<i>Bacillus proteolyticus</i> Enhances the Biocontrol of Needle Cast in <i>Cathaya argyrophylla</i> by Modulating Physiological Pathways and the Phyllosphere Microbiome

<i>Cathaya argyrophylla</i>, a relict species endemic to China in the Pinaceae family, is classified as endangered on the IUCN Red List of Threatened Species. As a rare and endangered plant classified under Grade I protection in China, <i>C. argyrophylla</i> possesses significant scientific reference value. Needle cast caused by the fungal pathogen <i>Neofusicoccum parvum</i> is one of the most prevalent diseases affecting <i>C. argyrophylla</i>, leading to needle discoloration and necrosis, which poses a serious threat to its growth. Currently, there is a lack of effective biological control methods for this disease. In this study, we isolated an endophytic bacterium, <i>Bacillus proteolyticus</i> X6-1, from healthy needles of <i>C. argyrophylla</i>. This strain demonstrated a 65.5% inhibition of <i>N. parvum</i> in vitro. The pot experiment demonstrated that after inoculation with <i>B. proteolyticus</i> X6-1, the disease incidence and index of needle cast in <i>C. argyrophylla</i> were reduced by 42 and 20.3%, respectively. Given its potential efficacy against needle cast in <i>C. argyrophylla</i>, we employed coculture techniques alongside transcriptomics and high-throughput sequencing to elucidate the biological control mechanisms of strain X6-1. Furthermore, X6-1 enhanced the photosynthetic activity and antioxidant enzyme levels of <i>C. argyrophylla</i>. Inoculation with the <i>B. proteolyticus</i> strain further altered the phyllosphere microbiome by promoting the enrichment of beneficial microorganisms while decreasing the abundance of <i>Neofusicoccum</i> spp. Transcriptomic analysis revealed that <i>B. proteolyticus</i> regulates key biological pathways associated with growth in <i>C. argyrophylla</i>. Moreover, strain X6-1 produces protease and β-glucanase enzymes, which may contribute to its antifungal activity. Collectively, these findings suggest that <i>B. proteolyticus</i> may serve as an effective biocontrol agent for managing needle cast in <i>C. argyrophylla</i>.