Plants Use Mycorrhizal Fungi to Warn Each Other of Incoming Threats

The March 2015 issue of New Phytologist is a Special Issue focusing on the “ecology and evolution of mycorrhizas.” This is the second of two articles from that issue that I am reviewing. Read the first review here.

Interplant signalling through hyphal networks by David Johnson and Lucy Gilbert

When an individual plant is attacked by an insect or fungal pest, it can warn neighboring plants – prompting them to produce compounds that either repel the pests or attract beneficial organisms that can fight off the pests. There are two main pathways for a plant to send these communications: through the air by way of volatile organic compounds (VOC’s) or through the soil by way of a vast collection of fungal hyphae called mycelium. Plant communication by aerial release of VOC’s has been well documented; communication via mycelium, however, is a fairly recent discovery, and there is much left to learn.

“The length of hyphae in the soil and the ability of mycorrhizal fungi to form multiple points of entry into roots can lead to the formation of a common mycelial network (CMN) that interconnects two or more plants.” These CMN’s are known to play “key roles in facilitating nutrient transport and redistribution.” We now understand that they can also “facilitate defense against insect herbivores and foliar necrotophic fungi by acting as conduits for interplant signaling.” The purpose of this research is to explore the “mechanisms, evolutionary consequences, and circumstances” surrounding the evolution of this process and to “highlight key gaps in our understanding.”

interplant signaling

An illustration of plant communication (aka interplant signaling) by air and by soil form the article in New Phytologist.

If plants are communicating via CMN’s, how are they doing it? The authors propose three potential mechanisms. The first is by signal molecules being transported “in liquid films on the external surface of hyphae via capillary action or microbes.” They determine that this form of communication would be easily disrupted by soil particles and isn’t likely to occur over long distances. The second mechanism is by molecules being transported within hyphae, passing from cell to cell until they reach their destination. The third mechanism involves an electrical signal induced by wounding.

If signal molecules are involved in the process, what molecules are they? There are some molecules already known to be transported by fungal hyphae (lipids, phosphate transporters, and amino acids) and there are also compounds known to be involved in signaling between plants and mycorrhizal fungi. Exploring these further would be a good place to start. We also need to determine if specific insect and fungal pests or certain types of plant damage result in unique signaling compounds.

It has been established that electrical signals can be produced in response to plant damage. These signals are a result of a process known as membrane depolarization. “A key advantage of electrical-induced defense over mobile chemical is the speed of delivery.” Movement of a molecule through cells occurs significantly slower than an electrical charge, which is important if the distance to transport the message is relatively far and the plant needs to respond quickly to an invasion. Various aspects of fungal physiology and activity have been shown to be driven in part by membrane depolarization, so involving it in interplant signaling isn’t too far-fetched.

photo credit: wikimedia commons

photo credit: wikimedia commons

How and why does a system of interplant communication involving mycorrhizal fungi evolve? And what are the costs and benefits to the plants and fungi involved? Determining costs and benefits will depend largely on further establishing the signaling mechanisms. Exploring real world systems will also help us answer these questions. For example, in a stable environment such as a managed grassland where CMNs are well developed, a significant loss of plants to a pest or disease could be devastating for the mycorrhizal community, so “transferring warning signals” would be highly beneficial. Conversely, in an unstable environment where a CMN is less established, assisting in interplant signaling may be less of an imperative. Regarding questions concerning the degree of specialization involved in herbivore-plant-fungal interactions: if a “generic herbivore signal” is sent to a neighboring plant that is not typically affected by the attacking herbivore, the cost to the plant in putting up its defenses and to the fungus in transporting the message is high and unnecessary. So, in an environment where there are many different plant species, species-specific signals may be selected for over time; in areas where there are few plant species, a generic signal would suffice.

As research continues, the mysteries of “defense-related” interplant communication via CMN’s will be revealed. Field studies are particularly important because they can paint a more accurate picture compared to “highly simplified laboratory conditions.” One exciting thing about this type of communication is that it may mean that some plants are communicating with each other across great distances, since “some species of fungi can be vast.” CMNs can also target specific plants, and compared to communication via aerial VOC’s, the signal will not be diluted by the wind.

Since I am in the process of reading Robin Wall Kimmerer’s book, Braiding Sweetgrass, I have decided to include her description of a tree-mycorrhizal fungi relationship:

The trees in a forest are often interconnected by subterranean networks of mycorrhizae, fungal strands that inhabit tree roots. The mycorrhizal symbiosis enables the fungi to forage for mineral nutrients in the soil and deliver them to the tree in exchange for carbohydrates. The mycorrhizae may form fungal bridges between individual trees, so that all the trees in a forest are connected. These fungal networks appear to redistribute the wealth of carbohydrates from tree to tree. A kind of Robin Hood, they take from the rich and give to the poor so that all the trees arrive at the same carbon surplus at the same time. They weave a web of reciprocity, of giving and taking. In this way, the trees all act as one because the fungi have connected them. Through unity, survival. All flourishing is mutual.

Using Plant Root and Mycorrhizal Fungal Traits to Predict Soil Structure

The March 2015 issue of New Phytologist is a Special Issue exploring the “ecology and evolution of mycorrhizas.” A mycorrhiza is a symbiotic association between a fungus and the roots of a plant. The introductory editorial of this special issue asserts that “almost all land plant species form a symbiosis with mycorrhizal fungi.” Generally, the association benefits both plant and fungus. The plant gains greater access to water and mineral nutrients by the way of fungal hyphae, and the fungus recieves carbohydrates (glucose and sucrose) that have been synthesized in the leaves of the plant and transported down into its roots. We have been aware of this relationship since at least the middle of the 19th century, but recent advances in technology have given us new insight into just how extensive and important it is . “Plants cannot be considered as isolated individuals anymore, but as metaorganisms or holobionts encompassing an active microbial community re-programming host physiology.”

However, there are still “critical gaps” in our understanding of mycorrhizas, hence the special issue of New Phytologist. In this issue they endeavor to address the following questions: “How is the balance of mutualism maintained between plants and fungi? What is the role of mycorrhizal fungi in the soil ecosystem? What controls fungal community composition, and how is diversity maintained?” There is so much more to learn, but the research presented in this issue has us moving in the right direction. If you are interested in this sort of thing, I encourage you to check out the entire issue. I have picked out just 2 of the 32 articles to present here – one this week and the other next week.

photo credit: wikimedia commons

photo credit: wikimedia commons

Plant root and mycorrhizal fungal traits for understanding soil aggregation by Matthias C. Rillig, Carlos A. Aguilar-Trigueros, Joana Bergmann, Erik Verbruggen, Stavros D. Veresoglou, and Anika Lehmann

Soil structure is determined by the size, shape, and extent of soil aggregates and the resulting pore spaces found between them. The arrangement of soil aggregates and pore spaces helps determine the availability and movement of water and air and also has an influence on the growth and movement of micro- and macroorganisims, including fungi, plant roots, bacteria, and arthropods. The authors state that “soil aggregation is important for root growth and for a wide range of soil features and ecosystem process rates, such as carbon storage and resistance to erosion.”

Soil aggregates are composed mainly of clay particles, organic matter (including plant roots), organic compounds (produced by bacteria and fungi), and fungal hyphae. There has been plenty of research on soil aggregation, but much of it is focused on management practices and physical chemical factors. Less is known about the contribution of plant roots and mycorrhizal fungi to the formation and stabilization of soil aggregates. We know they play a role, but we lack understanding about the extent to which soil aggregation can be predicted not just by abiotic factors but also by the presence of plants and mycorrhizal fungi. The authors of this paper propose a widespread, trait-based approach to researching this topic, recognizing that “summarizing ecological characteristics of species by means of traits has become an essential tool in plant ecology.”

Possible traits to be considered were grouped into two categories: formation-related traits and stabilization-related traits. Formation refers to “the initial binding together of particles” to form an aggregate. Stabilization is a process in which aggregates are “increasingly resistant to the application of disintegrating forces, such as water penetrating into pores.” These two processes (along with disintegration) are occurring simultaneously in virtually all soils, but they “may be executed by different organisms expressing different traits.” Some of the formation traits include length, extension ability, and relative growth of roots and hyphae; root and hyphae exudate quality and quantity; and the “ability of roots or hyphae to bring soil particles together by moving them, leading to potential aggregation.” Stabilization traits include tensile strength, density, and “entangling ability” of roots and hyphae; water repellency of the aggregates and cementation capability of the exudates; and the life span, palatability, and repair capacity of roots and hyphae.

photo credit: wikimedia commons

photo credit: wikimedia commons

The amount of time and effort it will take to measure the traits of each and every plant and mycorrhizal fungi species and to determine the extent to which those traits contribute to soil aggregation will be considerable. The authors acknowledge that “some of these traits will be relatively easy to measure,” while “others will be quite challenging.” However, as technologies advance, the mysterious world under our feet should become easier to explore. As the traits of each species of plant and fungi are measured, a database can be constructed and eventually used to determine the plant/fungi combinations that are the best fits for restoring and conserving the soils of specific regions.

Ultimately, this research may help us answer various questions, including whether or not we can use a survey of plant and mycorrhizal fungi (along with soil type, climate, and management) to predict soil aggregation. Ecosytem restoration efforts may also benefit if we are able to produce “tailor-made mycorrhizal fungi inocula and seed mixes” in order to “enhance soil aggregation.” Better understanding of these traits could also be applied to sustainable agriculture in areas such as crop breeding and cover crop selection. This research is in the hypothesis phase right now, and “only controlled experiments employing a range of plant and fungal species” can reveal the role that certain plant root and mycorrhizal fungal traits play in soil aggregation as well as the full range of applications that this information might have.

Speaking of soil, did you know that the 68th United Nations General Assembly declared 2015 the International Year of Soils? The purpose of this declaration is to “increase awareness and understanding of the importance of soil for food security and essential ecosystem functions.” You can read a list of “specific objectives” on their About page.