Prospects for Significant Nitrogen Fixation in Grasses From Bacterial Endophytes

Abstract

Since the discovery of biological nitrogen fixation by Hellreigel and Willfarth (1888), there have been two primary goals of nitrogen-fixation research. The first has been to understand the mechanism of action of nitrogenase as a catalyst for N2 reduction to ammonia. One long-term objective of this goal is the chemical synthesis of an efficient catalyst for N2 reduction that could be used for the lowtemperature low-pressure synthesis of ammonia. The second goal involves the indepth understanding of the various associations between nitrogen-fixing bacteria and plants so that (i) the symbiotic association between the legume host and rhizobia can be improved for increased agricultural production and (ii) the delivery of fixed-N to non-legumes can be optimized. Recent progress in understanding the legume-rhizobia symbiosis has been very impressive and will not be reviewed here. It may be that this knowledge will lead to the induction of nitrogen-fixing root nodules on grasses, such as wheat and rice. However, the discovery of nitrogen fixation in sugarcane over a decade ago has captured the imagination of a growing group of scientists around the world and has led to the study of endophytic nitrogen-fixing bacteria. These are microbes that inhabit the interior of grasses without causing either disease or the induction of any organized symbiotic structures. The progress made to date in the nitrogen-fixation system of sugarcane was reviewed recently (Boddey et al., 2003; see also Chapter 10 of this volume) and will only be briefly summarized here. Several important questions are now being asked about both the sugarcane system and other endophyte-grass associations that provide high levels of fixed-N to