RECENT ADVANCES IN BNF WITH NON-LEGUME PLANTS

It is now well accepted that nitrogen-fixing bacteria colonising grami naceous plants can be grouped into three categories: 1, rhizosphere or ganisms; 2, facultative endophytes and 3, obligate endophytes. In the first category are included all species that colonise the root surface such as Azotobacter paspali, Beijerinckia spp. Facultative endophytes are those nitrogen-fitting bacteria that can colonise the surface and interior of the roots principally the four species of Azospirillum, e xcept Azospirillum halopraeferans. The third category is constituted m ainly by diazotrophs isolated more recently such as Acetobacter diazot rophicus, Herbaspirillum spp and Azoarcus spp which are able to coloni se the root interior and aerial tissues of the plants. Although most o f the studies related to nitrogen fixing bacteria have been concentrat ed on Azospilillum spp, it is the obligate endophytes, isolated more r ecently, that have attracted the attention of scientists working in th is 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 graminace ous and other non-leguminous plants. In this review we compare these t hree groups of nitrogen-fixing bacteria, their interaction with the ho st plants and discuss the potential of their use in agriculture. (C) 1 997 Elsevier Science Ltd.