Endophytic Occurrence of Diazotrophic Bacteria in Non-Leguminous Crops

Abstract

The recent discovery of obligate endophytic diazotrophs, like Acetobacter diazotrophicus, Herbaspirillum seropedicae and H. rubrisubalbicans colonizing sugar cane roots, stems and leaves, may finally explain the high contributions of BNF to this crop. A. diazotrophicus so far has only been isolated from sugar cane, sweet potatoe and Cameroon grass, all three with high sugar contents and propagated by stem cuttings. Azoarcus spp., another obligate endophytic genus, isolated from Kallar grass (Leptochloa jusca), has been found to establish within wetland rice roots when inoculated. Herbaspirillum seropedicae has been isolated from many graminaceous plants, forage grasses as well as from maize, sorghum and rice, but it does not seem to occur in other plant families. H. rubrisubalbicans so far has only been isolated from sugar cane roots, stems and leaves. These two species, even though clearly identified as two distinct species by DNA homology, are extremely similar and can only be distinguished by growth on n-acetyl-glucosamine (H. seropedicae) and that on meso-erythritol (H. rubrisubalbicans). Transmission of H. seropedicae may occur within seeds or via graminaceous weed plants because even high numbers of this organism inoculated into soil, do not survive for more than one month. Inoculation of endophytic diazotrophs, especially A. diazotrophicus is better achieved by the use of VA mycorrhizae together with bacterial cultures or with spores containing the bacteria. Spores of various VAM species were found to contain this organism and also A. lipoferum and Klebsiella sp. The infection of the root cap zone in micropropagated sugar cane plants with A.diazotrophicus and its colonization of xylem, has been confirmed by optical and electron microscope observations with immunogold-silver-enhanced techniques. Also Herbaspirillum spp. have been localized in the metaxylem after artificial inoculation of sugar cane and sorghum, using the same technique.