Soil microbes help plants communicate

With the evolution of technology, scientists constantly renew our understanding of the plant world through increasingly precise probing and measuring instruments. One of the most fascinating recent discoveries with particular significance for farming is the knowledge that plants are capable of communicating with each other via extensive and complex networks, and can warn each other of the presence of pests. In response, the plants will mount natural defenses against the infestation, much in similar as animals do.

Scientists reported their findings in the journal Oecologia of a study they conducted to measure a plant’s response to the vibrational sound of a caterpillar chewing its leaf [ref. 1]. By placing a reflective tape on the leaf and a laser beam, they recorded the vibrational sound made by the chewing caterpillar through the reflected laser beam.

They found that:

  1. When a bug such as a caterpillar chews on a plant’s leaf, the plant produces more of certain phytochemical. These phytochemicals are also what give the plant many of its medicinal qualities, such as glucosinolates, which have anti-cancer properties, and other antioxidants. Insects usually will not feed on plants with high levels of such chemicals.
  2. They also played the recording of the near-inaudible vibrational sound of a caterpillar chewing to plants and found that plants that had been previously exposed to these feeding sounds released higher amounts of chemicals that deter bugs.

In a nutshell, the vibrational sound of a bug chewing on a plant’s leaf causes a change in the cellular metabolism of the plant, creating chemicals that repel the attacker. Plants can actually “hear” that their leaves are being chewed and mount their chemical defense in response to minimize damage.

Another 2012 article in the Journal of Chemical Ecology describes mycorrhizae-induced resistance as part of plants’ systemic “immune response,” protecting them from pathogens, herbivores, and parasitic plants [ref. 2]. One of the studies used bean plants and aphids and found that when a bean plant becomes infested with a pest like aphids, it warns surrounding plants of the attack via this network of mycorrhizal fungi. This “heads up” gives the other plants time to mount their chemical defenses in order to repel the aphids. In bean plants where the researchers had removed the mycorrhizae connecting them together, the plants quickly succumbed to the infestation, presumably because they didn’t receive the warning to mount their defenses.

Even more interestingly, the alerted bean plants deployed not only aphid-repelling chemicals, but also produced other chemicals that attract wasps, which are aphids’ natural predators!

In 2010, Song et al published a report about the interplant communication of tomato plants suggesting that CMNs [common mycorrhizal networks] may be considered as a plant-plant underground communication conduit whereby disease resistance and induced defense signals can be transferred between the healthy and pathogen-infected neighboring plants. Plants may have the ability to ‘eavesdrop’ on defense signals from the pathogen-challenged neighbors through CMNs to activate defenses before being attacked themselves.

Other studies find that mycorrhizae fungi can even connect plants of different species allowing even interspecies communication.

The various relevant studies seem to suggest that plants can communicate with other plants—even with plants of other species—through a complex network that includes:

  1. The plants’ rhizosphere (root ball)
  2. Mycelial networks in the soil
  3. Aerial emissions (volatile gasses emitted by the plants)

These three systems work together to form a network of biochemical highway where information about each plant’s status is constantly exchanged. These findings help us recognize the importance of microorganisms in soil, with mycorrhizae in particular. These fungi form a symbiotic relationship with the plant, colonizing the roots and sending extremely fine filaments far out into the soil that act as root extensionsNot only do these networks sound the alarm about invaders, but the filaments are more effective in nutrient and water absorption than the plant roots themselves. Studies show that mycorrhizae can increase the nutrient absorption of plants 100 to 1,000 times. In one thimbleful of healthy soil, several MILES of fungal filaments can be found, all releasing powerful enzymes that help dissolve tightly bound soil nutrients, such as organic nitrogen, phosphorus, and iron. In other words, healthy and nutritional plants come from healthy soil.

The interconnectedness between soil, microbes, plants, pests, and ultimately human health is a fascinating kind of balance Nature strives hard to maintain. In reality, pests are not unavoidable nuisance similar to disease. They serve the valuable role of destroying sick or damaged plants and feed those animals that help plant production. Insects and weeds also have their place. They only really reach “pest” status when the balance is shifted in such a way that they’re able to get the upper hand. Healthy plants typically don’t have a problem with insects and pests. Mycorrhizae aren’t the only organisms important for plant health. Plants need bacteria, fungi, protozoa, nematodes and earth worms. They are the building blocks of all life on Earth. When we are beautifully plugged into this system of interconnectedness and begin to appreciate the depth of Nature’s intelligence, we have to learn to seek the balance that leads to healthy soils, healthy plants, a healthy environment, and, ultimately, healthy bodies.

We, as caretakers of the earth, have the free will to either protect and nurture it or destroy it. If we choose to poison the earth, we poison ourselves. This is an expression of Nature’s self-regulatory capacity. Since soil health connects to everything up the food chain, our health originates in the soils in which our food is grown.

References

[1] H. M. Appel1 and R. B. Cocroft2, “Plants respond to leaf vibrations caused by insect herbivore chewing”, in Oecologia, 10.1007/s00442-014-2995-6 Plant-microbe-animal interactions – Original research, © The Author(s) 2014
(1) Bond Life Sciences Center and Division of Plant Sciences, University of Missouri, 1201 East Rollins St., Columbia, MO 65211, USA
(2) Division of Biological Sciences, University of Missouri, Columbia, MO 65211, USA

[2] Sabine C. Jung, Ainhoa Martinez-Medina, Juan A. Lopez-Raez, Maria J. Pozo, “Mycorrhiza-Induced Resistance and Priming of Plant Defenses”, in Journal of Chemical Ecology, Volume 38, Issue 6, June 2012, pp 651-664

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