Nitrogen fixation in grasses - gluconacetobacter activates genes in sugarcane

Abstract

Nitrogen-fixing bacteria have been isolated from sugarcane (Saccharum spp.) and other grasses in an endophytic and beneficial interaction that promotes plant growth. In this interaction, bacteria colonize the intercellular spaces and vascular tissues of most plant organs without causing disease symptoms. The best characterized sugarcane endophytic diazotrophs are Gluconacetobacter diazotrophicus, clustered in the alpha subclass of Proteobacteria, and Herbaspirillum seropedicae, Herbaspirullum rubrisubalbicans and Burkholderia sp, clustered in the beta subclass of the Proteobacteria (Reis et al. 2000). These particular types of endophytic plant growth promoting bacteria (PGPB) can offer several benefits to host plants such as to provide nitrogen trough Biological Nitrogen Fixation (BNF) and to produce plant growth hormones (eg. auxins and gibberellins), promoting plant development, increase in biomass, defense against pathogens and tolerance to abiotic stresses [1,2]. In Brazil, BNF plays a fundamental role in sugarcane cultivation by reduction of the use of nitrogen fertilizers, making Brazilian sugarcane culture more competitive in global markets. It has been suggested that the relatively low mineral N-inputs used for Brazilian sugarcane production over the last 100 years has historically selected for varieties with a low response to applied mineral N and a high N2-fixing ability [3]. Studies on the quantification of BNF to the Brazilian sugarcane varieties, using 15N isotope dilution and 15N natural abundance, have demonstrated that the amount of fixed N2 can be highly variable. Very large BNF-inputs were observed in several sugarcane varieties, especially the wild noncommercial species Krakatau (Saccharum spontaneum) used in plant breeding in Brazil, as well as the commercial varieties SP 70-1143 and CB 45-3, that exhibit high yields in lowfertility soils. SP70-1143 could obtain 72% of N-requirement from BNF, while Chunee (Saccharum barberi), a wild noncommercial species, obtained only 14% [3,4]. An important biotechnological challenge of this century is to develop tools to apply for a sustainable agriculture, that would increase productivity using less fertilizers, pesticides, water and cultivated area. The associations that occur between sugarcane and other grasses with nitrogenfixing endophytic bacteria have raised a large interest in their use in agriculture, in view of the positive effects on root development, and the increase in biomass and productivity. Experiments carried out at EMBRAPA Agrobiologia have shown significant results in biomass increase and grain yield due the use of diazotrophic endophytic bacteria inoculation in sugarcane, maize and rice inoculants. However, studies have also shown that the plant genotype and the environment where the association is established can influence the degree of beneficial results obtained by the plant caused by the association with endophytic bacteria. This plant / nitrogen-fixing endophytic bacteria interaction represents a novel system of beneficial plantmicroorganism association, which has unique features that remain to be characterized. The studies on BNF quantification have indicated that the selection of the best combination of endophytic diazotroph strains as well as sugarcane varieties needs to be exploited to obtain the maximum benefit from this association in agriculture. The signaling pathways by which sugarcane plants can decipher bacterial signals and respond properly for a successful association, by controlling endophyte recognition, colonization and nitrogen fixation rates, are still not clearly understood. Therefore, the major goal of our research is to develop biotechnological tools to help to obtain novel plant varieties more responsive to bacteria Laboratorio de Biologia Molecular de Plantas, Instituto de Bioquimica Medica, Centro de Ciencias da Saude, Universidade Federal do Rio de Janeiro, 21.941-590, Rio de Janeiro, RJ, Brazil Full list of author information is available at the end of the article Carvalho et al. BMC Proceedings 2014, 8(Suppl 4):O20 http://www.biomedcentral.com/1753-6561/8/S4/O20