Dual Benefits of Endophytic Bacillus velezensis Amzn015: Growth Promotion and Root Rot Control in Atractylodes macrocephala

<i>Atractylodes macrocephala</i> Koidz. (<i>A. macrocephala</i>), a medicinal plant extensively used in traditional Chinese medicine, is greatly susceptible to root rot under continuous monoculture, leading to serious yield and quality losses. To develop a sustainable control strategy, we isolated the endophytic bacterium <i>Bacillus velezensis</i> (<i>B. velezensis</i>) Amzn015 from healthy <i>A. macrocephala</i> plants and assessed its biocontrol efficacy and underlying mechanisms. In vitro assays showed that Amzn015 significantly inhibited <i>Fusarium oxysporum</i> and other phytopathogenic fungi by disrupting hyphal morphology and reducing spore viability. Pot experiments confirmed its effectiveness in reducing disease incidence and promoting plant growth. Mechanistically, Amzn015 induced reactive oxygen species accumulation and upregulated key defense responsive genes involved in salicylic acid, jasmonic acid/ethylene, and phenylpropanoid signaling pathways. The findings imply that Amzn015 synchronously activates systemic acquired resistance and induced systemic resistance in <i>A. macrocephala</i>. This dual activation contributes to enhanced immunity and plant vigor under pathogen challenge. Our findings offer fresh perspectives on the biocontrol potential of endophytic <i>B. velezensis</i> Amzn015 and support its application as an eco-friendly agent for managing root rot in medicinal crops.