Workin' Over Time: Rhizobium and Legumes

Introduction: Rhizobium are motile, rod shaped, gram negative bacteria.  They are particularly interesting because of their ability to form nodules with the roots of leguminous plants.  Legumes are herbaceous plants that produce seeds in pods.  Examples include peas, beans, trefoils, and mimosa.
This relationship is common in nitrogen limited environments.  [1]This relationship occurs hen the rhizobium responds chemotactically to flavonoids produced by the plant.
The rhizobium cause conformational changes in the root itself, asides from the nodule.  It causes the root hairs to change the direction of their growth, which ultimately plays a large role in infection.  The root hair curls around the bacterium and traps it and brings it into the root [2, 3].
Rhizobium are able to fix nitrogen with the aid of the enzyme nitrogenase.  The presence of oxygen would decrease the efficiency of the nitrogenase.  Leghemoglobin protects the nitrogenase from oxygen.  There is dispute as to the leghemoglobin's origin.  In the school of thought we will follow, the protein portion is contributed by the plant and the heme group is contributed by the bacteria. [3].
It should be noted that rhizobium has been shown to be an enivronmentally friendly and effective fertilizer and has great potential for the agricultural industry [5].

Cost Benefit:  Both species benefit from this relationship, but they can exist independently of each other as well.  The roots of the legume plants provide energy to the rhizobium in the form of nutrients and carbohydrates [4].  While the plant gives energy to the rhizobium, it does not give so much so that the plant itself suffers. The rhizobium benefit the plant by fixing nitrogen to ammonia. Some species on fix up to 220 lbs of N2 per agricultural acre per year [1].  The lack of negative aspects of this relationship leads me to conclude that this non-obligatory relationship should continue.

Sources
1.) http://filebox.vt.edu/users/chagedor/biol_4684/Microbes/rhizobium.html
2.) Bisseling, T; Geurts, R. Rhizobium Nod Factor Perception and Signalling. The Plant Cell, Vol. 14, 239-249. May 2002.
3.) Van Rhijn, P; Vanderleyden, J. The Rhizobium-Plant Symbiosis. MICROBIOLOGICAL REVIEWS. Vol 59(1). 124–142.  Mar. 1995.
4.) Combes, C. Parasitism: The Ecology and Evolution of Intimate Interactions.
5.) Mia, M; Shamsuddin, Z.  African Journal of Biotechnology Vol. 9 (37), pp. 6001-6009, 13 September, 2010

Photos: http://onestiphoto.net/wp-includes/theme-compat/rhizobium-bacteria