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I know that most nitrogen is fixed through industrial processes and bacterial symbiotic relationships. However, are there any plants that can fix their own atmospheric nitrogen?

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Fun Fact: 80% of the nitrogen in human tissues was fixed synthetically via the Haber-Bosch process –  Nick T Jan 17 '12 at 15:26
    
There soon may be: cbgp.upm.es/en/noticias/NFIX_LRubio_BMGF.html –  JoeHobbit Apr 17 '13 at 5:27

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up vote 9 down vote accepted

There are no known plants that fix their own nitrogen. However, there soon may be!

Because N is the major limiting factor in agricultural productivity, there is huge interest in plant systems which can fix their own. It's important enough that the Bill & Melinda Gates foundation have started a project aimed at reducing dependence on fertilisers by giving plants the ability to fix their own nitrogen. There are several potential strategies, the most obvious being encouraging more species to form symbioses with nitrogen fixing bacteria.

However, there's another proposal which builds on recent discoveries about the mechanism of nitrogen fixing in bacteria: we now know how the crucial enzyme complex is made (Rubio & Ludden, 2008). As a result, there are many people calling for efforts to engineer the system directly into a plant organelle (e.g. Beatty & Good, 2011; Godfray et al., 2010).

So, in 10-15 years time you can check back and the answer to this question might have changed! The most likely answer then will be "just the usual suspects: rice, maize, wheat".

References:

  • Beatty, P.H. & Good, A.G. (2011) Future Prospects for Cereals That Fix Nitrogen. Science. [Online] 333 (6041), 416 –417. Available from: doi:10.1126/science.1209467 [Accessed: 2 February 2012].
  • Godfray, H.C.J., Beddington, J.R., Crute, I.R., Haddad, L., Lawrence, D., Muir, J.F., Pretty, J., Robinson, S., Thomas, S.M. & Toulmin, C. (2010) Food Security: The Challenge of Feeding 9 Billion People. Science. [Online] 327 (5967), 812 –818. Available from: doi:10.1126/science.1185383 [Accessed: 2 February 2012].
  • Rubio, L.M. & Ludden, P.W. (2008) Biosynthesis of the Iron-Molybdenum Cofactor of Nitrogenase. Annual Review of Microbiology. [Online] 62 (1), 93–111. Available from: doi:10.1146/annurev.micro.62.081307.162737 [Accessed: 2 February 2012].
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Allowing plats to fix their own N2 sounds like the most dangerous form of genetic engineering possible. There are already so many noxious weeds - once a "food" can fix its own nitrogen, it will quickly become an uncontrollable weed. Innovation is not always good. –  JoeHobbit Feb 5 '12 at 23:26
    
I would be more comfortable with giving phytoplankton the ability to fix nitrogen: they might not be able to transfer their new N fixing organelles to land plants as easily, while they would increase the global fish supply- probably more than giving traditional plants N2 fixing capabilities would increase food production. Even then there would be the possibility of creating "dead" zones. –  JoeHobbit Feb 5 '12 at 23:53
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Well, by definition a food plant can't be a noxious weed, because they aren't noxious. But you raise a valid concern - mostly engineered traits such as vitamin A synthesis or herbicide tolerance are maladaptive and so they pose little risk of surviving outside deliberate cultivation for long. N fixation would be an adaptive trait in many habitats. Still, I don't think that's a reason not to do it, it's a reason to address the problem with a solution. –  Richard Smith Feb 6 '12 at 0:30
    
Right @JoeHobbit, dead zones was the first concern in my mind. The ocean would look green from space. Don't other species need the ability to fix nitrogen too? –  Gabriel Fair Mar 28 '12 at 12:08
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@JoeHobbit Well, it doesn't have to be 100% foolproof - that's an unrealistic goal. What matters is, do the benefits outweigh the risks, and are the risks manageable? In both cases, the answer is yes. There are various methods of preventing transgene introgression, but too much to write in a comment. Feel free to ask a new question about it and I'll answer to the best of my ability. –  Richard Smith Jun 6 '12 at 22:04

As far as I know, all biotic nitrogen fixation is performed by prokaryotic organisms such as Rhizobium. I don't know of any plants which can carry out this function on their own.

Plants can't use atmospheric N2 because it is held essentially inert by the nitrogen triple bond. The process of reducing N2 to NH3 which is usable by plants can be summarized:

N2 + 8e- + 8 H+ + 16 ATP -> 2 NH3 + H2 + 16 ADP + 16 Pi

(where Pi is a phosphate group)

Nitrogenase catalyses the reaction reducing N2 to NH3 by adding H+ and electrons. The whole process requires 8 ATP and is therefore energy intense.

In order to perform this coversion, bacteria require sufficent carbohydrates from decaying matter or plant vascular tissues (this is how Rhizobium derives energy from the host plant).

However, I should add that bacteria often have a mutualistic relation with the plant to perform this function, so in this sense you could say that plants can fix their own nitrogen.

There are also "free living" ammonifying bacteria in soils.

Ref

Science and the Garden, eds. Ingram, D.S., Gregory, P.J., Blackwell, 2008

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thanks biocs. I couldn't work out how to do subscripts on the ipad –  Poshpaws Jan 17 '12 at 15:03

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