Shall we dance?…

29 Sep

This post was chosen as an Editor's Selection for… says the rhizobial bacteria to the leguminous plant root.  In response, the plant warmly embraces her partner, literally wrapping herself fully around the bacteria, and so the intricate and highly choreographed ballet begins.

Image of a root hair embracing it's bacterial consort, by Euan James, via Life Sciences at the University of Dundee.

But how can a minuscule little bacterium communicate to a “higher plant” in a way to elicit such an overwhelming positive response?  A response which catalyzes this fundamental and truly amazing partnership, on which the world as we know it has depended for millenia?

The answer is simple, these bacteria developed special signaling molecules called nodulation (Nod) factors to communicate with certain plants.  These nodulation factors are diffusible signals, which mimic plant hormones, and stimulate very specific developmental processes within the plant during the initiation of the symbiotic relationship (Oldroyd & Downie, 2008).  They provide the choreography of the dance.

By simply altering the length and degree of saturation (think, saturated vs. unsaturated fats) of the nodulation factors, bacteria can target very specific plants.  This species-specific tuning of the nodulation signals allows an individual bacterium to “speak to” the host it prefers and form a relationship with, for example, a soybean plant rather than an alfalfa plant.

Once the plant recognizes the nodulation factor of the rhizobial bacteria, infection occurs through root hair cells which curl around and entrap the attached bacteria.  The bacteria, in turn, begin to break down the plant cell walls which entrap them, and subsequently initiate the formation of a tube-like structure called the intracellular infection thread or IIT (Ivanov, et al. 2010).

Intracellular infection threads containing fluroescently labeled rhizobial bacteria.  (By Dan Gage)

Intracellular infection threads (IIT) containing fluroescently labeled rhizobial bacteria. (By Dan Gage)

The IIT serves as a biological inoculation needle, so to speak, penetrating the root primordial cells and releasing bacteria into the cytoplasm of the plant.  At this point, the plant cells form a protective membrane around the mass of bacterial cells, where the bacteria differentiate into their nitrogen-fixing stage.  This special intercellular plant membrane, also called the peribacteroid membrane (PBM), allows nutrient exchange and helps protect the bacteria from oxygen, which would destroy the nitrogenase enzyme crucial to the process of gaseous N2 fixation (the process which is central to the nitrogen cycle, controls nitrogen availability, and consequently supports life on this earth).

At every point in this intricate dance, the bacteria and plant are engaged in a complex dialogue; this constant communication prevents activation of the plant defensive systems and the final formation of a functional symbiotic relationship (i.e. an active root nodule).  It’s a beautiful and fascinating process on which we rely, and should duly appreciate.

So, when, in my previous post, I mentioned the fact that it perturbs me when people dismiss the role bacteria play in symbiotic nitrogen fixation, I hope you can see that saying “plants fix nitrogen” for me, is comparable to saying Da Vinci’s paintbrush was responsible for the Mona Lisa, or that Einstein’s pen introduced the Theory of Relativity.  It just doesn’t give credit where credit is due.

ResearchBlogging.orgIvanov, S., Fedorova, E., & Bisseling, T. (2010). Intracellular plant microbe associations: secretory pathways and the formation of perimicrobial compartments. Current Opinion in Plant Biology, 13 (4), 372-377 DOI: 10.1016/j.pbi.2010.04.005

Oldroyd, G., & Downie, J. (2008). Coordinating Nodule Morphogenesis with Rhizobial Infection in Legumes. Annual Review of Plant Biology, 59 (1), 519-546 DOI: 10.1146/annurev.arplant.59.032607.092839


3 Responses to “Shall we dance?…”

  1. Hannah Waters October 1, 2010 at 10:34 am #

    Congrats on winning 2 separate editors’ selections for this singular post! Very impressive


  1. News » Blog Archive » Editor’s Selections: Penguins Tuxes, Turtle Outrage, Coral Danger, and Microbial Dances - October 1, 2010

    […] How microbes incorporated themselves into plant roots is truly a genteel and elegant dance. […]

  2. News » Blog Archive » Editor’s Selections: Bacterial fossils, shall we dance, and commuting to work - October 1, 2010

    […] Shall we dance? says the rhizobial bacteria to the leguminous plant root.  In response, the plant embraces her partner, literally wrapping around the bacteria. […]

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