Nitrogen-fixing bacteria in non-legumes

The process of biological nitrogen fixation, in which bacteria pass fixed nitrogen on to a plant host, is well known in the legume-rhizobia system. Less well known, but equally intriguing, is the ability of some grasses to harbor nitrogen-fixing bacteria. The plant-bacteria association is less distinct in the case of grasses, since the bacteria may be dispersed in low numbers in or near roots (associative fixation) or ensconced within cell walls inside the plant (endophytic fixation). It has been difficult to define the significance of nitrogen fixation in these systems. The most studied system in this regard is the association between sugarcane and endophytic bacteria such as Gluconoacetobacter diazotrophicus. This association appears to be responsible for the ability of sugarcane to produce large crops for many years without the addition of nitrogen fertilizers. Experiments comparing inoculated and uninoculated sugarcane plants show that nitrogen-fixing bacteria provide a strong benefit for the host plant. However, some of those benefits may arise from the ability of the bacteria to produce phytohormones. Other similar systems include: 1) various tropical grasses such as Digitaria and Paspalum with bacteria such as Azospirillum; 2) Kallar grass (Leptochloa fusca) with Azoarcus; and 3) a wide range of host plants (coffee, maize, dune grasses) with the widespread bacteria Burkholderia. PCR-based screens of DNA isolated directly from environmental samples suggest that associative/endophytic nitrogen fixation is more widespread than generally acknowledged.