Strategies for enhancement of switchgrass (Panicum virgatum L.) performance under limited nitrogen supply based on utilization of N-fixing bacterial endophytes

Sustainable agricultural production in the 21st century requires new approaches to reduce the use of synthetic nitrogen fertilizers. A newly recognized option is biological nitrogen fixation by commensal bacterial endophytes. The aim of this project was to explore strategies for supplying biologically fixed nitrogen to a bioenergy crop, switchgrass cv. Alamo. The tested strategies were: 1) harnessing the ability of horizontal gene transfer between a known N-fixing bacterium, Burkholderia phymatum STM 815, and a switchgrass growth promoting endophyte, Burkholderia phytofirmans strain PsJN, and 2) isolation and utilization of naturally occurring N-fixing endophytes from seeds of switchgrass cv. Alamo. The ability to grow on nitrogen free medium was successfully transferred from B. phymatum STM 815 to B. phytofirmans strain PsJN. The resulting bacterium, PsJN+, outperformed PsJN in switchgrass growth promotion in vitro on a low nitrogen (75 mg/L) medium (69 % increase). An endophyte with FAME and 16S sequence most similar to Sphingomonas sp. was isolated from seedlings derived from surface sterilized seeds germinated and grown in nitrogen-free hydroponic medium, and was also able to promote switchgrass growth under low nitrogen conditions (27 % increase over control). A plant growth promoting endophyte Burkholderia phytofirmans strain PsJN transformed with genomic DNA containing the nif operon from Burkholderia phymatum STM 815, and Sphingomonas sp. strain NSL, a naturally occurring switchgrass seed endophyte capable of nitrogen fixation, were able to promote in vitro growth of switchgrass under low nitrogen conditions.