Exploiting Natural Processes to Reduce the Need for Petroleum-Based Fertilizers: The Role of Bacterial Chemotaxis in Nitrogen Fixation of Leguminous Plants

Biological nitrogen fixation is a natural process
that supplies nitrogen to the soil and reduces the need for petroleum-based
fertilizer. Soil bacteria belonging to the genus Rhizobium form a symbiotic relationship with leguminous plants to
fix atmospheric nitrogen into ammonia, a form that can be taken up through the
vascular system of all plants once released into the soil. This process
operates through a specific signaling cycle that induces the migration of
rhizobia and formation of nitrogen-fixing nodules on the roots of the host
legume.² It has been shown experimentally that the directed
migration of the bacteria is governed by a mechanism called chemotaxis.^3,4 Chemotaxis is a physical motile response of
bacteria to environmental signals and concentration gradients. It occurs when chemical
cues bind with receptor molecules of the rhizobia and alter the swimming
pattern and bias the direction of movement towards the source. This chemotactic
behavior of the rhizobia is an important trait that facilitates the initial
contact and adsorption of symbiotic rhizobia to the host root surface, increases
the efficiency of nodule initiation, and increases the rate of infection
development.^4,5

The aim of this study is to quantify the effect that
soil moisture content has on rhizobia chemotaxis and nodule formation. A series
of rhizotron experiments were conducted to demonstrate that moisture content
impacts nodule formation, with special attention given to nodule primordia
formation. Vigna unguiculata,
commonly known as the cowpea, was grown in hydroponic conditions on horizontally
oriented, rockwool layered petri dishes. Bacteria (Bradyrhizobium spp) were inoculated via a peat
slurry at the end opposite the seed, as shown in figure 1. This rhizotron
design provided a 144 cm² observation area for root growth, bacterial
migration, and nodule formation. Observation of nodule primordia formation at
root locations farthest from the inoculation site indicated chemotactic migration
had occurred. By systematically reducing the amount of water supplied, a water
content level was reached, at which there was an absence of nodule formation.
This suggested a percolation threshold at which a continuous liquid pathway for
bacterial motility no longer existed. The next step of this project is to
quantify the impact of moisture content. This will be done using a Buchner-funnel
tensiometer. With this device, the water content can be controlled in certain
pore sizes by adjusting the hydrostatic pressure. With this experiment, it will
be possible to identify the desired levels of moisture in soil to facilitate
nodule formation. These results will be compared to climate conditions in order
to assess the potential for optimizing this natural process for a more
sustainable alternative to petroleum-based fertilizers for crop nutrition.

Figure 1: Example of rhizotron set-up from above
view point. Chemotactic migration can be determined from location of nodule
formation.

1.      Food and Agricultural Organization (2008). Retrieved November
26, 2015, from http://www.fao.org/soils-portal/soil-degradation-restoration/en/

2.      Bergman, K. (1990, January). The Role of Bacterial Chemotaxis
in Initiation of Rhizobium Legume Symbiosis for Nitrogen-Fixation. Journal of Chemical Ecology, 16(1), 114-115.

3.     
Caetano-Anollés, G., Wall, L. G., De Micheli,
A. T., Macchi, E. M., Bauer, W. D., & Favelukes, G. (1988). Role of
motility and chemotaxis in efficiency of nodulation by Rhizobium meliloti. Plant Physiology, 86(4), 1228-1235.

4.     
Soby, S., & Bergman, K. (1983). Motility
and chemotaxis of Rhizobium meliloti in soil. Applied
and environmental microbiology, 46(5),
995-998.