The infection thread is an intracellular tunnel which develops in the first steps of the nodulation process, through which the invading rhizobia infect the legume roots. Several plant components identified in the lumen of infection threads suggest a role of reactive oxygen species in regulating the rate of colonisation by Rhizobium and in controlling the success of the symbiosis. The purpose of this proposal is to study the role of pea diamine oxidase, an abundant enzyme in legume root tissues, as the potential source of hydrogen peroxide. The objectives are to demonstrate by a combination of approaches involving biochemistry and in situ localisation studies that diamine oxidase is implicated in the oxidative cross-linking of some components of the matrix of infection threads. Reporter-gene fusions and genetic modification of diamine oxidase gene expression will be achieved to investigate the importance of this gene as a regulator of nodule development.