NOTE / NOTE Association of Gluconacetobacter diazotrophicus with roots of common bean (Phaseolus vulgaris) seedlings is promoted in vitro by UV light Alejandra Trujillo-Lo ´ pez, Oscar Camargo-Zendejas, Rafael Salgado-Garciglia, Horacio Cano-Camacho, Vı ´ctor M. Baizabal-Aguirre, Alejandra Ochoa-Zarzosa, Joel E. Lo ´ pez-Meza, and Juan J. Valdez-Alarco ´n Abstract: Gluconacetobacter diazotrophicus is a diazotrophic endophyte that is a potential biofertilizer. Little is known about the mechanisms of G. diazotrophicus interaction with its host plants. We tested the effect of UV light, as an inducer of secondary metabolite accumulation, on the association between common bean (Phaseolus vulgaris L.) seedling roots and G. diazotrophicus. UV light irradiation of seedlings 4 h prior to bacterial inoculation increased the number of bacterial cells associated with the roots by 5.65-fold with respect to a nonirradiated control (p < 0.05). Gluconacetobacter diazotro- phicus associates with root hairs and root border cells. Aggregation of bacterial cells was observed in root structures from UV-induced seedlings. Secondary metabolite accumulation was also observed in roots from UV-irradiated seedlings. Key words: Gluconacetobacter diazotrophicus, secondary metabolism, plant-diazotroph interaction. Re ´sume ´: Le Gluconacetobacter diazotrophicus est un endophyte diazotrophe ayant un potentiel comme biofertilisant. On connaı ˆt peu de choses a ` propos des me ´canismes de l’interaction du G. diazotrophicus avec ses plantes ho ˆtes. Les auteurs ont ve ´rifie ´ l’effet de la lumie `re UV, comme inducteur de l’accumulation de me ´tabolites secondaires, sur l’association des racines de plantules du haricot commun (Phaseolus vulgaris L.), avec le G. diazotrophus. L’irradiation des plantules avec des UV, 4 h avant l’inoculation bacte ´rienne, augmente le nombre de cellules bacte ´riennes associe ´es aux racines par un fac- teur de 5,65, par rapport au te ´moin non irradie ´(p < 0,05). Le G. diazotrophicus s’associe aux poils absorbants et aux cel- lules de bordures racinaires. On observe une agre ´gation des cellules bacte ´riennes dans les structures des racines, chez les plantules stimule ´es par les UV. Mots cle ´s : Gluconacetobacter diazotrophicus, me ´tabolisme secondaire, interaction plante–diazotrophe. [Traduit par la Re ´daction] Introduction Gluconacetobacter diazotrophicus (syn. Acetobacter diaz- otrophicus; Yamada et al. 1998) is a diazotrophic endophyte associated with sugarcane and other grammineae used for human and animal feeding. Gluconacetobacter diazotrophi- cus has a wide host range, is tolerant to extreme osmolarity and pH, and does not have nitrate reductase activity. It also produces indoleacetic acid, cytokinins, and gibberellins. Its capacity to synthesize phytohormones and to fix nitrogen provides beneficial effects to sugarcane (Muthukumarasamy et al. 2002). Taken together, these properties suggest that G. diazotrophicus could be used as a biofertilizer for a range of agronomically important crops. The molecular mechanisms by which G. diazotrophicus interacts with its host plants have not been described. A search of sugarcane sequence tags (ESTs) differentially ex- pressed during the interaction with G. diazotrophicus re- vealed a cDNA that encodes chalcone synthase, a key enzyme of the flavonoid biosynthetic pathway (Nogueira et al. 2001). This suggests that secondary metabolite accumula- Received 22 September 2005. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 11 April 2006. A. Trujillo-Lo ´pez. Instituto de Ciencias, Beneme ´rita Universidad Auto ´noma de Puebla, Apdo. Postal 1622, 72000, Puebla, Me ´xico. O. Camargo-Zendejas, H. Cano-Camacho, V.M. Baizabal-Aguirre, A. Ochoa-Zarzosa, J.E. Lo ´pez-Meza, and J.J. Valdez-Alarco ´n. 1 Centro Multidisciplinario de Estudios en Biotecnologı ´a, Facultad de Medicina Veterinaria y Zootecnia, Universidad Michoacana de San Nicola ´s de Hidalgo, Apdo. Postal 53, Administracio ´n Chapultepec, 58262, Morelia, Michoaca ´n, Me ´xico. R. Salgado-Garciglia. Instituto de Investigaciones Quı ´mico Biolo ´gicas, Universidad Michoacana de San Nicola ´s de Hidalgo, Edificio B1, Cd. Universitaria, C.P. 58040, Morelia, Michoaca ´n, Me ´xico. 1 Corresponding author (e-mail: jjvaldez@zeus.umich.mx). 321 Can. J. Bot. 84: 321–327 (2006) doi: 10.1139/B05-169 # 2006 NRC Canada