-factor tests and response surface methodology, the fermentation conditions of MAA were then optimized using rice root length as the primary index, while the possible mechanisms driving this growth promotion were initially explored. The results showed that both MAA and its fermented supernatant exhibited notable growth-enhancing impacts on rice. Sucrose and peptone were found to be the best carbon and nitrogen substrates, respectively. The most remarkable growth promotion was observed when the fermented supernatant—obtained under conditions of pH 6.0, 0.2% β-Ala, and 100 μmol/L aluminum sulfate—was applied, leading to a 21.7% rise in rice root length relative to the control group, which further has an impact on the metabolites of MAA. In addition, the relative abundance of MAA increased by 400-fold in rice roots, strongly suggesting root association, and significantly restructured the root endophytic fungal community by suppressing pathogenic genera (e.g., Fusarium sp.) and enriching plant growth-promoting beneficial genera (e.g., Trichoderma sp.). Meanwhile, marked increases in chlorophyll content (a 28.17% rise compared to the control) and more abundant root hairs were also observed in rice treated with the MAA fermented supernatant. Collectively, these hydroponic-based findings provide a theoretical basis for the development and utilization of environmentally friendly and high-efficiency MAA-derived bio-inoculants in rice production, though field validation is still needed. IMPORTANCE Metarhizium robertsii is a promising entomopathogenic and endophytic fungus with biocontrol and plant growth-promoting functions. Its growth-promoting effects and mechanisms on rice remain poorly understood, and fermentation conditions tailored for rice have not been optimized. This study optimized fermentation parameters, revealed the growth-promoting mechanism, and provided a technical basis for developing green, efficient microbial inoculants to support sustainable rice production.