Azospirillum brasilense colonizes plant roots and improves productivity, but the molecular mechanisms behind its phytostimulation properties remain mostly unknown. Here, we uncover an important role of TARGET OF RAPAMYCIN (TOR) signaling on the response of Arabidopsis thaliana to A. brasilense Sp245. The effect of the bacterium on TOR expression was analyzed in the transgenic line TOR/tor-1, which carries a translational fusion with the GUS reporter protein, and the activity of TOR was assayed thought the phosphorylation of its downstream signaling target S6K protein. Besides, the role of TOR on plant growth in inoculated plants was assessed using the ATP-competitive inhibitor AZD-8055. A decrease in growth of the primary root correlates with an improved branching and absorptive capacity via lateral root and root hair proliferation 6 days after transplant to different concentrations of the bacterium (103 or 105 CFU/mL). Bacterization increased the expression of TOR in shoot and root apexes and promoted phosphorylation of S6K 3 days after transplant. The TOR inhibitor AZD-8055 (1 μM) inhibited plant growth and cell division in root meristems and in lateral root primordia, interfering with the phytostimulation by A. brasilense. In addition, the role of auxin produced by the bacterium to stimulate TOR expression was explored. Noteworthy, the A. brasilense mutant FAJ009, impaired in auxin production, was unable to elicit TOR signaling to the level observed for the wild-type strain, showing the importance of this phyhormone to stimulate TOR signaling. Together, our findings establish an important role of TOR signaling for the probiotic traits elicited by A. brasilense in A. thaliana.

中文翻译：

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雷帕霉素信号的靶点在巴西固螺菌 Sp245 促进拟南芥生长中发挥作用

Azospirillum brasilense 定植于植物根部并提高生产力，但其植物刺激特性背后的分子机制仍然未知。在这里，我们发现了雷帕霉素靶点 (TOR) 信号对拟南芥对巴西拟南芥 Sp245 的反应的重要作用。在携带与 GUS 报告蛋白的翻译融合蛋白的转基因品系 TOR/tor-1 中分析了细菌对 TOR 表达的影响，并通过其下游信号靶标 S6K 蛋白的磷酸化来检测 TOR 的活性。此外，使用 ATP 竞争性抑制剂 AZD-8055 评估了 TOR 对接种植物中植物生长的作用。移植到不同浓度的细菌（103 或 105 CFU/mL）后 6 天，主根生长的减少与通过侧根和根毛增殖提高的分枝和吸收能力相关。移植后 3 天，杀菌增加了地上部和根尖中 TOR 的表达并促进了 S6K 的磷酸化。TOR 抑制剂 AZD-8055 (1 μM) 抑制根分生组织和侧根原基中的植物生长和细胞分裂，干扰 A. brasilense 的植物刺激。此外，探索了细菌产生的生长素刺激 TOR 表达的作用。值得注意的是，生长素产生受损的巴西曲霉突变体 FAJ009 无法将 TOR 信号传导至野生型菌株观察到的水平，显示了这种植物激素对刺激 TOR 信号的重要性。总之，我们的研究结果确立了 TOR 信号在 A. thaliana 中由 A. brasilense 引起的益生菌性状的重要作用。