Optimization of antifungal and nitrogen-fixing activities of Niallia circulans YRNF1 using response surface methodology to control Fusarium root rot of pepper and promote the plant growth

Sweet pepper crop is exposed to Fusarium root rot, caused by Fusarium solani (Mart.) Sacc., resulting in great economic losses. In this study, culturing conditions for nitrogen-fixing and biocontrol activities of Niallia circulans YRNF1 were optimized in vitro using the response surface methodology. Results indicated that the optimum culturing conditions for both activities were initial pH = 6, glucose concentration of 27.5 mgL⁻¹, temperature of 13.5 °C, and incubation time of 12.3 days. In the greenhouse, inoculating the infected pepper plants with N. circulans YRNF1, grown under optimized conditions, reduced the disease severity to 24.8%, compared to 37.3% in case of the un-optimized N. circulans YRNF1. In addition, three defense-related genes (CHI II, C4H, and H3F) were highly overexpressed more than in case of the un-optimized bioagent. Treating the infected plants with N. circulans YRNF1, grown under the optimized conditions, triggered the plant defense responses through inducing the phenolic compounds accumulation and activities of two antioxidant enzymes; POD and PPO. Inoculating the infected plants with the optimized N. circulans YRNF1 promoted their shoot height and root length (22.00 and 9.00 cm, respectively) and the flowers number, compared to the untreated infected plants (17.33 and 4.00 cm, respectively). Moreover, application of the optimized N. circulans YRNF1 on the pepper plants led to a significant increment in the total nitrogen content in pepper roots (206.97%), compared to those inoculated with the un-optimized bioagent (181.39%). Optimization of the culturing conditions of N. circulans YRNF1 represents a promising and eco-friendly approach for supporting the organic and sustainable agriculture.