Trees, Truffles and Mycorrhizae - Beneficial Fungi
Fungi are interesting organisms that have many effects on our lives. They make tasty and nutritious foods for us, decompose the bodies of dead organisms and recycle their nutrients, produce medications to treat diseases or make us sick themselves.
There is another important application of fungi in our lives, however. Some live on or in plant roots. This association between a fungus and a plant is known as a mycorrhiza. Both organisms benefit from the association, which helps them grow.
Biologists say that plants which normally have mycorrhizae either couldn't grow without their fungal companions or would grow much less successfully. This makes mycorrhizae very important both economically and ecologically.
Mycorrhizae have another fascinating function. The mycorrhizae in a community of plants form a communication network, passing information between the plants. In forests the network is sometimes called the "Wood-Wide Web".
Most types of plants have mycorrhizae, including trees. The most common kinds of Christmas trees - fir, Douglas fir, spruce and pine trees - grow in association with mycorrhizal fungi. Cacao trees, which give us the cocoa used to make chocolate, also have mycorrhizae.
Truffles are flavorful gourmet mushrooms. A truffle is the reproductive structure of a mycorrhizal fungus belonging to the genus Tuber. Chanterelles, morels, porcini mushrooms (or boletes) and matsutake mushrooms also develop from mycorrhizal fungi.
The Structure and Life of a Fungus
A fungus consists of thread-like structures called hyphae. The hyphae branch as they grow to form a tangled web known as a mycelium.
The mycelium of a fungus produces reproductive structures called "fruiting bodies", which make spores. Once a spore is released into the environment it can produce new hyphae. Fruiting bodies may be small structures, but in some cases - such as mushrooms - they are large and noticeable.
The hyphae of a fungus release digestive enzymes into the substrate (the material in which they're growing). These enzymes break the substrate down, producing molecules that are absorbed by the fungus and used as nutrients. Fungi aren't plants. They don't contain chlorophyll and can't make their own food by photosynthesis.
Although a mycorrhizal fungus gets its food from a plant's roots, it doesn't destroy the plant. The two organisms live as partners, with each giving the other something that it lacks or has difficulty obtaining.
Mycorrhizae and Trees - An Example of Mutualism
It's estimated that around ninety percent of vascular land plants (those with water and food conduction vessels) have mycorrhizae. The relationship between the fungus and the plant in a mycorrhiza is classified as mutualism, since both organisms benefit from the relationship.
The presence of the fungus greatly increases the surface area of the roots, enabling more water and nutrients to be absorbed. The fungus absorbs important chemicals that the plant needs, including phosphorus and nitrogen. The nutrients are then absorbed by the root cells. Research has shown that mycorrhizal fungi increase the concentration of phosphorus in their host by up to forty percent. The fungi benefit from their association with the plant by having access to sugars stored in the root, which they use for food.
Types of Mycorrhizae
Ectomycorrhizae develop mainly on the surface of plant roots. The fungus forms a mycelial net over the plant's rootlets (the fine divisions of the roots). This net is called a fungal sheath or a mantle.
The mycelial net sends hyphae into the outer layers of the root. These hyphae extend through the spaces between the outer root cells but don't enter the cells.
Endomycorrhizae develop mainly or completely inside plant roots. There is usually no mantle around the outside of the roots. If one exists, it's generally made of only a few hyphae. The hyphae of the fungus are located inside the outer root cells and may or may not be located between the cells as well.
Most trees have ectomycorrhizae. A few have ectendomycorrhizae, which have characteristics of both ectomycorrhizae and endomycorrhizae.
The Wood-Wide Web in Forests
Researchers have discovered that some of the ectomycorrhizal hyphae around the root of one plant travel to the root of a neighbouring plant and surround and enter it, too. In addition, one plant may form mycorrhizae with several fungi.
In a community of plants, such as a forest, a network of plants connected by hyphae is formed. A plant-fungus network is referred to as a wood-wide web or a common mycelial network (CMN).
Investigating the Wood-Wide Web
Communicating Through Mycorrhizae
Plants are able to communicate with each other via chemicals. For example, when some plants are injured by insects, they release airborne chemicals that travel to nearby plants. The chemicals stimulate the plants to defend themselves in some way, such as by producing substances that repel the insects or attract predators of the insect.
For some time scientists have suspected that plants can also communicate through their connected mycorrhizae. Now some exciting research has shown that this is the case, at least in one type of plant.
Researchers have found that when broad bean plants are attacked by aphids, they can "warn" other plants about the danger through mycorrhizae.
In an interesting experiment, scientists allowed some bean plants to form mycorrhizal connections with each other but prevented others from doing so. They covered the plants with bags so that no plant chemicals could enter the air.
The scientists placed aphids on some of the bean plants. These plants produced chemicals to repel the aphids. If a plant with aphids was connected to others via mycorrhizae, the connected plants also made chemicals to prevent the aphid attack. If the plants weren't connected, only the plant with the aphids mounted a chemical attack. The other plants apparently received no signal from the injured plant and made no defensive chemicals.
What are Truffles?
A truffle is the mushroom of an ectomycorrhizal fungus. Like other mushrooms, truffles contain spores. However, truffles are formed underground. Their spores are distributed when animals dig up the mushroom to eat it. True truffles belong to the genus Tuber, but there are truffle-like fungi in other genera.
The mycelium of a truffle forms mycorrhizae with the roots of several types of trees, including hazel, birch, poplar, beech, oak and pine trees. Truffles can be found in several parts of the world, including North America, but only certain species are eaten by humans. These are highly valued by some people.
Guinness World Records
The largest truffle on record was an Alba white truffle weighing 2 lbs 8 oz. It was found in Croatia on November 2nd, 1999.
Truffles - Gourmet Mushrooms
Truffles look very unimposing but have a flavor that many people love. They also have a strong aroma. Intact truffles can be very expensive, ranging from around a hundred dollars a pound for lesser known varieties to several thousands or even many thousands of dollars per pound for the most popular ones.
The two most famous truffles are the Alba white truffle (Tuber magnatum), named after the Italian city of Alba, and the Perigord black truffle (Tuber melanosporum), named after the Perigord region of France.
There is a burgeoning truffle market for the local truffles in North America. In addition, some North American truffle farmers have imported truffle spores from other countries, as have farmers in other parts of the world. A would-be truffle farmer needs patience, however. Seven to nine years are needed after inoculation of a host plant with spores before truffles are ready to harvest.
Truffles are served as shavings placed on food such as eggs, meat, chicken, fish, cheese and salads. They are also chopped or grated and added to butter, stuffing and sauces. (Truffle oil generally contains no truffles.) Alba white truffles taste best when raw, while the flavor of Perigord black truffles intensifies with heating.
A Truffle-Hunting Pig
Hunting for Truffles
Pigs and dogs are used to find truffles. Female pigs are attracted to the aroma of truffles because the mushrooms contain a chemical which smells like the male pig's pheromone, or mating attractant.
Dogs can be trained to detect truffles and then indicate their presence. Some people prefer to use dogs to hunt for truffles because dogs are less likely to eat the mushrooms.
Some farmers hunt for truffles by themselves by raking the soil in the area where truffles are likely to be found.
A Lagotto Hunts For Truffles
Mycorrhizal Fungi and Plants - An Important Relationship
There is still a lot to learn about mycorrhizal fungi and their relationship with plants. It's an important area of study, since we are so dependent on plants and so many of them live in partnership with fungi.
Research is showing that mycorrhizal fungi play an important role in maintaining plant health. Keeping plants healthy is important for our future and for the future of the planet.
© 2013 Linda Crampton
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