Chapter 3: A plant is many
Plants engage in lively conversations below ground and communicate with millions of fungi, bacteria, viruses, and other living organisms. They release soluble signal molecules from their roots into their surroundings and they create a nutrient-rich oasis in their root stock. They “sweat” nutrients (sugars, amino acids, organic acids, enzymes, and various messenger compounds) to feed cohabitating fungi, bacteria, and viruses. In exchange, these organisms help plants unlock nitrogen and other nutrients from the soil and air and resist disease, heat, flooding, and drought. The microorganisms also enhance root growth, neutralize toxic compounds in the soil, and deter pathogens and predators. For millions of years, microbes and plants have been intimate partners in virtually all life processes.
Plants are quite willing to invest in this cooperation with organisms in the soil. Some plants deliver up to 70 per cent of their self-produced sugar compounds to cohabitants in the soil. Wheat and barley contribute 20 to 30 per cent of their sugars to this underground network.
Experiments by the research team of Russell J. Rodriguez at the University of Washington in Seattle demonstrate the importance of cohabitation. The researchers examined a rare grass (Dichanthelium lanuginosum) that thrives in temperatures up to 70 degrees Celsius at the hot springs in Yellowstone National Park. They discovered a fungus living inside the grass, and amazingly, after removing it, the grass was unable to tolerate the heat and died immediately. Then they isolated spores from the fungus and inoculated regular wheat seeds with them. Normally, wheat cannot tolerate more than 38 degrees Celsius. But with the fungus living inside, the wheat plant thrived in temperatures of up to 70 degrees Celsius and now needed half the water to grow.
Other microorganisms in soil help plants tolerate saline soils. For example, when fungi were extracted from the rootstock of the salt-loving dune grass Leymus mollis and sprayed onto rice plants, they actually thrived on saline soils. They also grew bigger and needed half the water than rice plants not sprayed with the fungi. Similarly, plants can survive drought or cold thanks to help from their fellow cohabitants in the soil.
Ann Reid, former director of the American Academy of Microbiology, writes: “We think that optimizing the microbial communities of plants offers an entirely new approach to enhancing productivity while reducing the demand on fertilizers and pesticides. Indeed, such an approach is the opposite of past management strategies that targeted microbes, in the mistaken belief that they all cause disease. Producing more food with fewer resources may seem too good to be true, but the fact is, the world’s farmers have trillions of potential partners that can help achieve that ambitious goal. Those partners are microbes."
Our understanding of these highly dynamic, invisible network systems inside the rootstock is limited. Presently, only 2 per cent of all soil microorganisms are known. The root area is terra incognita, literally “unknown territory”. Over 500 years ago, Leonardo da Vinci wrote: “We know more about the movement of celestial bodies than about the soil underfoot.” Little has changed since then. If plant roots are to collaborate with the tens of millions of microorganisms in the soil, they must communicate with their tiny cohabitants somehow to coordinate their behaviour. One possible way plants communicate is by secreting fragrances, dissolved in water. Admittedly, few examples of such molecular dialogues have been documented. We are very much in the infancy of understanding this amazing collaboration. What we know: A plant is a huge, interconnected whole, not just a creature that “vegetates” alone. A plant is many.
622-kilometre roots: the root system of a four-month-old rye plant
Total root surface area (with root hairs): 639 square metres
Of these, 402 square metres are root hairs
Total length of the root system (without root hairs): 622 kilometres
Total length of root hairs: 10, 620 kilometres
Growth of the root system per day (mean, without root hairs): 4.99 kilometres
Growth of root hairs per day: 89 kilometres
Example: The clever strategies of the coyote tobacco
The seeds of coyote tobacco (Nicotiana attenuata) survive decades on the desert floor of the Great Basin in the western United States, before quickly germinating after a bush fire. Smoke is their signal for germination. Few plants grow so quickly after a fire – a great advantage; however, caterpillars, beetles, grasshoppers, and other insects quickly pounce on her. How does this plant survive? A group of researchers led by Ian Baldwin of the Friedrich Schiller University in Jena examined this question. They observed that after three hours, herbivores suddenly stopped eating the coyote plant and moved on. The researchers then froze nibbled leaves for testing. After being pulverized, purified, and examined chromatographically, the tobacco leaves showed surprisingly high concentrations of nicotine – up to ten milligrams per leaf. Similarly, a cigarette contains approximately the same amount but, if consumed, will make a person ill. The researchers found that the coyote tobacco recognizes the saliva of attacking herbivores and immediately produces large amounts of nicotine to repel them.
But there is one exception. Nicotine does not harm the tobacco hornworm (Manduca sexta), an insect that lives symbiotically with coyote tobacco.
At dusk, these insects search for nectar from the tobacco plants and simultaneously pollinate the plants and deposit their little eggs on the underside of leaves. Soon enough, caterpillars hatch from the eggs, equipped with a special enzyme to break down and excrete the tobacco plant’s neurotoxic nicotine. Each day, these caterpillars eat more leaf mass than their body weight. The coyote tobacco plant once again tastes who is chewing away at her, recognizes the tobacco hornworm’s saliva, and then reduces nicotine production since that would be a waste of energy. Instead, she emits a special fragrance, an “SOS signal”, to attract big-eyed bugs (Geocoris) that destroy the voracious Manduca sexta larvae. However, the process must happen quickly, before the caterpillars grow larger than one centimetre. At that size, they are too large for the predatory bugs to stop them. But wait – the coyote tobacco has a strategy against this scenario too. At a certain point, all the larvae suddenly leave their victim plant and migrate to intact tobacco plants. Researchers still do not know how the tobacco plant manages to repel the caterpillars, but feel certain that the caterpillars do not leave on their own initiative. The tobacco plant prompts them to leave somehow.
One problem remains: Those coyote tobacco plants that have been attacked and repelled by the hornworm must find new pollinators and pursue an unusual strategy. Instead of blooming at dusk, which is normal for the coyote tobacco, they now bloom during the day. This change allows them to attract a new pollinator – the hummingbird. The hummingbird uses his beak to drink nectar from the plant’s white calyx and thus pollinates the tobacco plant.
These are excerpt from the book “Plant Whispers | A journey through new realms of science.” by Florianne Koechlin, translation to English by Thomas Rippel.