(Actually Microscopic Fungus and Bacteria in the Soil!)
By Dr. David Boyle
Retired Professor of Mycology
with Dr. Elizabeth Spence, writing assistant and beginner fungus student
Plants are amazing in that they can grow just about anywhere. But they don’t do it alone. Most of them support and are supported by a complex interconnected network of – – –
Helper Microbe Associates or HMAs for short.
These are found in the plants’ roots and in the soil surrounding them. They are generally too small to see but they are doing a lot of the work that makes plant growth possible.
HMAs collectively make up a huge biomass. The HMAs from a hectare of garden (about 2½ acres), if all clumped together, could be as big as a full-sized elephant!
As a gardener, you can adjust conditions so this elephant helps the plants do what you want them to do. Understanding how it works is fundamental to the practice of organic gardening.
The magic power of plants is photosynthesis.
Plants use light energy to drive chemical reactions that ‘fix’ carbon dioxide with water to form sugars. In other words, photosynthesis is:
Sunlight + Carbon dioxide + Water -> Sugar + Oxygen
The oxygen is released into the air, and the sugars react via carefully orchestrated biochemical pathways to form all the other molecules that make up the plant.
The plants make more sugar and molecules than they need for their own growth, so they transport the excess into their roots and into the soil around their roots.
There, in a sort of barter system, the molecules from the plant supports the HMAs, and since they are in or close to the roots, they are in a perfect position to take advantage of these.
In return, the HMAs can help the plant in a variety of ways. Some obtain mineral nutrients for the plant, some get water, some defend against disease, and so forth.
The plants ‘talk’ to their various HMAs and vice-versa, through the ‘language’ of biochemistry in ways that science is just starting to recognize. Symbiosis in action!
Probably the best known and most prevalent of the HMAs are the mycorrhizae. ‘Myco’ is derived from the Greek word meaning fungus, and ‘rhiza’ means root in Greek. These root-fungi live inside and around essentially all plant roots.
You may have noticed that some potting mixes like Promix and Myke contain mycorrhizae.
These are fine in container gardening, but in natural soil the fungi are already present, so you need not add them. (Elizabeth’s note: well, well, well!)
You can get a glimmer of how important the mycorrhizae are to the plant by looking at a prepared root under a microscope.
In the photomicrograph below, the fungal structures are stained blue, while the plant’s cells are just dimly visible in the background. It is of a section of the root, magnified 400X.
The round structures are “vesicles”, where nutrients are stored.
The fuzzier objects are small bush-shaped structures called “arbuscules.” These are where materials are transferred from the fungus to the plant cells.
The filamentous, stringy, structures are “hyphae”, which is the name for fungal cells.
The hyphae are much thinner than root hairs and can therefore extend much further into the soil, and they use fewer materials and less energy than the plant root hairs do.
In this way they can access more distant nutrients which they then transport back into the plant roots where they enter the arbuscules to feed the plant.
In a similar way, the mycorrhizae help the plant take up water.
Other fungi are also HMAs although their relationship with the plant is not as close as that of mycorrhizae. Included here are various species of Trichoderma, Fusarium, Aspergillus, and many others.
When some of these filamentous organisms colonize the soil around the plant roots, they create enzymes and other metabolites that make the soil more suitable for plant growth. Here are some of the things they do:
- They break down the residues of dead plants, insects, nematodes and other
organisms. After further microbial digestion, the nutrients that are released can be used by the plants. - They change the texture of the soil so it is less prone to erosion and better at holding water. Fungal hyphae agglomerate the small pieces of sand, organic
debris, etc. into larger pieces. - They also make a glue-like material that holds the agglomerates together. Without the fungi, the soil is much more prone to blowing or washing away. This is one reason why erosion can be much higher in fungicide-treated farm fields.
- Some can control fungal diseases. Trichoderma is one of the most well-studied of these fungi and it has the ability to kill and digest other fungi that are pathogens.
- Some fungi can degrade lignin, cellulose, hemicellulose and proteins, these being the large molecules that make up the bodies of plants, animals and
microorganisms. These fungi are nature’s scavengers. Together with some
bacteria they break down the bodies of other organisms (including people!) so
their nutrients can be re-used. - Other fungi can kill various pest insects. Notable here are Beauveria sp., Metarhizium sp. and Cordyceps sp.
These fungi invade the insect body, and in natural systems they play a big role in
keeping insect populations from getting out of control. They can be highly
sophisticated in their function.
If you google search “Zombie Fungi “or “Cordyceps Ants” you will find videos showing how some fungi infect insects, take over their nervous system and make them climb up nearby vegetation. There, the fungus’s spore-producing structure emerges and it spews its spores far and wide. The insect does not survive this.
Pretty good trick for a lowly fungus! I highly recommend everyone take a few minutes to watch some of these videos!
Another big HMA group is bacteria.
- The most thoroughly studied of these are the nitrogen fixing bacteria. Some of them grow in close association with roots of a variety of plants where they make characteristic nodules. Pull up a legume (e.g. a pea or bean plant) and you may see these:
- Some nitrogen fixing bacteria grow in a looser association with non-leguminous plant roots, but may still provide ammonia which the plants require.
- Many bacteria work together with the fungi and break down organic debris (leaves, dead insects, other microbes, etc.) making the nutrients in the debris
available to the plant.
- Plants cannot use complex forms of nitrogen like proteins or amino acids but
some of the bacteria transform these large molecules into ammonia and nitrate that the plant can use. - Some bacteria also make various polysaccharides similar to xanthan gum which we use in ice cream, tooth paste, paint, etc. These polysaccharides can thicken the water that lands in the soil so it remains in the root zone for longer.
- The polysaccharides also, like the materials that fungi produce, can help control erosion by impeding the movement of the soil particles by water or wind.
Bacterial HMAs can also have a big effect on a plant’s health.
Some, like Bacillus subtilis, make a wide variety of compounds that are broad spectrum biocides. These can inhibit the growth of many disease-forming fungi and bacteria.
Other very common soil bacteria are the Streptomyces.
These are bacteria, but they look like fungi since their cells link together to form long filaments that resemble fungal hyphae.
As the name suggests, some of these bacteria can make streptomycin and a variety of other antibiotics. These have been used in medicine, but they can also help plants ward off disease.
Some of the antibiotics can be systemic, circulating through the plant.
Their ability to make antibiotics also helps the bacteria to outcompete other microbes around the plant roots.
To summarize, these fungal and bacterial HMAs, along with many others that are not discussed here – the elephant – carry out all sorts of functions that allow your plants to grow and remain healthy.
For Part Two Click here