Mycorrhizae

We’re made of dirt. We eat plants that grow in dirt. Foods like eggs, meat, and dairy come from animals that eat plants. The iron in our blood, the calcium in our bones, and the phosphorus in our DNA all come from soil.

Getting those nutrients from the soil into our bodies, though, isn’t a trivial matter. Some nutrients, like water, potassium, and nitrogen move easily through the soil. Nutrients like iron and phosphorus, on the other hand, are prone to oxidation and mineralization, and are a lot harder. Phosphorus is vital for all life, and especially for plants, but it’s not very mobile. Once a root has absorbed all the phosphorus around it, the zone of depletion remains depleted for a long while, since the phosphors is slow to diffuse in and replenish it.

And that brings us to the point of this post: mycorrhizae. Mycorrhizae are fungal symbiotes that colonize plants’ roots. Their relationship with plants is ancient: fossil mycorrhizae have been found dating back at least 400 million years. [1] The symbiosis is very intimate. Plants allow mycorrhizae to penetrate their root cells, where they form structures called arbuscules and vesicles, which allow them to exchange water and nutrients with the host plant. And plants, in turn, willingly exude substances for mycorrhizae to feed on, and accept chemical signals from mycorrhizae that alter their root growth to better suit the mycorrhizae.

Mycorrhizae, like many fungi, grow filaments called hyphae. Mycorrhizal hyphae act as an extension of the host plant’s root system, dramatically increasing its reach and its surface area. In the case of phosphorus, mycorrhizal hyphae increase the volume of soil roots can draw from. They also excrete organic acids and enzymes that convert phosphorus into a mobile form. [2] In some cases, mycorrhizae alone can supply as much of 80% of a plant’s phosphorus demand.

Phosphorus, though, is only one of the vital nutrients. Mycorrhizae have been documented to help absorb a slew of others, including nitrate, ammonia, ammonium, calcium, iron, potassium, and zinc. [3]

All other things being equal, mycorrhizae-colonized plants tend to be larger, healthier, more productive, and more resistant to stress and disease than plants grown in sterile soil.

This last point is important: mycorrhizae can provide significant protection against disease. [4] First of all, since they grow so close to the roots, they create a physical barrier. Second, fungi (mycorrhizae included) are constantly at war with other microbes, and so they’ve got powerful chemical defenses. They’re also in chemical communication with their hosts, so they can mobilize those defenses when the host sends out a distress signal.

Unfortunately, the modern world isn’t always kind to mycorrhizae. Soils see a fair bit of abuse in urban and agricultural soils. Construction, foot traffic, vehicle traffic, and heavy equipment compact the soil. This crushes the soil’s pore spaces, which makes the soil more prone to water-logging and oxygen deficiency, and more difficult for roots and hyphae to penetrate.

In agriculture, soil also sees very intensive use. That often leaves it dry and depleted, and so, to keep their plants healthy, farmers must resort to chemical fertilizers and pesticides. When over-applied or mis-applied, chemical fertilizers can chemically damage the soil, which can kill beneficial microbes. And broad-spectrum fungicides often kill mycorrhizae just as easily as disease-causing fungi. The result is a soil that might have all the nutrients a plant needs, but lacks the mycorrhizae that help plants absorb them.

But the news isn’t too bad: In the last few decades, understanding of the role and importance of mycorrhizae has grown rapidly. And with the rising interest in organic food, and with the increasing cost and regulation of chemical fertilizers, farmers, landscapers, and plant-care professionals are looking for ways to help make plants more resilient and self-sufficient. Mycorrhizae allow plants to grow and be productive with less labor and chemicals. With our growing population and growing demand for food, that’s more important than ever.

Citations

[1] Four hundred-million-year-old vesicular arbuscular mycorrhizae.
[2] Roles of Arbuscular Mycorrhizas in Plant Phosphorus Nutrition
[3] Nutrient uptake in mycorrhizal symbiosis
[4] Arbuscular mycorrhiza protect an annual grass from root pathogenic fungi in the field.