Tag Archives: Mycorrhizas

First Steps Toward Learning the Language of Mycorrhizal Communication

I’ve already talked about mycorrhizal associations numerous times (here and here), so if you’re not already used to hearing about mycorrhizae, you will if you continue to read this blog.  In this recent paper, entitled “Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza“, published online in the journal Nature, the authors Maillet et al address plant and fungal interactions of arbuscular mycorrhizal associations.  Using the Glomus intraradicesMedicago truncatula model system, the researchers identify diffusible chemical signals produced by the fungus during initiation of the mycorrhizal association with the plant.

It has been hypothesized that both fungi and bacteria interacting with plant roots do so using similar genetic mechanisms.  It has already been shown that rhizobial bacteria – particularly the nitrogen fixing microbes associated with leguminous plants – produce lipochitooligosaccharide (LCO) signals used in the  communication with host plants.  The authors of this study discovered that the fungus Glomus intraradices, like the nitrogen-fixing bacteria, secretes an array of sulfated and non-sulfated simple LCOs which stimulated the formation of arbuscular mycorrhizae in disparately related plants, such as Medicago (Fabaceae), Daucus carota (Wild Carrot; Apiaceae), and Tagetes patula (French Marigold; Asteraceae).  These compounds were found in Glomus intraradices both interacting with plant roots and in free-living resting spores in the soil.

Comparing the genes involved in the transduction of the LCO signals in both rhizobial bacteria associated with legumes and arbuscular mycorrhizal fungi associated with land plants yielded similar gene expression pathways.  In order to validate the role of LCOs in mycorrhizae formation, the researchers genetically engineered non-plant interacting bacteria to produce the LCOs from Glomus.  These engineered bacteria increased mycorrhizae formation in plants already associated with Glomus.  Fungal LCOs were also found to induce root branching, a trait long associated with the formation of mycorrhizae in plants.  There is a nice commentary on this research article located here.

Lessons Learned from Ectomycorrhizal Fungal Genome Sequencing

Genome sequencing has provided us with an amazing amount of information regarding organismal biology and mycorrhizal fungi are some of the most interesting of the organisms who have had their genomes sequenced.  Maybe I am a little partial to these fungi because I study them intimately, but new sequencing technology has made this an exciting time for people like me.

Ectomycorrhizal fungi are a polyphyletic group of organisms which form a symbiotic association with the roots of tree species (the word ‘mycorrhizal’ literally means plant-fungal unions).  This association has typically been recognized by the exchange of nutrients and water from the fungus to the plant and the exchange of sugars derived from photosynthesis from the plant to the fungus.  Although ectomycorrhizal fungi only form mycorrhizae  with 3% of plant species (arbuscular and other mycorrhizal fungi associate with approximately 92% of plants), these associations are with a diverse array of plant lineages, including the Betulaceae, Cistaceae, Dipterocarpaceae, Fabaceae, Fagaceae, Myrtaceae, Pinaceae, and Saleceae.  Plants in these families cover almost the entire portion of boreal, temperate, Mediterranean, and sub-tropical woodlands, so their importance is significant.  It’s very interesting to note that these associations have independently arisen at least eight times within the angiosperms and between six and eight times in the gymnosperms.  Mycorrhizal associations are thought to have originated when plants and fungi climbed onto land together (more on that here).

The sequencing of both fungal and plant genomes over the last few years has led to greater understanding of how these organisms interact during their mutualistic associations.  Although genome sequencing has addressed some long established questions, there are many more questions that have arisen from these sequencing efforts.  This recent review in Trends in Genetics by Jonathan Plett and Francis Martin of INRA-Nancy, two of my collaborators, addresses the current state of our knowledge of  the ectomycorrhizal symbiosis and poses directions for future research in this vital research area.

Currently, only two ectomycorrhizal fungal genomes (Basidiomycete mushroom Laccaria bicolor & Ascomycete truffle Tuber melanosporum) have been sequenced, but other fungi (see the Mycorrhizal Genomes Project) are scheduled to be sequenced by the Martin Lab through JGI.  With a genome size of 65 Mb Laccaria bicolor has the largest amount of protein coding regions of any sequenced fungus, and Tuber melanosporum has the largest genome of any sequenced fungus at 125 Mb but has one of the least dense genomes.

The nature of mutualistic symbiotic relationships imply that both organisms benefit from the association and both ectomycorrhizal fungi and their host plants fulfill this criteria.  Unlike saprotrophic fungi, ectomycorrhizal fungi are very poorly suited to degrade cellulosic plant material, but they are able to access soil nutrients via a large biological toolbox of secreted proteases and phosphorus transporters.  Both Laccaria bicolor and Tuber melanosporum, which have very different genomes, exhibit a very similar suite of symbiosis-induced nutrient cycling enzymes, which suggest that providing nutrients to the host plant is a key defining feature of ectomycorrhizal fungi.  Interestingly, Laccaria bicolor and Tuber melanosporum rely on differing mechanisms of interacting with their host and acquiring carbon from the environment.  Laccaria bicolor appears to be less dependent on the host and more active at acquiring carbon from the soil substrates and, as a result, may act as a weak saprotroph in the environment.  Tuber melansporum is more aggressive in its colonization of plant roots and does not appear to be able to acquire carbon from the soil and therefore is more dependent on the host for its survival.

Information gathered from fungal genomes suggests that a majority of the biochemical and genetic control over the initiation of the mycorrhizal association comes from the fungal partner, which makes sense given that the fungus has more energy to gain from the association.  Most mycorrhizal fungi are unable to acquire carbon from the environment so they are completely reliant on hand outs from their host plants.  It appears that mutualistic fungi share similar mechanisms with pathogenic fungi and bacteria when interacting with plants, including the use of small secreted proteins which interact directly with plant cells.

With the sheer amount of genomic data being generated it’s an exciting time to be a scientist, especially one who studies mycorrhizal fungi.  Over the next few years, especially with sequencing projects scheduled for completion, we will have even more data to shed light on the amazing biological associations of plants and microbes.

(Above Photo: section of Populus/Laccaria ectomycorrhizal root – JM Plett © INRA)

Mycorrhiza-like Associations In Early Land Plants

Life is moving a little too fast for me right now, so I’ve been a little slow at posting here.  At least I feel like I have been spending my time in the laboratory, which is probably where my focus should lie anyway.  That being said, I did get caught up on reading some newer papers this weekend.This paper by Humphreys et alMutualistic mycorrhiza-like symbiosis in the most ancient group of land plants” is an article in Nature’s fairly new Nature Communications electronic journal and was published online on November 2nd of this year.  This paper is significant because it attempts to experimentally address hypotheses of ecological selection for symbiotic associations of early land plants and the fungi these plants harbor on their roots.  Through both fossil evidence and genetic clock estimations it is believed that these symbiotic associations were present 400 million years ago when the first plants made it onto land.  These hypotheses of early plant and fungal symbiosis were introduced more than 35 years ago in a classic paper by Pirozynski & Malloch in the journal Biosystems (“The origin of land plants: a matter of mycotropism”).  The authors address these hypotheses by using a system of studying a complex thalloid liverwort, Marchantia paleacea (image link), believed to be a member of an ancient extant basal clade of plants, and members of the fungal phyla Glomeromycota (Glomus group Ab).  Because these small liverworts have limited root mass, the use of mycorrhizal fungi to acquire nitrogen and phosphorus could be extremely beneficial in the early colonization of land.

Using a comparison of mycorrhizal and non-mycorrhizal liverwort plants at ambient and elevated CO2, the research group showed that mycorrhizal liverworts exhibited an increase in host plant photosynthesis, growth, and fitness (in the case of liverworts, more asexual propagules).  This increase could be attributable to an increase in nitrogen, and most notably, phosphorus.  These findings have been shown in both arbuscular and ectomycorrhizal plants in previous research, but this is the first example of finding this phenomenon in “early” plants, as well as elevated levels of CO2 (1500 ppb) believed to represent the atmosphere of the Paleozoic.  The researchers propose that the elevated atmospheric CO2 during the Paleozoic actually amplified the benefits from mycorrhizal associations, notably the acquisition of phosphorus, at a time when land was colonized by early plants.