INFECTION AND COLONIZATION OF SUGAR-CANE AND OTHER GRAMINEOUS PLANTS BY ENDOPHYTIC DIAZOTROPHS

Agriculturally important grasses such as sugar cane (Saccharum sp.), r ice (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 a re associated and the more numerous of them, for example, Herbaspirill um 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, veg etative propagation, dead plant material, and possibly by insect sap f eeders. By contrast, Azospirillum spp. are not wholly endophytic but a re root-associated, soil-dwelling bacteria that are also often found w ithin plants, probably entering host plants via seeds or via wounds/cr acks at lateral root junctions. Endophytic diazotrophs have been isola ted from a number of grasses in which significant biological N-2 fixat ion (BNF) has been demonstrated, particularly Brazilian sugar cane var ieties, but also in rice, maize, and sorghum. However, although the en dophytic diazotrophs are held to be the causative agents of the observ ed BNF, direct evidence for this is lacking. Therefore, in this review we examine probable sites of bacterial multiplication and/or BNF with in endophyte-containing grasses and discuss these in terms of potentia l benefits (or not) to both host plants and bacteria. In particular, w e examine how potentially large numbers of bacteria, especially Herbas pirillum spp., A. diazotrophicus, and Azospirillum spp., can exist ext racellularly within non-specialized (for symbiotic purposes) regions s uch as xylem vessels and intercellular spaces. The processes of infect ion and colonization of various grasses (particularly sugar cane) by d iazotrophic 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.