The symbiosis between plants and fungi is often oversimplified. Even I usually say something like, “Fungi find and deliver scant soil nutrients to the plant in return for a sugar reward.” The truth is, these mycorrhizal relationships are far more complex, as countless factors interplay. Scientists now are looking into these intricate species interactions and are finding some noteworthy results. A relatively new publication looked into what ecological changes are associated with plants recognizing the relatedness of their neighbors. This same study found plants growing near closely related plants benefit from a common mycorrhizal network (CMN) and invest more sugars to fungal mutualists more so than plants living near distantly related individuals. It is findings like these that change the way I see the forest floor. It really is a dynamic, multispecies entity inhabited by individuals driven to maximize their fitness.
Several studies have shown that plants can recognize their neighbors. The mechanism is a little blurry, but a large subset of these papers has found that plant chemicals released by one individual’s roots, can be absorbed by neighboring roots. The secondary compounds released have a unique chemical signature that closely related plants ban pick up on. Plants that have identified their closely related neighbors that share the same CMN interact with their fungal symbiote in a different manner compared to non-related plants.
Amanda File and her team of researchers used ragweed, (Ambrosia artemisiifolia) a plant that utilizes arbuscular mycorrhizal fungi (AMF) and Glomus intraradices, an agressive mycorrhizal fungus to help describe these interactions taking place beneath the forest floor. Two, fully factorial greenhouse experiments with several treatments and an impressive number of sample sizes were carried out to do this.
The first of these two experiments showed a significant response of root colonization in related vs. unrelated plant seedlings. Although stem elongation and aboveground biomass remained the same in both seedling relatedness habitat, seedlings grown in mesocosms with a sibling had significantly enhanced arbuscular and hyphal colonization. This reveals that related plants less than 4 weeks old recognize each other’s root exudates, and respond by teaming up with the same fungal mutualist.
Similarly, the second experiment looked at older juvenile plants harvested at 15 weeks. Again, stem elongation, branchiness, and leaf biomass didn’t differ between the plants relatedness environment. Though, hyphal length was significantly longer in plants grown next to kin compared to not only plants grown next to a genetic stranger but solitary plants too. This study showed that for ragweed growing in this experimentally selected environment, seedlings and juveniles didn’t nutritionally benefit from their enhanced mycorrhizal network. This could be because the soil mix used in the experiments had plenty of soil nutrients that roots could readily access without help of a fungal symbiote. Even without benefiting nutritionally, these closely related plants acquired other ecological fitness enhancements through their improved mycorrhiza.
The quantification of fungal root colonization was not the only aspect looked at once roots were analyzed. Accompanied by plant growth and mycorrhizal data, these scientists also quantified root parasitism. Root parasites including nematodes and antagonistic fungi increase the number of root lesions, which, in total was reduced with enhanced mycorrhizae. Even if mycorrhizae didn’t enrich plants nutritionally, this portion of the study revealed that closely related plants invest more sugars to their fungal symbiote which in return provide protection against soil parasites and pathogens.
Common mycorrhizal networks also known as the ‘wood wide web’ are formed more strongly between closely related plants. This evolutionary, ecological response has been driven by ‘cheaters,’ over millions of years. It is common for plants, especially juvenile plants living in the light limited understory, to take advantage of mycorrhizal fungi. These cheating organisms receive nutrients from fungus without paying a sufficient sugar reward. Without receiving enough plant sugars, the mycorrhizal network disconnects from unproductive plant roots to find a greater payoff. That is, unless the cheating plant is connected to and recognized by a closely related individual through a CMN.
In this case, a plant may invest excess sugars to mycorrhizae to support its kin. Since closely related plants share much of the same genes, the more of them that make it to adulthood to reproduce, the more fit they become. In addition to kin selection helping to enhance their fitness, the fitness of forest mycorrhizae is heightened once a plant can recognize closely related individuals. Theses genetically similar stands invest more sugar to their mutualists that ultimately increases mycorrhizal biomass.
The way plants interact with fungal mutualists depends on a wide array of both living and non-living factors. For example, certain woody species living in more nutrient poor, later successional ecosystems may pair up with closely related individuals through CMN in hopes of increasing their chances to acquire scant soil nutrients. Other, more ephemeral species like ragweed may increase their mycorrhizal investments to offer root protection from parasites and pathogens. These mycorrhizal strategies are as diverse as the fungal kingdom itself and we are just scratching the surface. Mycorrhizal functioning is a difficult, time consuming aspect of forest ecology to study, but studies like these prove we are making headway. We now know that plants interact with their fungal symbiote differently if they grow alone, are surrounded by strangers, or live near closely related kin. Plants not only send signals to mycorrhizae, but communicate to plants nearby to ‘decide’ how much to invest into their symbiote. Pretty fascinating stuff, right? The forest floor really is an ancient, calculated place, chock-full of undiscovered species interactions.