背景回顾：To overcome nitrogen deficiency, legume roots establish symbiotic interactions with nitrogen-fixing rhizobia that is fostered in specialized organs (nodules). Similar to other organs, nodule formation is determined by a local maximum of the phytohormone auxin at the primordium site. 

提出问题：However, how auxin regulates nodule development remains poorly understood.

主要发现：Here, we found that in soybean, (Glycine max), dynamic auxin transport driven by PIN-FORMED (PIN) transporter GmPIN1 is involved in nodule primordium formation.

结果1-表达模式和亚细胞定位：GmPIN1 was specifically expressed in nodule primordium cells and GmPIN1 was polarly localized in these cells. 

结果2-诱导表达：Two nodulation regulators, (iso)flavonoids trigger expanded distribution of GmPIN1b to root cortical cells, and cytokinin rearranges GmPIN1b polarity.

结果3-功能缺失突变：Gmpin1abc triple mutants generated with CRISPR-Cas9 showed impaired establishment of auxin maxima in nodule meristems and aberrant divisions in the nodule primordium cells. 

结果4-过表达：Moreover, overexpression of GmPIN1 suppressed nodule primordium initiation. 

结果5-协同因子：GmPIN9d, an ortholog of Arabidopsis thaliana PIN2, acts together with GmPIN1 later in nodule development to acropetally transport auxin in vascular bundles, fine-tuning the auxin supply for nodule enlargement. 

结论：Our findings reveal how PIN-dependent auxin transport modulates different aspects of soybean nodule development and suggest that establishment of auxin gradient is a prerequisite for the proper interaction between legumes and rhizobia.

 摘 要 

为了克服氮缺乏，豆科植物的根与具有固氮能力的根瘤菌建立了共生相互作用，固氮的根瘤菌存在于豆科植物根中的一个特定的器官，即根瘤组织中。与其他器官相似，根瘤的形成是由植物激素生长素在原基部位的局部最大值决定的。然而，生长素是如何调控根瘤的发育仍不清楚。本文中，作者发现大豆PIN转运蛋白GmPIN1驱动的生长素动态转运参与了根瘤原基的形成。GmPIN1在根瘤原基细胞中特异性表达，并存在极性定位。两种结瘤调节因子黄酮和异黄酮诱导了GmPIN1b向根皮层细胞的扩张，并且细胞分裂素重新介导了GmPIN1b的极性。由CRISPR-Cas9介导的Gmpin1abc三突植株中，根瘤分生组织中生长素最大值的建立出现问题，并且根瘤原基细胞出现异常分裂。此外，GmPIN1的过表达抑制了根瘤原基的起始。GmPIN9d是大豆中与拟南芥PIN2直系同源的蛋白，与GmPIN1共同作用于根瘤后期发育，在维管束中向地性运输生长素，精细调控生长素地供应以促进根瘤的扩大。本文的研究结果揭示了PIN依赖的生长素运输是如何作用于大豆根瘤发育的不同方面，并表明生长素梯度的建立是豆科植物与根瘤菌互作的前提。