Saturday, November 19, 2011

More on mycorrhizas

Well, I was hoping to have this up before the article was published, but it simply didn’t work out that way. In any event, better late than never. In the process of preparing for a Myco Digest article, I always read far more than I can cover in any kind of “further reading” list, and often must for reasons of space put aside information on the subject that I think would be interesting to include. But thankfully, I do have a blog (that unfortunately has gone largely unused for several years now), that serves as the perfect place for such addenda.

First off, though I did briefly introduce the idea of mycorrhiza, I presumed a certain degree of familiarity with the idea, and didn’t spend a whole lot of time introducing the concept of mycorrhiza and explaining its various subtypes. Good intros can be found on Wikipedia and the wonderful CSIRO site, Mycorrhizal Associations: The Web Resource.

Also, both sites have good intros to AM fungi, here and here.

Similarly, a good introduction to myco-heterotrophs, that is, parasitic mycorrhizal “cheater” plants can be found in this Wikipedia article, plus this article by David Hibbett, and one I wrote for Myco Digest a few years back.

I can vouch for its accuracy of the Wiki article, having written the majority of the article and having had Martin Bidartondo, a leading researcher on the topic, proof it. The article could use some updating based on research that’s been done over the last several years (in particular, I know Nicole Hynson published quite a bit of work on various aspects of myco-heterotrophy when she was doing her grad work in the Bruns lab), but it’s a topic I need to get caught up on. (Of course, if there any volunteers, Wikipedia is open to all…).

On the topic of the multiple lines of evolution of myco-heterotrophic plants, including myco-heterotrophs parasitizing the AM fungi, a good review can be found here.

I also mentioned the fact that fossils that of fungal associations with the roots of the early plant Aglaophyton found in the Rhynie chert of the Early Devonian epoch (about 410 million years ago). Although the plant is quite different from modern vascular plants, the mycorrhiza is quite clearly identifiable as AM mycorrhiza (and with ancient plants responding to fungal symbiosis in much the same way as modern plants do), with the fungal component, Glomites, being noticeably similar to modern members of the Glomeromycota. This was quite a breakthrough discovery back in 1995, when it was first reported. Key papers on this topic can be found here, here, and here.

Another recent experiment that I did not have space to discuss might shed some light on how the plant may control its relationship with AM fungi at the level of gene expression. A hot-off-the-press paper by Gaude, et al. of the Max Planck Institute for Molecular Plant Physiology (news article here, abstract here) describes how plant cells penetrated by fungal arbuscules express (among many other things) a protein that is built into channels on the cell membrane that allow phosphate compounds to cross over from the fungal arbuscules into the plant cell itself. And while this is not surprising, it was also noted that nearby plant cells also strongly expressed this protein, as if the plant was priming those cells to actively engage in nutrient exchange once fungi show up. Although it is not mentioned in the article, such a strong priming on the part of the plant cell for active exchange with mycorrhizal fungi could be key to how plants regulate exchange with more- vs. less-cooperative Glomus partners.

What I find especially cool about how the research was done is the use of (pardon the pun, Sherman) cutting-edge laser capture microdissection techniques to isolate individual root cells (both with and without arbuscules) so that gene expression could be looked at on a cell-by-cell basis. Laser microdissection is done using a microscope with a built-in microlaser controlled from a computer screen. One selects an area of cells or tissue onscreen, much like drawing a selection in Photoshop, then a microlaser cuts that area out. One can isolate very tiny parts of the organism for further study or analysis.

Also worth a mention is another recent paper by Toby Kiers (lead author of the study I wrote about in the Myco Digest article), “Mutualisms in a changing world: an evolutionary perspective”. This paper is a review of examples of change in mutualistic relationships driven by global climate change and other anthropogenic changes to out environment. Changes include abandonment of established mutualistic relationships by one or both partners, mutualists turning to antagonists or exploiters, and switches to novel mutualistic partners, often invasive entrants to ecosystems. We can see this dynamic taking place in everything from the pollinator extinction crisis to the emergence of Amanita phalloides as one of the most common visible mycorrhizal partners of California oaks. It is driven by many of the same dynamics as the spread of novel pathogens I wrote about last year for Myco Digest. These are important environmental issues we must pay close attention to, and mitigate where we can.

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