Plants emerged before human life in the timeline of evolution. When human beings first evolved from other species, plants were already flourishing in their own kingdom and today, they continue to dominate the earth. In the past, philosophers like Aristotle highlighted the dogmatic view that plants were passive organisms that did not have the ability to sense their environment (Baluška, 2009). Over the centuries, this dogmatic view persisted as human cultures and countries evolved. Darwin was one of the first scientists to talk about dynamic plant life in his book, The Power of the Movement of Plants. Over the last few decades, botanists and scientists are beginning to appreciate that plants are actually highly sensitive living things that interact with other plants as well as their environment in more than one domain of life. Therefore, a lot of scientific interest has been generated in trying to understand better how plants interact and communicate with each other.
The levels at which plants can interact with their environment are multiple. Witzany and Baluska classify these forms of communication in plants at four different levels (Witzany & Baluška, 2012). The first level is interpreting abiotic influences such as light, pressure, touch, gravity etc. and the second is trans-organismic communications of plants with other living species such as bacteria, fungi, insects and other animals. The third level of communication is inter-organismic with other non-related or related plant species and the fourth is intra-organismic when plants communicate at the intracellular or intercellular level. It is a scientifically proven fact that plant communication and interaction is active at all four levels, which alters our previously rigid view that plants are immobile living things with no ability to sense their environment.
It has long been known that plants respond to abiotic stimuli such as light, touch, gravity, but this has never been thought of as a method of communication. However, plants not only sense light, but can move the growth of their leaves and their light requiring parts towards the sunlight. This response indicates a level of cognition and evolutionary response that shows that plants are communicating with their environment to improve their chances of survival. Even though plants are sessile and it has long been thought that intelligence means altering or moving within the environment, the bending of plant leaves towards the sun and the roots traveling in search of water can all be construed as signs of an intelligent cognition (Baluška, 2009). Root tips function almost as the brains of the plant as they move in hunt of minerals and water, just as an animal moves in search for nutrition. Various parts of the plant don’t just use chemical or mechanical stimuli to respond to their environment.
Plants can communicate with other living beings through electrical stimuli as well. Many bacteria and fungi exist with plants in a symbiotic relationship. The establishment of this relationship depends on the chemical exudates secreted from the roots in attracting the zoospores towards themselves, but at the same time, also depends on electrical signals that can even overshadow the effect of the chemotaxis (Bais, Park, Weir, Callaway, & Vivanco, 2004). Even in interaction with other plants, these chemical signals are important for establishing a relationship. Several host plant factors like quinones, hydroxyl acids and flavonoids interact with each other at a chemical level to produce an environment that acts as an inducement for the haustoria of the parasitic plant to develop (Yoder, 1999). Through this chemical signaling, a dynamic relationship is established which communicates to the parasitic plant that it can invade the host plant. Therefore, the way in which plants communicate with the elements of the environment and other species is a lot more complex than thought previously.
Even when it comes to the development of the plant itself and the growth of its various parts, the botany theories have changed and have begun to appreciate that the plant is able to communicate on an intracellular level and an intercellular level as well. Initially, phyllotaxis was thought to be purely a function of transducing mechanical stimuli through stretch activated signals. But now, biochemistry and genetics are thought to play a bigger role in the development of the plant and its various parts. Hormones like auxin and cytokinin are thought to feedback to each other about the positioning of the various plant parts and alter signaling mechanisms to control exactly how and where a new leaf or a root stem appears in the plant (Braybrook & Kuhlemeier, 2010). It is also being increasingly recognized that these hormones do not act independently, but are regulated by small peptides so that asymmetrical auxin gradients necessary for root growth are maintained (Murphy, Smith, & De Smet, 2012). Even the auxin and cytokinin hormones regulate the transcription of these small peptides indicating that there is significant cross talk between the hormones and the small peptides. Therefore, pure mechanical transduction has given way to signaling mechanisms that are mediated by hormones and control various transcription factors in the plant at the intracellular level. This impacts each and every step of the development of the plant and each part communicates with the other via these mobile hormones and signaling factors that can transport themselves from cell to cell with ease and communicate the necessary biochemical messages for the plant’s growth.
Overall, botanical science is moving away from the purely mechanical transduction based theory of how plants respond to and communicate with the external environment and are beginning to understand that plant communication involves a complex signaling mechanism and mediators that are not unlike those found in animal biology. Electrical signals similar to those in the animal neurons are being discovered in many levels of plant-environment communication, whether it is the growth of a root towards gravity or the leaf traps of carnivorous plants (Baluška, 2009). The ability of plants to generate action potentials has been known for over 150 years now and the knowledge about this aspect of plant communication is increasing every year. Plant communication is being linked more and more with the field of neurobiology since botanists are quickly recognizing that the ways in which plants communicate with themselves and with other plants and living beings are not as different from the way animals communicate with their environment. It will be interesting to see how this field of plant neurobiology evolves over the years. Undoubtedly, we will understand more about how plant cells talk to each other, how plants talk to parasites, bacteria, fungi, pollinators and how different parts of the plant communicate with each other.
References
Bais, H. P., Park, S.-W., Weir, T. L., Callaway, R. M., & Vivanco, J. M. (2004). How plants communicate using the underground information superhighway. Trends in Plant Science, 9(1), 26-32. doi: 10.1016/j.tplants.2003.11.008
Baluška, F. (2009). Plant-environment interactions: from sensory plant biology to active plant behavior. Berlin: Springer Berlin Heidelberg.
Braybrook, S. A., & Kuhlemeier, C. (2010). How a plant builds leaves. The Plant cell, 22(4), 1006-1018. doi: 10.1105/tpc.110.073924
Murphy, E., Smith, S., & De Smet, I. (2012). Small signaling peptides in Arabidopsis development: how cells communicate over a short distance. The Plant cell, 24(8), 3198.
Witzany, G., & Baluška, F. (2012). Biocommunication of Plants: Springer-Verlag.
Yoder, J. I. (1999). Parasitic plant responses to host plant signals: a model for subterranean plant-plant interactions. Current opinion in plant biology, 2(1), 65-70. doi: 10.1016/S1369-5266(99)80013-2
