The role(s) played by plant hormones in the regulation of symbiotic structures

我研究了某些有益细菌、根瘤菌和豆科植物之间存在的相互作用。这种相互作用的结果是，根瘤菌穿透植物根毛，触发一种被称为根瘤的新结构的形成。如果共生成功，根瘤菌现在寄生在根瘤内，就可以固定大气中的氮。当氮固定过多时，植物就可以利用氮素。早期感染阶段被认为需要跨越根部皮质的两个相反的化学梯度；伙伴成功地整合这些梯度导致根瘤的启动。第一个梯度本质上是根瘤菌；被称为结瘤因子的复杂分子被认为指向根中心。这种梯度触发了内皮层的第一次细胞分裂和根毛的第一次形态变化。另一种指向根表面的梯度被认为涉及植物激素，特别是生长素和细胞分裂素(CK)。在这个应用中，我提出，为了响应根瘤菌的感染，来自根中心的维管组织的CKs穿过几个细胞层，积累到内部皮质细胞中，在那里它们促进细胞分裂。我推测，聚集在这些细胞中的CKs为根瘤菌提供了位置信息，即它们起到了灯塔的作用，将根瘤菌吸引到细胞群中。为了证明Cks是结核形成的重要信号，我将(1.)创造CK水平改变的植物嵌合体；接种嵌合体，在细胞水平上追踪根瘤菌的感染情况；遵循特定感染步骤的分子标志物的表达。这里使用的方法可能会促进豌豆作为模式植物系统的使用，因为这种植物现在可以接受分子技术。在遥远的将来，我的研究计划中所获得的结果可能会导致开发出一种方法，这种方法可以超越阻止根瘤菌共生的适当建立的机制。从农学角度来看，这一发展是非常有价值的，因为根瘤菌对植物的成功感染是减少作物上使用化肥的关键一步。

I study the mutualistic interaction which exists between certain beneficial bacteria, the rhizobia, and legume plants. As the result of this interaction, the rhizobia penetrate the plant root hairs and trigger the formation of a novel structure known as the nodule. If the symbiosis is successful, the rhizobia, now hosted within the nodule, fix atmospheric nitrogen. When fixed in excess, the nitrogen becomes available to the plant. The early infection stages are thought to require two opposing chemical gradients across the cortex of the root; the successful integration of these gradients by the partners results in the initiation of the nodule. The first gradient is rhizobial in nature; complex molecules known as Nod factors are thought to be directed towards the root centre. This gradient triggers the first cell divisions of the inner cortex and the first morphological changes of the root hairs. The other gradient oriented towards the root surface is thought to involve plant hormones, specifically auxin and cytokinin (CK). In this application, I propose that, in response to the rhizobial infection, CKs coming from the vascular tissues located in the centre of the root move across several cell layers and accumulate into the inner cortical cells where they promote cell divisions. I postulate that the CKs pooled in these cells give positional information to the rhizobia, i.e., they act as a beacon, "attracting" the rhizobia towards the cells cluster. To demonstrate that CKs are important signals for nodule initiation, I will (1.) create plant chimeras with altered CK levels; (2.) inoculate the chimeras and track the rhizobial infection at the cellular level; and (3.) follow the expression of molecular markers specific for certain steps of the infection. The methods used here will likely boost the use of pea as a model plant system as this plant is now amenable to molecular techniques. In the far future, results obtained in my research program could lead to the development of methods that could override mechanisms which prevent the proper establishment of the rhizobial symbiosis. Agronomically, this development would be extremely worthwhile since successful infections of plants by rhizobia are a key step to reducing the use of chemical fertilizers on crop plants.