The Secret Life of Plants

. . . Since the development of time-lapse photography, it has been possible to document the dances and scuffles in densely populated plant communities: saplings on the forest floor compete for space to stretch their roots and shoots; fallen trees provide young ones with nourishment; vines lash around desperately searching for a trunk they can climb to reach the light; and wildflowers race each other to open their blooms in springtime and compete for the attention of pollinators. To truly understand the secret social life of plants, however, you must look and listen more closely.
A good place to start is underground in the rhizosphere - the ecosystem in and around plant roots. Beneath the forest floor, each spoonful of dirt contains millions of tiny organisms. These bacteria and fungi form a symbiotic relationship with plant roots, helping their hosts absorb water and vital elements like nitrogen in return for a steady supply of nutrients.
Now closer inspection has revealed that fungal threads physically unite the roots of dozens of trees, often of different species, into a single mycorrhizal network. These webs . sprawled beneath our feet are genuine social networks. By tracing the movement of radioactive carbon isotopes through them,
"Plants don't go out to parties or to watch the movies, but they do have a social network"
Simard has found that water and nutrients tend to flow from trees that make excess food to ones that don't have enough. One study published in 2009, for example, showed that older Douglas firs transferred molecules containing carbon and nitrogen to saplings of the same species via their mycorrhizal networks. The saplings with the greatest access to these networks were the healthiest (Ecology, vol 90, p 2808).
As well as sharing food, mycorrhizal associations may also allow plants to share information. Biologists have known for a while that plants can respond to airborne defense signals from others that are under attack. When a caterpillar starts to munch on a tomato plant, for example, the leaves produce noxious compounds that both repel the attacker and stimulate neighboring plants to ready their own defenses.
Yuan Yuan Song of South China Agricultural University in Guangzhou and colleagues investigated whether similar chemical alarm calls travel underground. They exposed one group of tomato plants to a pathogenic fungus and monitored the response in a second group connected to the first via a mycorrhizal >
26 March 20111 NewScentist 147
network. The diseased plants were sealed inside airtight plastic bags to prevent any communication above ground. Nevertheless the healthy partners began producing defense chemicals, suggesting that the plants detect ' each other's alarm.,calls via their mycorrhizal networks (PLoS One,\vol 5, p e13324).
Another recent discovery, one which may be connected with Song's finding, is that some plants recognize members of their own species and apparently work together for the common good. Amanda Broz of Colorado State University in Port Collins paired spotted knotweed plants inside a greenhouse either with other knotweeds or with blue bunchgrass. She then simulated an attack by spraying them with methyl jasmonate, a chemical many plants release when wounded. The knotweed's response depended on its neighbours. When growing near members of its own species, it produced leaf toxins to boost its defences. But it chose to focus on leaf and stem growth when its neighbours were bunchgrass {BMC Plant Biology, vol 10, p 115).
Such discrimination makes sense because, in the knotweed's native environment, dense clusters of a single plant tend to attract large numbers of insects to an all-you-can-eat buffet. So cooperating with other knotweed plants helps stave off an attack. However, when knotweed is surrounded by bunchgrass, a better strategy is to leave defense to its neighbours and concentrate on aggressive growth -which might also help explain why knotweed is such an effective invasive species.
Broz's research was published just last year,
and it remains unclear how knotweed, or any other plant, could be recognizing members
of its own species. However, one instance of a plant with family values has been more thoroughly explored.
In a landmark paper published in 2007, Susan Dudley from McMaster University in Ontario, Canada, reported the first case of plants recognizing and favoring their kin (Biology Letters, vol 3, p 435). Her studies of American sea rocket, a scraggly weed that grows along the shorelines of the Great Lakes, showed that a plant potted with an unrelated individual did not hesitate to spread its roots and soak up as much water and nutrients as it could. However, when Dudley planted sea- rocket siblings in the same pot, they exercised restraint, taming their eager roots to better share resources. Siblings and strangers that grew near each other but did not share pots showed no differences in root growth, indicating that sea rocket relies on underground chemical signaling to identify its kin. They don't seem to be using mycorrhizal networks, though.
In subsequent research with Meredith Biedrzycki from the University of Delaware in Newark, Dudley discovered that the signals take the form of' exudates" - a cocktail of soluble compounds including phenols, flavonoids, sugars, organic acids, amino acids and proteins, secreted by roots into the rhizosphere. How these indicate relatedness is still a mystery, though (Communicative & Integrative Biology, vol 3, p 28).
In the past few years, kin recognition has been discovered in of "Arabidopsis and a kind of lmpatiens called pale jewelweed. This has led some botanists to argue that plants, like animals, are capable of kin selection- behaviours and strategies that help relatives reproduce. Kin selection has an evolutionary rationale because it increases the chances that the genes an individual shares with its relatives will be passed to the next generation, even if altruistic behaviour comes at a cost to one's own well-being.
"There's no reason to think plants wouldn't get the same benefits from kin selection that animals do," says Dudley.
Recognizing siblings and restraining one's growth in response certainly looks like kin selection, but that still leaves the question of whether such interactions also improve the survival prospects of related plants. Research by Richard Karban at the University of California, Davis, goes some way to answering that.
Karban studied a desert shrub called sagebrush, which emits a pungent bouquet of chemicals to deter insects. When he clipped an individual plant's leaves to simulate an attack, he found that it mounted a more robust defence if it was growing next to its own clone than if its neighbour was unrelated. What's more, for a period of five months afterwards, the neighbouring clones suffered far less damage from caterpillars, grasshoppers and deer than did unrelated neighbours (Ecology Letters, vol 12, p 502).
Studying kin selection and other plant interactions doesn't just improve our knowledge of basic plant biology and ecology. "There are a lot of people really interested in it, because it's not just an intellectually neat puzzle," says James Cahill at the University of Alberta in Edmonton, Canada. "There are many potential applications, especially for agriculture."
One obvious area is in companion planting - the strategic positioning of different crops or garden plants so they benefit one another by deterring pests, attracting pollinators and improving nutrient uptake. This ancient technique, which traditionally relies on trial and error and close observation, can be highly effective. For example, beans fix nitrogen that boosts growth in some other plants, and when Europeans arrived in America in the 15th century, they discovered that Native Americans used corn as a natural trellis for bean plants. Our modern understanding of plant interactions suggests we could find new, more subtle and potentially beneficial relationships, which could help us overcome a major drawback of modern monoculture farming. Since a single pathogen can wipe out an entire crop of genetically similar- and therefore equally vulnerable - plants, farmers make heavy use of pesticides. But instead of picturing an endless stretch of corn or wheat, imagine something more like a jungle of diverse species that work together above and below ground.
Breeding cooperation
Cahill has another idea. "Fertilizers aren't always spread evenly," he says. "Maybe we could breed plants to cooperate more effectively with their neighbours to share fertilizer." Meanwhile, Simard thinks the recent discoveries about mycorrhizal networks have implications for both agriculture and forestry. Hardy old trees should not be removed from forests so hastily, she says, because saplings depend on the mycorrhizal associations maintained by these grandparent trees. She also suggests that farmers should go easy on fertilization and irrigation because these practices can damage or destroy delicate mycorrhizal networks.
Clearly, we do not yet have all the information we need to start deploying such tactics. "What we want to do next is develop more advanced techniques to watch roots grow, to really see what they do with each other and how they interact in space," Dudley says. She also wants to figure out what genetic factors control plant interactions and look at how they change survival and reproduction. "The molecular aspects are perhaps the most challenging," she adds, "but we have made some big leaps."
The idea that plants have complex relationships may require a shift in mindset. , "For the longest time people thought that plants were just there," says Biedrzycki. "But they can defend themselves more than we thought and they can create the environment around them. It turns out they have some control over what is going on through this chemical communication." Passive and silent though plants may seem, their abilities to interact and communicate should not come as such a shock. "Some incredibly simple organisms - even one-celled organisms - can recognize and respond to each other," says Broz. "Why is it so bizarre to think that plants could have this same kind of ability?"
March 25, 2011 NewScientist
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Perhaps the forest is being grown by a benevolent fungus?
Steve
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Paging Mr. Aldis!
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