Dual Roles of Heterotrophic Ammonia-Oxidizing Bacteria in Enhancing Compensatory Growth upon Post-Drought in Maize

This study investigates the mechanisms driving maize compensatory growth upon post-drought, to reveal how the root's original cytokinins are regulated by the two-fold roles of heterotrophic bacteria with ammonia-oxidizing (HAOB) capabilities. The HAOB' dual roles encompass influencing root cytokinin synthesis and transport through nitrification and a direct pathway. Experiment 1 involved introducing the application of varying amounts of NO₃^- to the roots to examine how nitrification affects cytokinin roots-to-leaves transport. Results demonstrate that the 30-40 mmol·L^-1 NO₃^- concentration had ideal effects on enhancing post-drought growth in maize by facilitating cytokinin synthesis and transport. In experiment 2, an HAOB strain, S2_8_1, was utilized and NO₃^- was supplemented alongside HAOB inoculation to assess the joint impacts of nitrification and the direct pathway on the production and transportation of cytokinins. Results demonstrate that the HAOB strain S2_8_1 increases nitrification rates in rhizosphere soil, thereby promoting the transport of cytokinins from roots to leaves. In addition, the HAOB strain promotes root cytokinin transport to leaves autonomously, showcasing its direct pathway. Inoculation with the HAOB strain increased leaf cytokinin content and improved water use efficiency compared to the addition of NO₃^-; however, the combination of NO₃^- and HAOB strains resulted in a synergistic effect and further improvement. These findings elucidate how HAOB can enhance maize compensatory growth through its dual roles, presenting promising applications in agriculture.