Acetobacter diazotrophicus is a nitrogen‐fixing endophytic bacterium, originally isolated from sugarcane. Its colonizing ability was evaluated in high and low N‐fertilized sugarcane plants by inoculating stem‐cuts with a β‐glucuronidase marked A. diazotrophicus strain. Bacterial quantification by the most probable number technique showed a severe decrease of A. diazotrophicus cells in plants fertilized with high levels of nitrogen. The inoculated strain was detected inside low N‐fertilized sugarcane plants by histochemical staining of β‐glucuronidase and scanning electron microscopy. A. diazotrophicus was found mainly inside cortical cells of stems and inside xylem vessels. No β‐glucuronidase activity was observed in non‐inoculated plants. High nitrogen fertilization of fields might be a threat to maintaining naturally occurring endophytic associations.

Different experiments have estimated that the contribution of biological nitrogen fixation (BNF) is largely variable among sugarcane cultivars. Which bacteria are the most important in sugarcane-associated BNF is unknown. However, Gluconacetobacter diazotrophicus has been suggested as a strong candidate responsible for the BNF observed. In the present study, bacteria-free micropropagated plantlets of five sugarcane cultivars were inoculated with three G. diazotrophicus strains belonging to different genotypes. Bacterial colonization was monitored under different nitrogen fertilization levels and at different stages of plant growth. Analysis of the population dynamics of G. diazotrophicus strains in the different sugarcane varieties showed that the bacterial populations decreased drastically in relation to plant age, regardless of the nitrogen fertilization level, bacterial genotype or sugarcane cultivars. However, the persistence of the three strains was significantly longer in some cultivars (e.g., MEX 57-473) than in others (e.g., MY 55-14). In addition, some strains (e.g., PAl 5(T)) persisted for longer periods in higher numbers than other strains (e.g., PAl 3) inside plants of all the cultivars tested. Indeed, the study showed that the inoculation of G. diazotrophicus may be beneficial for sugarcane plant growth, but this response is dependent both on the G. diazotrophicus genotype and the sugarcane variety. The most positive response to inoculation was observed with the combination of strain PAl 5(T) and the variety MEX 57-473. Although the positive effect on sugarcane growth apparently occurred by mechanisms other than nitrogen fixation, the results show the importance of the sugarcane variety for the persistence of the plant-bacteria interaction, and it could explain the different rates of BNF estimated among sugarcane cultivars.

Azospirillum isolates were obtained from rhizosphere soil and roots of three cactaceae species growing under arid conditions. All Azospirillum isolates from rhizosphere and roots of Stenocereus pruinosus and Stenocereus stellatus were identified as A. brasilense; isolates of surface-sterilized roots from Opuntia ficus-indica were both A. brasilense and A. lipoferum. Azospirilla per g of fresh root in the three species ranged from 70 x 10 3 to 11 • 10 3. The most active strains in terms of C2H2 reduction (25-49.6nmol/h-ml) and indoleacetic acid (IAA) production (36.5-77/~g/ml) were those identified as A. brasilense and isolated from Stenocereus roots. A. lipoferum isolated from Opuntia roots produced low amounts of IAA (6.5-17.5/tg/ml) and low C2H2-reduction activity (17.8-21.2 nmol/h, ml).

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