Agriculturally important grasses su

Agriculturally important grasses such as sugar cane (Saccharum sp.), rice (Oryza
sativa), wheat (Triticum aestivum) sorghum (Sorghum bicolor), maize (Zea mays), Panicum maximum,
Brachiaria spp., and Pennisetum purpureum contain numerous diazotrophic bacteria, such
as, Acetobacter diazotrophicus, Herbaspirillum spp., Azospirillum spp. These bacteria do not
usually cause disease symptoms in the plants with which they are associated and the more numerous
of them, for example, Herbaspirillum spp. and A. diazotrophicus, are obligate or facultative endophytes
that do not survive well (or at all) in native soil; these are thought to be spread from plant
generation to plant generation via seeds, vegetative propagation, dead plant material, and possibly
by insect sap feeders. By contrast, Azospirillum spp. are not wholly endophytic but are rootassociated,
soil-dwelling bacteria that are also often found within plants, probably entering host
plants via seeds or via wounds/cracks at lateral root junctions. Endophytic diazotrophs have been
isolated from a number of grasses in which significant biological N2 fixation (BNF) has been
demonstrated, particularly Brazilian sugar cane varieties, but also in rice, maize, and sorghum.
However, although the endophytic diazotrophs are held to be the causative agents of the observed
BNF, direct evidence for this is lacking. Therefore, in this review we examine probable sites of
bacterial multiplication and/or BNF within endophyte-containing grasses and discuss these in
terms of potential benefits (or not) to both host plants and bacteria. In particular, we examine how
potentially large numbers of bacteria, especially Herbaspirillum spp., A. diazotrophicus, and Azospirillum
spp., can exist extracellularly within non-specialized (for symbiotic purposes) regions
such as xylem vessels and intercellular spaces. The processes of infection and colonization of
various grasses (particularly sugar cane) by diazotrophic endophytes are also described, and these
are compared with those of important (nondiazotrophic) endophytic sugar cane pathogens such
as Clavibacter xyli subsp. xyli and Xanthomonas albilineans.