Data_Sheet_1_A PilZ-Containing Chemotaxis Receptor Mediates Oxygen and Wheat Root Sensing in Azospirillum brasilense.pdf

Chemotactic bacteria sense environmental changes via dedicated receptors that bind to extra- or intracellular cues and relay this signal to ultimately alter direction of movement toward beneficial cues and away from harmful environments. In complex environments, such as the rhizosphere, bacteria must be able to sense and integrate diverse cues. Azospirillum brasilense is a microaerophilic motile bacterium that promotes growth of cereals and grains. Root surface colonization is a prerequisite for the beneficial effects on plant growth but how motile A. brasilense navigates the rhizosphere is poorly studied. Previously only 2 out of 51 A. brasilense chemotaxis receptors have been characterized, AerC and Tlp1, and only Tlp1 was found to be essential for wheat root colonization. Here we describe another chemotaxis receptor, named Aer, that is homologous to the Escherichia coli Aer receptor, likely possesses an FAD cofactor and is involved in aerotaxis (taxis in an air gradient). We also found that the A. brasilense Aer contributes to sensing chemical gradients originating from wheat roots. In addition to A. brasilense Aer having a putative N-terminal FAD-binding PAS domain, it possesses a C-terminal PilZ domain that contains all the conserved residues for binding c-di-GMP. Mutants lacking the PilZ domain of Aer are altered in aerotaxis and are completely null in wheat root colonization and they also fail to sense gradients originating from wheat roots. The PilZ domain of Aer is also vital in integrating Aer signaling with signaling from other chemotaxis receptors to sense gradients from wheat root surfaces and colonizing wheat root surfaces.

趋化细菌通过特异性受体感知环境变化：此类受体可结合胞外或胞内信号分子，并传递该信号，最终改变运动方向，朝向有益信号移动、远离有害环境。在诸如根际（rhizosphere）的复杂环境中，细菌需能够感知并整合多种信号分子。巴西固氮螺菌（Azospirillum brasilense）是一种微需氧运动细菌，可促进禾本科与谷类作物生长。根系表面定殖是其发挥促植物生长有益效应的前提，但运动型巴西固氮螺菌如何在根际中定向移动的相关研究仍较为匮乏。此前，在51种巴西固氮螺菌趋化受体中，仅AerC与Tlp1两种得到了功能鉴定，且仅Tlp1被证实对小麦根系定殖至关重要。本研究报道了另一种趋化受体Aer，其与大肠杆菌（Escherichia coli）的Aer受体同源，大概率结合黄素腺嘌呤二核苷酸（FAD）辅因子，并参与趋氧运动（aerotaxis，即空气梯度中的定向运动）。本研究同时发现，巴西固氮螺菌的Aer受体参与感知源自小麦根系的化学梯度。巴西固氮螺菌Aer受体除带有推定的N端FAD结合PAS结构域（PAS domain）外，还拥有一个C端PilZ结构域（PilZ domain），该结构域包含所有与结合环二鸟苷单磷酸（c-di-GMP）相关的保守残基。缺失Aer受体PilZ结构域的突变株，其趋氧运动能力发生改变，且完全丧失小麦根系定殖能力，同时无法感知源自小麦根系的化学梯度。Aer受体的PilZ结构域还在整合Aer信号通路与其他趋化受体的信号通路中发挥关键作用，以此感知小麦根系表面的化学梯度并完成根系定殖。