The Key to Higher Quality Fruit: Fungus!

Primary Source: Cao, Ming-Ao, Peng Wang, Abeer Hashem, Stephan Wirth, Elsayed F. Abd_Allah, and Qiang-Sheng Wu. 2021. “Field Inoculation of Arbuscular Mycorrhizal Fungi Improves Fruit Quality and Root Physiological Activity of Citrus” Agriculture 11, no. 12: 1297. ,

Agricultural researchers found that treatment of tree roots with multiple species of mycorrhizal fungi improved quality of Mandarin Oranges, a popular citrus fruit eaten throughout the world. Credit: Dieter Albrecht Source:
The Root of the Matter

Eating a healthy snack like a piece of fruit can get you thinking about where exactly the fruit came from. Often, you’d probably picture the leafy boughs of a tree in an orchard, but to get the full picture of where those nutrients you’re enjoying came from, you’ve got to dig a little deeper… literally. Tree roots grow ever outward, searching for nutrients scattered in the soil, but perhaps not as efficiently as you may think. Frequently, plant roots are too slow-growing and too large to find the small pockets of nutrients hidden in-between the soil. Lucky for the plants, plenty of underground organisms are willing to help, for a modest price. In exchange for sugars only photosynthesis can produce, many species of fungus will deliver nutrients directly to the roots of struggling plants. These fungi are known as mycorrhizal fungi, with the Greek myco meaning fungus and rhiza meaning roots.  Mycorrhizal fungi go far beyond the web-like structure of plant roots, with long strands throughout the soil, only a few cells thick. Forming a wiry mesh through the ground, few nutrients escape the reach of these fungi, allowing their photosynthetic partners to grow faster. This pairing is so successful, 80% of land plants have been noted to partake in fungal colonization with one or more species of fungi. While humanity has made use of nature’s works countless times to improve our agricultural efforts, this key relationship, by virtue of existing underground in easily disturbed conditions, has eluded us until now.

A Growing Field

Since the unearthing of fungi’s key role in plant growth, these underappreciated underground dwellers have been at the center of agricultural research. Some of said research, coming from several Chinese universities, sought to see the effect that colonization by different mycorrhizal fungi had on the quality of Mandarin Oranges. These researchers set up three treatment groups in March of 2018, each containing five Mandarin Orange trees, all of which were comparable in age and size, grown in an orchard in Hubei, China. From this point, each tree of the three groups had one of three mixes of fungal cultures applied to its roots. The first, a control mixture, contained sterile fungi that would be unable to partner with the roots. The second mixture contained three species of mycorrhizal fungi, Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices, while the final mixture was composed solely of F. mosseae. After a year and seven months of field management, in November of 2019 , samples of each tree’s roots and fruits were collected and measured for various attributes associated with quality. When studying the fruit of the different study groups, the researchers measured fruit diameter, fruit coloration, and percent of soluble solids in fruit juice.  When studying the roots of the different study groups, aside from simply the health of the collected roots, the researchers measured the levels in the surrounding soil for easily extracted and difficult to extract glomalin-related soil proteins. Glomalin-related soil proteins, or GRSP’s are not well understood byproducts of mycorrhizal fungi but have been shown to be heavily correlated with soil quality. Easily extracted GRSP’s increase the soil’s ability to aggregate and hold moisture, reducing erosion. Difficult to extract GRSP’s, on the other hand, inhibit plant-growth, as they may contain toxic substances.

A cross-section of two tomato plants. The plant on the left is smaller and has roots that do not spread far into the soil, whereas the one on the right is larger, bearing more fruit, with far reaching roots due to assistance from mycorrhizal fungi.
Because the work of mycorrhizal fungi occurs below ground, it can be difficult to visualize just how much more soil they give a plant access to. Cross sections such as this can be useful in understanding the effects of mycorrhizal fungi. Credit: Mycorrhizal Applications, Inc.© Source:
Orange You Glad We Have Fungi?

Unsurprisingly, the results showed signs of quality to be higher in trees from groups treated with mycorrhizal fungi. However, an interesting development found that the trees from the group treated with mixed mycorrhizal fungi had several measurements of quality that were significantly higher than those of the group treated solely with the single species F. mosseae. While the mechanism behind the improved ability of mixed species over individual species of mycorrhizal fungi is not yet understood, this phenomenon makes it clear that the potential for fungi use in agriculture is even greater than previously thought. Though mycorrhizal fungi were already center stage in the field of agriculture, research like this demonstrates that our understanding has hardly scratched the surface of this important interaction. As such, to truly utilize these fungi, we have to do some literal and metaphorical digging.

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Cypress Novick

Cypress Novick

I am a recent graduate of Occidental College in Los Angeles, California, where I studied for my Bachelor's in Biology. My main research interests are wetlands ecology, mycology, estuary ecosystem interactions, and plant-based trophic interactions. I have always been passionate about making science more available and understandable, and am always trying to improve my writing so I may help myself and others be better understood.

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