Dual function of heterotrophic ammonia-oxidizing bacteria in facilitating maize compensatory growth under limited rewatering after drought.

Water scarcity threatens global food security, making drought resilience in crops like maize crucial. In response to this challenge, this study investigates the potential of heterotrophic ammonia-oxidizing bacteria (HAOB) to enhance maize compensatory growth under post-drought limited rewatering conditions. Specifically, we focus on the dual mechanism of HAOB in modulating cytokinin synthesis and transport, aiming to develop an innovative agricultural biotechnology to support sustainable crop production. The S2_8_1 HAOB strain was used across two experiments. Experiment 1 investigated varying NO levels' effects on cytokinin translocation from roots to leaves under limited rewatering. Experiment 2 combined NO supplementation with HAOB inoculation to assess HAOB's twofold function in promoting compensatory growth under limited rewatering. The results showed that optimal NO levels (20-30 mmol·L for limited rewatering) enhanced maize growth, root-to-shoot cytokinin translocation, and leaf cytokinin levels under limited rewatering. Notably, inoculation with HAOB outperformed these effects, demonstrating a more robust impact on cytokinin delivery and plant growth. This confirmed HAOB's twofold mechanism: Nitrification pathway - HAOB enhances rhizospheric NO₃⁻ availability, thereby stimulating cytokinin biosynthesis in roots and its translocation to leaves. Non-nitrification pathway - HAOB further promotes cytokinin translocation through mechanisms independent of soil NO₃⁻ increase. Sufficient rewatering increased rhizosphere nitrification rates, boosting root cytokinin translocation to leaves, thereby supported compensatory growth. Limited rewatering reduced rhizosphere nitrification, cytokinin translocation, and compensatory growth. However, HAOB overcame these constraints through its twofold function, enhancing cytokinin translocation and improving water use efficiency by more than fourfold, successfully promoting compensatory growth even under limited rewatering. Additionally, NO supplementation alleviated some limitations by increasing rhizosphere NO, but HAOB inoculation proved more effective, highlighting its superior role. This twofold function of HAOB strain significantly elevated cytokinin levels in leaves, supporting compensatory growth under limited rewatering. This biotechnology offers high agricultural potential, particularly in water-scarce regions, by improving drought resilience and yield stability.