Proteomic analysis of legume–microbe interactions

Legumes are very special plants. They form symbioses not formed by most other plants, including Arabidopsis, and are important sources of protein for animals and humans. Legume roots are invaded and colonized by the nitrogen-fixing soil bacteria called rhizobia [29], and also with mycorrhizal fungi which contribute to plant phosphorus acquisition [16]. The development of the root nodule meristem is unique, as its site, timing of initiation, the target cell type and the ontogeny can be defined. The elicitor (the nod factor) for cell division is known and can be synthesized. The beauty of the interaction is that the precise cell biology changes have been documented and many bacterial and some plant genes affecting nodulation have been characterized. In addition, mutants are available in both symbiotic partners. Within the nodule the nitrogen-fixing rhizobia, known as bacteroids, are surrounded by a host-derived membrane called the peribacteroid membrane (PBM) which controls molecular exchanges between the bacteroid and the legume cell [26]. The elicitor, or Rhizobium nod factor responsible for nodule initiation, is a lipochitin oligosaccharide [LCO] and plays a pivotal role in the induction of symbiotic developmental responses in legumes, leading to the formation of a nodule [29]. Proteomics is an ideal tool for the dissection of plant microbe interactions. First, it provides a broad overview of the proteins produced by both partners during their constant signal exchange and, in particular, it enables the effect on gene product networks of gene knockouts, additions and specific growth states to be determined. Second, it allows the detection of signal transduction pathways by following phosphorylation changes of proteins [27] that are important for protein function. The recent discovery of several plant receptor kinases responsible for the early detection and signal transduction of Nod factor perception [10,30] and autoregulation of nodule numbers [17,23,28], suggests that many early plant–microbe signalling events are regulated by phosphorylation events and key receptor kinases.