Improving a “Generation 1.5” biofuel feedstock crop: Colonization and growth enhancement of energy beet (Beta vulgare L. Beta 5833R) by inoculation with Gluconacetobacter spp.

•Inoculation with Gluconacetobacter spp. resulted in enhanced of growth of sugar beet.•G. diazotrophicus colonized root tips, root hairs and lateral root junctions.•N2 fixation by Gluconacetobacter spp. was demonstrated within the plant.•Growth increases due to inoculation was greater at lower levels of nitrate supply.•Aside from N2 fixation, other mechanisms may have stimulated beet growth.

“Generation 1.5” biofuels are derived from biomass feedstocks that originated as food crops, but with selective breeding and modified production management, are more energy dense and have lower carbon footprints than their food crop progenitors. Energy beets, specialized genotypes of sugar beet (Beta vulgaris L.), grown under the proper low-carbon footprint conditions, is an example of such a Generation 1.5 feedstock crop. In this study, Beta vulgaris variety-Beta 5833R was tested for its responses to 14 different strains of the growth-promoting bacterium, Gluconacetobacter spp. in greenhouse studies at low (1 or 2 mM) and high (10 mM) levels of NO3- supplied to the plants. Using a β-glucuronidase (GUS)-labeled strain of the bacterium, confirmed that G. diazotrophicus is able to colonize the plant with root tips, root hairs and lateral root junctions being the major infection sites. Using 15N-isotope dilution technique, the present work demonstrates for the first time that N2 fixation associated with Gluconacetobacter spp. increased N accumulation and lead to increases of up to 110% in sugar beet biomass. However, biomass increases in sugar beet were still evident even at higher levels of NO3- supply even though N2 fixation by the bacterium was quite low, suggesting that other mechanisms may also have been at work in the growth promotion of sugar beet by Gluconacetobacter spp.