Recent advances in BNF with non-legume plants

It is now well accepted that nitrogen-fixing bacteria colonising graminaceous plants can be grouped into three categories: 1, rhizosphere organisms; 2, facultative endophytes and 3, obligate endophytes. In the first category are included all species that colonise the root surface such as Azotobacter paspali, Beijerinkia spp. Facultative endophytes are these nitrogen-fixing bacteria that can colonise the surface and interior of the roots principally the four species of Azospirillum, except Azospirillum halopraeferans. The third category is constituted mainly by diazotrophs isolated more recently such as Acetobacter diazotrophicus, Herbaspirillum spp and Azoarcus spp which are able to colonise the root interior and aerial tissues of the plants. Although most of the studies related to nitrogen fixing bacteria have been concentrated on Azospirillum spp, it is the obligate endophytes, isolated more recently, that have attracted the attention of scientists working in this field. The ability to colonise the entire plant interior and locate themselves within niches protected from oxygen or other factors make them the most promising group of diazotrophs associated with graminaceous and other non-leguminous plants. In this review we compare these three groups of nitrogen-fixing bacteria, their interaction with the host plants and discuss the potential of their use in agriculture.