Plants are passive creatures. When it comes down to it, they’re not much more than nature’s pretty green backdrop. They stay in the same place their whole lives, silently soaking up water and sunlight, growing a little bit each day, pretty much defenseless against hungry herbivores. They just don’t do much—right?
If that’s what you think – and you wouldn’t be alone – take a look at this excerpt from David Attenborough’s spectacular The Private Life of Plants, produced with the BBC:
For most plant scientists, the notion of plants as passive, unresponsive and motionless organisms is simply insupportable in view of the vast evidence to the contrary. Time-lapse photography and film show that plants do in fact move, albeit at a much slower pace than most animals. Over time, for example, the roots and shoots of many developing plants twirl out into the environment, evaluating soil content and sources of light and water, searching for the best direction to grow, looking for a foothold and—in some cases—even sniffing out neighboring plants as friend or foe.
Sometimes, plants move exceptionally quickly: flowers rush to open in time to receive spring’s flurry of pollinators; seed pods explode, sending pollen grains flying; the leaves of Mimosa pudica (the Sensitive Plant) recoil instantly when touched; and, perhaps most famously, the Venus Flytrap snaps its green jaws whenever a six-legged meal crawls its way. See the proof for yourself:
The Sensitive Plant (Mimosa pudica)
Venus Fly Trap
The field of plant physiology, which concerns cellular and molecular functions, has discovered a wide array of active signaling mechanisms important for communication both in the individual plant and between plants—mechanisms that rely on hormones, proteins, peptides and electrical signals. When attacked by hungry insects, for example, potato and tomato plants respond by increasing the production of protease inhibitors—chemicals that inhibit digestion of leafy material—not only in the wounded leaf, but in all leaves.
Recently, plant science has witnessed the emergence of a new minority advocating even greater recognition of plants as organisms that respond directly to their environments with sophisticated signaling systems. This minority—associated with a novel field called “plant neurobiology”—argues for plants as “information processing” organisms capable of true behavior and adaptation, drawing attention to the potential neurobiology underlying these abilities. As the International Laboratory of Plant Neurobiology states on its web site, “The nascent field of Plant Neurobiology has been formed based on recognition that neurobiology of humans is a most rapidly breaking field in biology today, and the reality that much of the biochemistry, cell biology and electrophysiology known in classical neurobiology exists as well in plants.”
Many of the scientists in this minority are represented by The Society of Plant Signaling and Behavior, which has already held six international symposia. Here is an excerpt from their web site:
The goal of this field is to illuminate the structure of the information network that exists within plants. Plants are dynamic and highly sensitive organisms that actively and competitively forage for limited resources both above and below ground. Plants accurately compute inputs from the environment, use sophisticated cost-benefit analysis, and take action to mitigate diverse environmental insults. Plants are also capable of refined recognition of self and non-self, and are territorial in behavior. This view sees plants as information processing organisms with complex, long-distance communication systems within the plant body and extending into the surrounding ecosystem. Our Society was originally founded in 2005 as the Society for Plant Neurobiology to reflect these views of plant function. In May 2009 the Society voted to expand its view and change its name accordingly.
Why did the Society change their name in May 2009? Why remove the term ‘neurobiology’ from their title? Their decision reflects the controversy they have inspired and the resistance they have encountered from their scientific peers. Neurobiology—the detailed study of the nervous system and the brain—has nothing to do with plants, many of their colleagues argue; plants do not have nervous systems and making parallels between their signaling systems and those of animals is unwarranted, unscientific and misleading. Just two years before the Society changed its name, thirty-three researchers—from such universities as Oxford, Yale and the University of California Davis—signed a letter of opposition questioning the rationale of the entire plant neurobiology movement and claiming that it “does not add to our understanding of plant physiology, plant cell biology or signaling.”
However, there are several striking similarities between the cellular and molecular signaling mechanisms of plants and those of animals, namely: (1) the use action potentials—a change in voltage across an excitable cell membrane; (2) voltage-gated ion channels in cell membranes; and (3) evidence of neurotransmitter-like molecules. Plants have cells capable of sensing different aspects of their environment—especially light, temperature, humidity and gravity. They respond to these environmental stimuli with directly observable and measurable developmental changes. That action potentials and neurotransmitter-like molecules could be involved in these interactions and in internal plant communication is a valid claim supported by some preliminary evidence.
So the controversy continues, for now. Whether Plant Neurobiology will take root or not is unclear, but the evidence to which they point is too intriguing to simply ignore.