Understanding the legume-rhizobia symbioses: key role of microscopy techniques

The symbiotic association between certain soil bacteria (rhizobia) and most legumes leads to the formation of root nodules, symbiotic organs in which the fixation of atmospheric nitrogen takes place. This symbiosis is significantly important for the nitrogen input in both agricultural and natural ecosystems, but it can be affected by environmental stresses. Nodulation is a complex process not completely understood; some features differ depending on the legume, and they lead to distinct nodule types. After a specific recognition between the symbionts through the exchange of bacterial and plant signalling molecules, the nodule primordium is induced and rhizobia enter the root cortex and infect cells of this primordium. Bacteria are released into the host cell surrounded by a plantderived membrane and form a new cellular compartment, the symbiosome, which is a nitrogen-fixing microfactory. Based on several characteristics of the symbiosome including its metabolic behaviour, the composite origin of the membrane, the cytoskeletal dynamics and symbiosome segregation during the division of rhizobia-infected cells in certain legume nodules, symbiosome can be considered an organelle-like 1 . Legume nodules are probably one of the most fascinating plant organs. They have been studied for more than a century, and microscopy techniques have been and will be key tools to enhance our understanding of this symbiosis. We will present examples of how microscopy techniques (light, fluorescence, laser confocal, TEM, LTSEM and X-Ray microanalysis, etc.) have been essential to elucidate different aspects of nodule development, structure and functioning, as well as its response to abiotic stress.