Plant growth promoting rhizobacteria influence host functional and adaptive traits via complex mechanisms that are just started to be clarified. Azospirillum brasilense acts as a probiotic bacterium, but detailed information about its molecular mechanisms of phytostimulation is scarce. Three interaction systems were established to analyze the impact of A. brasilense Sp245 on the phenotype of Arabidopsis seedlings, and underlying molecular responses were assessed under the following growth conditions: (1) direct contact of roots with the bacterium, (2) chemical communication via diffusible compounds produced by the bacterium, (3) signaling via volatiles. A. brasilense Sp245 improved shoot and root biomass and lateral root production in the three interaction systems assayed. Cell division, quiescent center, and differentiation protein reporters pCYCB1;1::GUS, WOX5::GFP, and pAtEXP7::GUS had a variable expression in roots depending of the nature of interaction. pCYCB1;1::GUS and WOX5::GFP increased with volatile compounds, whereas pAtEXP7::GUS expression was enhanced towards the root tip in plants with direct contact with the bacterium. The auxin reporter DR5::GUS was highly expressed with diffusible and volatile compounds, and accordingly, auxin signaling mutants pin3 , slr1 , arf7arf19 , and tir1afb2afb3 showed differential phytostimulant responses when compared with the wild type. By contrast, ethylene signaling was not determinant to mediate root changes in response to the different interactions, as observed using the ethylene-related mutants etr1 , ein2 , and ein3 . Our data highlight the diverse effects by which A. brasilense Sp245 improves plant growth and root architectural traits and define a critical role of auxin but not ethylene in mediating root response to bacterization.

中文翻译：

---

固氮螺菌-拟南芥相互作用的性质决定根和植物生长促进的分子和形态变化

促进植物生长的根际细菌通过刚刚开始阐明的复杂机制影响宿主功能和适应性特征。Azospirillum brasilense 作为一种益生菌，但关于其植物刺激分子机制的详细信息很少。建立了三个相互作用系统来分析 A. brasilense Sp245 对拟南芥幼苗表型的影响，并在以下生长条件下评估潜在的分子反应：（1）根与细菌的直接接触，（2）通过化学通讯由细菌产生的可扩散化合物，（3）通过挥发物发出信号。A. brasilense Sp245 在所分析的三个相互作用系统中提高了地上部和根系生物量以及侧根产量。细胞分裂，静止中心，和分化蛋白报告基因 pCYCB1；1::GUS、WOX5::GFP 和 pAtEXP7::GUS 在根中具有可变的表达，这取决于相互作用的性质。pCYCB1;1::GUS 和 WOX5::GFP 随着挥发性化合物的增加而增加，而 pAtEXP7::GUS 表达在与细菌直接接触的植物中向根尖方向增强。生长素报告基因 DR5::GUS 在可扩散和挥发性化合物中高度表达，因此，与野生型相比，生长素信号突变体 pin3、slr1、arf7arf19 和 tir1afb2afb3 显示出不同的植物刺激素反应。相比之下，正如使用乙烯相关突变体 etr1、ein2 和 ein3 所观察到的那样，乙烯信号不是调节根系响应不同相互作用的决定因素。我们的数据突出了 A.