The contribution of nitrogen fixation to sugarcane (Saccharum officinarum L.), and the identification and characterization of part of the associated diazotrophic bacterial community

Rhizospheric, epiphytic and endophytic bacteria are associated with several non-legumes, colonizing their surface and inner tissues. Many of these bacteria are beneficial to their hosts, and are collectively termed plant growth-promoting rhizobacteria (PGPR). Recent interest has focused particularly upon PGPR that are endophytic (i.e. PGPE), and which have been reported to be associated with important crops such as rice, wheat and sugarcane. Different mechanisms are involved in bacteria-induced plant growth promotion (PGP), including biological nitrogen fixation (BNF), mineral solubilization, production of phytohormones and pathogen biocontrol. In Uruguay, sugarcane (Saccharum officinarum L.) is considered a strategic multipurpose crop, used for bioenergy, feed, sugar and bioethanol production. The aim of this work was to estimate the BNF contribution to Uruguayan sugarcane cultivars, as well as to identify and characterize the (culturable) putatively endophytic diazotrophic bacteria associated with these varieties. Results using the N-15-dilution technique have shown that these sugarcane varieties obtain significant inputs of N from BNF (34.8-58.8% Ndfa). In parallel, a collection of 598 isolates of potentially endophytic diazotrophs was obtained from surface-sterilized stems using standard isolation techniques, and nifH (+) isolates from these were the subject of further studies. The bacteria were shown to belong to several genera, including Pseudomonas, Stenotrophomonas, Xanthomonas, Acinetobacter, Rhanella, Enterobacter, Pantoea, Shinella, Agrobacterium and Achromobacter. Additionally, some PGP features were studied in 35 selected isolates. The data obtained in this study represent the initial steps in a program aimed at determining the mechanisms of PGP of non-legume crops in Uruguay (such as sugarcane) with potentially beneficial plant-associated bacteria.