Associative Nitrogen Fixation

 

Nitrogen (N) is the major limiting nutrient that promotes growth of most plants. Acquisition and assimilation of N is second in importance only to photosynthesis for plant growth and development. For this reason common agricultural practices make use of chemical inputs including fertilizers to maintain high crop yields. Excessive use of fertilizers, however, can have the adverse effects on the environment through extensive chemical runoff into the waterways. Legumes provide the advantage that, in symbiosis with soil rhizobial bacteria, they can obtain N through biological nitrogen fixation (BNF). However, most agricultural plants, especially grasses, lack this ability and, hence, there has been sustained interest in transferring BNF ability into grass crops, such as corn.

Plant growth promoting bacteria (PGPB) colonize roots and engage in associative symbiosis with various host plants, including bioenergy grass species. In most cases, the mechanism of plant growth promotion is unknown. In selected cases, plant growth promotion is attributed to antagonism toward phytopathogens and/or the induction of plant resistance. Other PGPB may act mostly by phytostimulation (e.g., release of phytohormones). Unlike rhizobia that form an intimate intracellular symbiosis with their legume hosts, PGPB do not induce the formation of observable plant structures. These associations are strictly defined by the lack of any evidence of intracellular infection.

Many PGPB are capable of BNF. However, the role of BNF in plant growth promotion has not been well documented. Most publications simply report the presence of nitrogen fixing PGPB or perhaps the in planta expression of bacterial nitrogenase protein or genes. Only a few rare studies have provided convincing data for fixation in planta and even fewer for incorporation of fixed nitrogen by the plant host. However, the levels of nitrogen fixation reported would provide little or no contribution to the overall nitrogen demand. In contrast, some field studies with wild grass species suggest that BNF can provide 30% or more of the plant nitrogen, attesting to the promise of this approach. What is clearly needed is a tractable experimental system that exhibits appreciable levels of associative nitrogen fixation in which more detailed, mechanistic studies can be conducted.

Associative nitrogen fixation is a relatively new project in the laboratory. We are utilizing the model, C4 grass species, Setariaviridis, as a system for the study of PGPB and the role of BNF. For example, in our recent paper (Pankievicz et al., 2015), we were able to demonstrate significant levels of BNF in S. viridis, as well as incorporation of this fixed nitrogen by the plant. With the appropriate plant and bacterial genotypes, we could demonstrate sufficient nitrogen fixation to support full growth of S. viridis seedlings under conditions of low nitrogen. We are moving forward to exploit this system to define the plant genetic components that support PGPB colonization and BNF.

Recent publications from the lab on this topic:

Pankievicz, Vania C.S., Fernanda P. Amaral, Karina F. D. N. Santos, Beverly Agtuca,
Youwen Xu, Michael J. Schueller, Ana Carolina M. Arisi, Maria. B.R. Steffens, Emanuel M. de Souza, Fabio O. Pedrosa, Richard A. Ferrieri and Gary Stacey (2015) Robust biological nitrogen fixation in a C4 model grass, Setariaviridis. Plant J. (in press; available online)