Abstracts of Presentations at IS-MPMI XVIII Congress

Rhizobia are a diverse group of Alpha- and Beta-proteobacteria that undergo a mutualistic relationship with legume plants. Intense chemical crosstalk between rhizobia and the plant host takes place during symbiotic establishment. One major bacterial signaling factor are secreted lipochito-oligosaccharides. These so-called Nod-factors trigger root hair curling in the host which entraps root hair-attached bacteria. From there rhizobia enter the root of host plants via infection threads. These are plant-derived structures which guide the bacteria into the root cortex where they are released into plant cells. Eventually, rhizobia will differentiate into nitrogen-fixing bacteroids and exchange the fixed nitrogen for reduced carbon sources and other nutrients. Within infection threads rhizobia are enclosed completely by plant tissue and hence, all nutrients needed for growth must be provided by the host. Likewise, rhizobia growing in the rhizosphere receive most of their nutrients from root exudates. So far, the microscopic nature of early infection stages has hampered biochemical experiments and the physico-chemical nature of these important structures remains unknown. A novel experiment using INseq has revealed genes of the pea symbiont Rhizobium leguminosarum bv. viciae 3841 which are essential or advantageous during the infection process. Here, we describe mutations in such metabolic genes and their effect on the symbiotic efficiency of R. leguminosarum.