Do plants really communicate with each other? What’s really going on in their speechless yet dynamic world? Do they like some of their neighbors better than others? Are they really able to ward off insect attacks? Over the past 10 years, some fascinating research has delved into what’s really going on in plant communities and the interaction within and among different species of plants and even insects. Studies have also documented some lively communication between plants and microbes – such as fungi and bacteria – that share living space in the root zone of a plant. Other studies have proven that there are also above-ground mechanisms plants use to signal one another, often in times of trouble.
The world of the underground is an interesting place indeed, and there’s a lot going on. In many cases, it’s a very crowded environment, like the streets of New York City or the freeways of Los Angeles, with steep competition for limited space. But, plants have a distinct disadvantage that we don’t have. They can’t flee to the solace of a country home when times get rough, when water and food is in short supply, or when an enemy is lurking. They’re literally rooted in the ground!
Lucky for them, they are able to take advantage many effective ways to communicate, albeit in a chemical rather than in a vocal way. How does all this happen? Who gets the last ‘word’? Who gets left out and why? To understand the interplay and rules of the game, it’s helpful to realize that there’s an intricate balance of nature at work. ‘Survival of the fittest’ mechanisms that pit stronger specimens against weaker ones often occur, sometimes even within the same species. And, it’s not uncommon for a mutually-beneficial (symbiotic) relationship between a plant and a beneficial fungus to take place right next to a hostile relationship between a disease-causing pathogen and an unsuspecting host.
Scientists have discovered that in many cases, chemical communicators are secreted from the root systems of plants. When I was a graduate student in horticulture, our research team looked at what turned out to be a positive outcome of this type of interaction. We grew seeds of 250 strains of old, never hybridized garden pea (Pisum sativum) in test tubes also containing common weed seeds. Voila! We were thrilled that 13 of the strains (native to countries such as Ethiopia, Nigeria, and Peru) killed off the invading weeds through natural herbicide-like chemicals given off by their roots.
It all makes sense when you think about it. Sometimes, in our quest for breeding bigger, more perfect looking fruit, higher yielding crops, or even longer-stemmed roses, we lose something valuable in the process. The resulting fruit might not taste quite as good, the bumper crop might succumb to an opportunistic insect that wasn’t a problem before, and the $10 a stem roses might lack the scent older varieties had. Fortunately, things aren’t so one-sided. There are many skilled public and private plant breeders out there that devote entire careers to identifying disease and insect resistant plants and, through modern scientific methods, are also able to take advantage of genetic engineering.
There are many other plants that give off toxins that ward off competing vegetation. Black walnut trees welcome lima beans as a neighbor but kill off tomato, eggplant and pepper plants and just about everything else! Lantana selectively kills milkweed. And, if you want to impress your neighbors, ask them if they are aware that their Tree of Heaven (Ailanthus altissima ) is allelopathic (the scientific term for “a positive or negative effect of one plant on another through a chemical release”). Allelopathy is nothing new. Theophrastus, a successor to Aristotle and early “father of botany”, penned an account around 300 B.C. of how chickpea “exhausts” the soil and kills weeds. But, it’s only been fairly recently that scientists have been able to identify many of the chemicals involved, thanks to new technology. In the future, look for environmentally friendly commercial products made from these natural weed killers that can reduce the need for synthetic herbicides.
Can plants also protect themselves from populations of potentially deadly microbes? While we still have a lot to learn about these complex interactions, we also have some answers. Plants have remarkable natural ways of fending for themselves. They often face an arsenal of attacking pathogens and win the battle by making their own chemical weapons that they secrete from their roots. On the battlefield, they form a ‘front line’ of detached but living cells (aptly called border cells) that give off a dizzying number of biological chemicals that thwart the advancing enemy.
Plants are dependent on ‘good guy’ fungi and bacteria that live in their root zones to provide biological control of diseases. That’s why it’s always a good idea to use non-chemical measures to prevent and reduce damage caused by diseases whenever possible, relying on chemical control only as a last resort. As we learn more about what makes plants ‘tick’ we can better appreciate the mutually-beneficial nature of plants and microscopic organisms. Fungi that form symbiotic relationships with plant roots in natural settings (mycorrhizae) have been found to enhance water and nutrient uptake of plants. The most common ones enter the root cell of an estimated 80-90% of trees, shrub, flower, grass and crop plant species found on earth.A fascinating form of plant-to-plant communication used to ward off insects involves ‘conversations’ between different plant species.
A recent example is an experiment conducted by scientists at the University of California, Davis, who performed surgery on the leaves of sagebrush to mimic that of insect damage. The clipped sagebrush responded by sending out a chemical SOS signal, picked up by neighboring tobacco plants. In turn, the tobacco plants cranked out large amounts of an enzyme that greatly decreased the tastiness of their foliage. Their cleverness paid off. Many hungry grasshoppers and caterpillars soon discovered that what had once been a favorite eatery had really gone downhill. Not overly discouraged, most tracked down far tastier tobacco growing next to unclipped sagebrush! (In fact, the tobacco plants next to the clipped sagebrush had 60% less damage than plants next to the intact sagebrush.)
Remember those lima beans that were favored by the black walnut tree? Well, lima beans, like it or not, might be around for quite awhile because these clever legumes can also outsmart spider mites. Scientists at Kyoto University in Japan discovered that not only did the lima beans in their experiment release chemicals that diminished their own flavor, but the same chemicals prompted a whole new wave of protection for their neighbors. The compounds warned unaffected lima bean plants that trouble was on the way. These plants, in turn, sent out their own arsenal of chemicals that not only deterred the spider mites, but called in the reserves – a species of mite that ate the offending ones! Scientists have determined that the signaling from damaged plants is triggered by chemicals in the saliva of the insect pests.While these examples illustrate how plants can avoid insect damage through some fancy chemical trickery, some insects aren’t settling for second place. Not to be outdone, a species of female parasitic wasp can coax the plants she calls home to produce a scent that attracts males of her species.
There has never been a better opportunity for scientists to apply these lessons of communication learned from nature to modern cropping systems and urban horticultural settings than in today’s world. The rapidly expanding knowledge base will likely lead to more biological pest control, reduced reliance on synthetic pesticides, plant breeding and selection for resistant varieties, and genetically engineered plants that are able to avoid or sustain attacking organisms.
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