On Sat, 12 Apr 2003 20:56:24 +0000 (UTC), "Robert Karl Stonjek"
 wrote:

>A diffusible factor from arbuscular mycorrhizal fungi induces
>symbiosis-specific MtENOD11 expression in roots of Medicago truncatula
>Plant Physiology 2003 131:952-962
>Commentary by Richard Staples
>
>"The roots of over 80% of land plants are colonized in a mutualistic
>association with fungi of the Glomeromycota, the arbuscular mycorrhizal
>(AM) symbionts that play an important role, in which plant carbon is
>traded for fungal phosphate and other minerals. The symbiosis is
>obligate for the fungal partner, but not for the plants. There are a
>number of reports that arbuscular mycorrhizal fungi, during the initial
>stages of infection, trigger some of the same defense responses or
>changes in gene expression in roots as do fungal parasites. However,
>some genes or responses elicited during pathogen attack are not induced
>by mycorrhizal fungi. On the contrary, colonization by some mycorrhizal
>fungi suppresses pathogen-induced defense-related root responses. Thus,
>there is convincing evidence that the AM-fungi do suppress the
>resistance responses they induce in roots upon colonization as they
>establish compatible symbiotic associations with plant tissues. To date,
>however, there is no evidence to indicate that there is plant
>recognition of the fungus before contact, nor is there evidence for the
>existence of fungal signals before root penetration."
>
>Read the rest of the article at BioMedNet
>http://update.bmn.com/rsearch/section/record?uid=UPDATE.Staples070420038
>1
>
>The paper is avaibale at 'Plant Physiology'
>
>A Diffusible Factor from Arbuscular Mycorrhizal Fungi Induces
>Symbiosis-Specific MtENOD11 Expression in Roots of Medicago truncatula
>Sonja Kosuta, Mireille Chabaud, Géraldine Lougnon, Clare Gough, Jean
>Dénarié, David G. Barker, and Guillaume Bécard
>
>Abstract:
>Using dual cultures of arbuscular mycorrhizal (AM) fungi and Medicago
>truncatula separated by a physical barrier, we demonstrate that hyphae
>from germinating spores produce a diffusible factor that is perceived by
>roots in the absence of direct physical contact. This AM factor elicits
>expression of the Nod factor-inducible gene MtENOD11, visualized using a
>pMtENOD11-gusA reporter. Transgene induction occurs primarily in the
>root cortex, with expression stretching from the zone of root hair
>emergence to the region of mature root hairs. All AM fungi tested
>(Gigaspora rosea, Gigaspora gigantea, Gigaspora margarita, and Glomus
>intraradices) elicit a similar response, whereas pathogenic fungi such
>as Phythophthora medicaginis, Phoma medicaginis var pinodella and
>Fusarium solani f.sp. phaseoli do not, suggesting that the observed root
>response is specific to AM fungi. Finally, pMtENOD11-gusA induction in
>response to the diffusible AM fungal factor is also observed with all
>three M. truncatula Nod/Myc mutants (dmi1, dmi2, and dmi3), whereas the
>same mutants are blocked in their response to Nod factor. This positive
>response of the Nod/Myc mutants to the diffusible AM fungal factor and
>the different cellular localization of pMtENOD11-gusA expression in
>response to Nod factor versus AM factor suggest that signal transduction
>occurs via different pathways and that expression of MtENOD11 is
>differently regulated by the two diffusible factors.
>
>http://www.plantphysiol.org/cgi/content/abstract/131/3/952
>
>Comment:
>I note that this paper was given as a poster at the fifth European
>Nitrogen Fixation Conference.  The interest for Evolutionary Biology is
>in the nature of the symbiotic relationship and the possible
>evolutionary progression that led to it eg why does some fungi cause a
>defensive response and some form symbiotic associations with the roots
>of some plants??

I am not sure just what you are asking.  Is it the same as the
question: why do some bacteria form symbiotic associations with us and
others cause disease?  There are a tremendous number of different
fungi with different ecological strategies to get by.  Some choose to
live in peace with their neighbors, cooperate, and get along nicely.
Others get nasty and greedy.  And some might even live change their
personality after a while. It takes all kinds!
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