Plant and Soil 154: 145-150, 1993. © 1993 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 9963 Acetobacter diazotrophicus, an indoleacetic acid producing bacterium isolated from sugarcane cultivars of M~xico L.E. FUENTES-RAMIREZ, T. JIMENEZ-SALGADO, I.R. ABARCA-OCAMPO and J. CABALLERO-MELLADO 1 Centro de lnvestigaciones Microbiol6gicas, lnstituto de Ciencias, Universidad Aut6noma de Puebla, Apartado Postal No. 1622, Puebla, Pue. M~xico, C.P. 72000; J To whom correspondence should be sent. Present address: Departamento de GenOtica Molecular, Centro de lnvestigaci6n sobre Fijaci6n de Nitr6geno, UNAM, Apartado Postal 565-A, Cuernavaca, Mor. M~xico Received 9 February 1993. Accepted in revised form 4 June 1993 Key words: auxins, diazotrophic bacteria, endophytic bacteria, sugarcane Abstract Thirteen cane cultivars grown on fields in M6xico were sampled to assess the occurrence of Acetobacter diazotrophicus, a recently identified N2-fixing bacterium. Results showed that the isolation frequencies extended over a broad range (1.1 to 67%), likely to be related to the nitrogen fertilization level. The lowest isolation frequencies (1.1 to 2.5%) were obtained from plants growing at high nitrogen doses (275-300 kg ha -~) and the highest values (10-67%) from plants cultivated with 120 kg N ha -~. All eighteen strains of A. diazotrophicus produced indoleacetic acid (IAA) in defined culture medium. Estimates obtained from HPLC analyses revealed that A. diazotrophicus strains produced from 0.14 to 2.42/xg IAA mL J in culture medium. Considering that A. diazotrophicus is found within the plant tissue, the biosynthesis of IAA suggests that the bacteria could promote rooting and improve sugarcane growth by direct effects on metabolic processes, in addition to their role in N, fixation. Introduction Nitrogen fixing bacteria are commonly found in association with the roots of diverse plants. Frequently several species of diazotrophs can be isolated from the same plant (Patriquin et al., 1983), but according to the technique used the type of bacteria differs (Balandreau, 1983). Thus, D6bereiner (1961) found Beijerinckia spp. in 95% of the rhizosphere soil samples of sugar- cane. Prevalence of Azotobacter chroococcum in the rhizosphere of sugarcane was reported (Zafar et al., 1986). Rennie et al. (1982) isolated Klebsiella pneumoniae, Enterobacter cloacae, Er- winia herbicola and Bacillus polymyxa, from inside sett and roots of sugarcane, but neither Beijerinckia nor Azotobacter spp. were found. Although several nitrogen-fixing bacteria have been isolated from sugarcane, it still remains unknown which of these bacteria are the most important in the plant-associated biological ni- trogen fixation. Recently, Acetobacter diazotrophicus, a N~- fixing bacterium has been isolated from sugar- cane roots and inside stems collected in various sites of Brazil (Cavalcante and D6bereiner, 1988) and of Australia (Li and Macrae, 1991). Since this bacterium is able to fix N 2 even in presence of nitrates and it seems best adapted to the sugarcane environment (Cavalcante and D6bereiner, 1988), it could have more economic importance compared with other diazotrophs associated with sugarcane. The ability to synthesize auxins has been