For anything to be considered “alive”, it must be distinct and separate from the environment. From the perspective of life on Earth, a cell is the fundamental unit of life; in fact, one of the defining traits of life is being formed of at least one cell. This definition is arbitrary, based on the concept of life “as we know it”. If you want to see a review of really good book on life as we do not know it, look here.
One of the essential characteristics of life is behavior. For a living organism to survive and eventually reproduce (which is the name of the game in evolution), it must be able to obtain information about the environment, process this information and finally “react” to it in order to enhance its chances of survival by capturing nourishment, by defending itself from a predator, etc. In multicellular organisms, certain cell types are specialized to perform these functions. One type of such cells are the neurons.
Groups of neurons (nerve cells) are capable of such feats because they belong to a class of cells called “excitable cells”. What makes an excitable cell. All cells, excitable or not, must keep their internal and external environment isolated from each other. This is essential to sustain life. One aspect of this involves the difference between the concentration of ions (charged atoms, positive or negative) in or out of the cell. The two major players in animals are sodium and potassium ions (in plants the main players are calcium and chloride).
When ions are present at different concentrations in and out of the cell, we say that the cell is “polarized”. All cells display this property. In the case of excitable cells, we call this the “resting potential”, which can be visualized as a voltage difference between the cell and its external environment. By convention, this voltage difference is assigned a negative value. These are really small voltages, in the milliVolt range (1/1000 Volts; for comparison, keep in mind that normal batteries, like the ones in a remote control, usually carry a charge of about 1.5 Volts).
Excitable cells are capable of a really nifty trick to use this potential difference to communicate with other cells. They use specific proteins in their membranes, called voltage-gated ion channels, which when opened, allow for the ions to flow across the membranes, making the voltage difference mentioned above display a more positive value. This flow of ions can be recorded as electrical currents. We call this phenomenon “depolarization”, and many living organisms have learnt to use this to transmit information from one cell to another. When information is transmitted in this way, this process is called the “action potential” (AP). This is the basis for the discipline of electrophysiology.
Behavior is usually associated with animals; however, make no mistake, plants and even microorganisms display behavior too! Bacteria and other unicellular organisms are able to react to their environment to obtain food and escape from toxic substances. This is also true of plants; think about the sensitive plant (Mimosa sp.) which is capable of rapidly closing its leaves when touched, and even more dramatically, think about the Venus flytrap (Dionea muscipola), which actively captures insects and even the occasional small vertebrate. People have always been fascinated by carnivorous plants, precisely because of their unusual behavior of hunting other organisms for nourishment. Hunting behavior is usually associated with animals. The first formal study on insectivorous plans was written by (who else?) Charles Darwin, in 1875.
There is a relatively new (although highly controversial) field of study called “plant neurobiology” which is currently intensively studied. The term “neurobiology”, like the term “behavior”, is historically associated with animal life; in animals, behavior is generated by the nervous system. At their most basic level, nervous systems must be able to change their fundamental properties in response to environmental stimuli; usually achieve this by producing action potentials (see above).
This presents a problem. Plants have no nervous system that we can recognize. How can they generate behavior?
Plants have no neurons, but ironically, the etymology of the term roughly translates to “vegetal fibers”. However, many types of plants possess cells that contain subcellular machinery very similar to neuronal cells. In fact, many plants produce substances which are bona fide neurotransmitters in animals, such as acetylcholine, glutamate and GABA among others. Moreover, action potentials were first recorded in plant cells! This was done in 1873 by an animal physiologist, John Burdon-Sanderson, who worked on the Venus flytrap. These findings were not further explored because of the emphasis on signaling research was shifted towards chemical as opposed to electrical transmission.
In the 1970s more evidence accumulated pointing out at electrical signaling as an important phenomenon in plant physiology. However, this trend was reversed by the publication of the book “The secret life of plants” by P. Tompkins and C. Bird. This book was not generally accepted and it is considered as pseudoscience by the scientific community. This substantially diminished the enthusiasm for the study of plant electrophysiology. Very few scientists are willing to work in a scientific area that is associated albeit unjustly, with pseudoscience.
It is indisputable that plants display electrophysiological properties and that these properties play an important role in the adaptation and survival of plants in challenging environments. It is unfortunate that in several cases, published papers in this area have been rightly criticized as overreaching when interpreting experimental results as well as such results being described as experimental artifacts by highly qualified experts in the field. The credibility of this area of research is further damaged by the fact that some researchers in this field chose the term “plant neurobiology” and phrases like “brain like” as a metaphors because it does not give the impression of seriousness, precisely because plants have no neurons! Additionally, some scientists are not helping the cause by starting to talk about plant intelligence and cognition, while these terms are not fully understood concepts even when viewed exclusively from the perspective of animal biology. Science is developed by people, with all the biases and imperfections inherent to Human behavior. It is entirely possible that some scientists dismiss the claims of the plant neurobiology field just because of a psychological barrier due to the way the field is named.
Despite all of this, I believe that the study of plant electrophysiology and its effects on plant behavior is a legitimate, very interesting research field, with the potential of producing significant scientific results. The tools of classical neurobiology, in conjunction with current advances in molecular biology should make the next few years a very interesting period for plant behavior research.
If you want to know more
Baluška, F. et al. (2004) Root apices as plant command centres: the unique ‘brain-like’ status of the root apex transition zone. Biologia (Bratisl.) 59, 9–17
Brenner E, Stahlberg R, Mancuso S, Vivanco J, Baluška F, Van Volkenburgh E (2006) Plant neurobiology: an integrated view of plant signaling. Trends Plant Sci 11: 413-419.
Stahlberg R. Historical Overview on Plant Neurobiology. Plant Signal Behav. 2006 Jan-Feb; 1(1): 6–8.