Mycorrhizae: The Good Guys in the Ground (Part 1)

The Many Ways Mycorrhizae and Rhizobia Benefit your Garden and your Crops

I know the word mycorrhizae is kind of a mouthful and sounds really science-y and intimidating, but it’s not. Hopefully after today’s post, you’ll understand what they are and why you want them in your garden.

What are Mycorrhizae?

Simply put, mycorrhizae are a class of fungi that form mutually beneficial relationships with plants via their roots. There is also a class of bacteria called Rhizobium that does much the same thing (think the inoculant you add to peas and beans).

It’s currently estimated that at least 80, and possibly as many as 95%, of all plants form these relationships, so clearly there’s a reason. Let’s explore the benefits and why we care.

Warning: I’m going to totally geek out on you today and delve a little deeper into the science of plants than I normally do. This post (and the next one) are excerpted from a semester-long, 30-page independent study I did in school called “Plant Mycorrhizal and Rhizobial Symbiotic Relationships and Their Role in Plant Defense Response“.

Yeah, not really light reading, but I’m going to try to translate it into actual plain English for you (and shorten it a whole lot!!) However, if you’re a total science geek like me, I’ll include a link to the whole paper at the end of this post for your reading pleasure (or to help with your insomnia, whichever you prefer).

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Enhanced Plant Nutrition

Based on the fact that host plants “pay” both the Rhizobia and the mycorrhizae with part of their carbon stores (that they gain through photosynthesis), one benefit to the host plant appears to be improved water, phosphorous and nitrogen uptake. This makes sense because it makes the plant healthier, thus increasing photosynthesis rates, giving the mycorrhizae access to more carbon from the host.  

Studies have found that plants associated with mycorrhizae have improved uptake of not only phosphorous and nitrogen, but also potassium, calcium, copper, manganese, iron and zinc. 

So, how do mycorrhizae and Rhizobia help plants mine enhanced nutrients from the soil?

Mycorrhizae do this by growing hyphae, or tendrils, that are attached to the plant’s roots. These hyphae are longer than the roots, and thus, are able to reach beyond what the roots alone could reach. 

The soil area directly around a plant’s roots is often depleted of nutrients due to what the plant uses. Mycorrhizal hyphae extend beyond this area and bring phosphorus and nitrogen, among other nutrients, back to the plants. Phosphorous, in particular, doesn’t move well through soil, so this added surface area of hyphae is vitally important.

Rhizobia benefit their hosts by forming nodules on the roots of plants (you can see these nodules on the roots if you pull a pea or bean plant at the end of the season, or check out the pic here). These nodules mine nitrogen gas from the air and turn it into a form usable by the plant host. Again, this makes the plant host healthier, giving the Rhizobia access to more of its carbon.

Pro tip: DON’T pull your pea or bean plants up at the end of the season. Cut them off at ground level and leave the roots in the ground. This leaves more of the beneficial bacteria right there in the soil, and won’t hurt anything for next season, assuming you’re a good little gardener and practice proper crop rotation techniques (no legumes in the same place the following season!)

If you remember from my posts on growing peas and green beans, I mentioned the importance of inoculating your seeds before planting. The inoculant is this Rhizobial bacteria. (By the way, if you’d like access to growing cheat sheets for almost a dozen common garden vegetables, head over here to sign up for my email list. You’ll join a growing community of gardeners and get access to lots of FREE resources).

If you need the inoculant for beans or fall-planted peas, you can find the kind I use here. Remember to keep it in the refrigerator until you use it.

Because the Rhizobial bacteria adds nitrogen to the soil, any crops planted directly after your peas or beans will also benefit from the enhanced nutrition, although unless they are legumes, they won’t form direct relationships with the bacteria.

Enhanced Plant Response to Pathogen Attack

You may have noticed I referred to mycorrhizae and Rhizobia as fungi and bacteria, and you might have wondered why they don’t injure the plants, as most fungi and bacteria are detrimental.

That’s true, and in the right circumstances (namely if there’s way too much phosphorus in the soil ~ very uncommon, by the way), even these can hurt plants. In typical conditions, though, they don’t. Still, the plants don’t initially “know” this and will actually mount a defense against the beneficial bacteria and fungi.

The cool thing is that the beneficial fungus or bacteria then mounts its own response that tamps down the host plant’s defenses and doesn’t allow it to go into a full-fledged “panic” response.

The plant does, however, stay in a somewhat heightened defensive state once the colonization has happened. Were this state a full-out defense response, it would eventually exhaust and damage the plant, but that’s not what happens.

In one study, inoculating tomato plants with mycorrhizae didn’t cause an increase in defense-related genes in the plants at all.  However, when insects were allowed to feed on the tomatoes, four different defense genes were shown to increase substantially, giving the tomatoes a head start on resisting these pests.

Let’s make it simpler:

When you get the flu, you get sick. Your body goes haywire ~ fever, chills, vomiting, respiratory symptoms, you name it. That’s a full-out defensive response to the virus invading your body.

But, when you get vaccinated against the flu, your body mounts a response to the pathogens in the vaccine, but it’s a muted one. You may feel a little “off” for a couple days, but it’s nothing like getting the actual flu.

This is very similar to what the plant’s immune system does. In practical terms, it stays on guard against fungi and bacteria, so when an actual pathogen attacks, the plant has a head start on beating it, just like you do if you’ve had the flu vaccine.

Note: I’m not trying to get into any kind of debate about the efficacies or advisabilities of getting vaccinations, it’s just a good analogy for what plants do in this situation.

Preventing Free Radical Damage in Plant Hosts

Much as human bodies produce free radicals under stress, plants produce substances like hydrogen peroxide, which is a free radical of sorts. 

Hydrogen peroxide has beneficial effects in plants ~ it’s needed for signaling and in some defense processes. Plants are even able to tolerate fairly high levels of it in their tissues. However, high enough levels will eventually cause cell death.

It’s been found that alliances with beneficial organisms like mycorrhizae have antioxidant effects for the host plant, lowering hydrogen peroxide to healthy levels and limiting damage.

Conversely, nitric oxide in a plant signals it to increase antioxidant levels, and can work directly as an antioxidant, removing some free radicals. Plants under pathogen attack show increased levels of nitric oxide, but plants colonized by mycorrhizae show even higher levels.

Another antioxidant, glutathione reductase, also aids in reducing levels of hydrogen peroxide in stressed plants.  It was shown to increase substantially in plants infected with a pathogen, but mycorrhizal plants had much higher levels even than their nonmycorrhizal counterparts.

Since humans are told to take Vitamin C to help with free radical damage, I guess that means mycorrhizae are the Vitamin C of plants. Cool, huh?

Cumulative Effects of Mycorrhizae and Rhizobia

Protection against pathogens can also be cumulative. That is, association with more than one mycorrhizal species, or with mycorrhizae and Rhizobia at the same time, can have multiplied positive effects on plant resistance. 

In lab tests, cucumber plants inoculated with both beneficial fungi and bacteria showed much higher levels of defense-related genes when attacked than those colonized by a single species only, leading to a more efficient and effective defense. 

When soybean plants were infected with red crown rot, plants colonized by both Rhizobia and mycorrhizae were always healthiest and showed lowest effects from the disease. 

In the plants with no beneficial fungi or bacteria, 79% showed symptoms of infection.  This dropped to 45% in those colonized with either Rhizobia or mycorrhizae, but further dropped to only 23% when both beneficial organisms were present. 

Interestingly, when looking at defense genes, plants colonized by Rhizobia showed higher levels in roots, while mycorrhizal plants showed higher levels in leaves. 

It’s therefore possible that host plants respond to different beneficial organisms in different ways, probably Rhizobia locally and mycorrhizae systemically (throughout the whole plant), thus priming them to have a more complete and efficient defense response when both are present and a pathogen comes calling.

Thanks for reading!

I hope you’ve enjoyed Part 1 of my two-part series on mycorrhizae (and that I haven’t put you to sleep yet!) Next time, I’ll talk about a couple more benefits and how you can be sure to encourage lots of mycorrhizae in your own garden so your plants stay happy and healthy.

If you’re brave enough to want to read the whole paper this post was excerpted from, you can find the link here. I’ll also add it to the Subscriber’s Resource Library so you can find it in the future.

As always, you’ll find some pinnable images below. Be sure to pin one to your Gardening or Garden Soil board so you can find it again. Smile and have a crazy organic day!

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The Many Ways Mycorrhizae and Rhizobia Benefit your Garden and your Crops