Recent achievements in the study of long-distance electrical signals in higher plants and the analysis of the relationship between the local action of stressors and the systemic physiological responses were summed up in a review article published in the journal Progress in Biophysics and Molecular Biology by Associate Professor Vladimir Sukhov and his colleagues from the Biophysics Department of Lobachevsky University.
“Electrical signals are reversible changes in the electrical potential difference across the plasma membrane of living cells, and such signals can propagate over large distances. It is a well-known fact that such signals (action potentials) travel through the nerve cells and transmit information within the bodies of animals and humans. Now, it is shown that such signals are also present in plant organisms,” says Vladimir Sukhov.
Signaling in plants is significantly different from that in humans and animals. While in animal organisms frequency coding of information (about the nature of the stressor, its strength, duration, etc.) is possible due to the different number of signals arriving constantly one after another, such coding is extremely rare in plants.
“We can assume that the information about the nature of stress action in plants is more likely to be linked to the existence of various types of electrical signals, the main types being: the action potential arising under the influence of mild stressors (weak cooling, touch, light), the variation potential arising in case of strong damage (destruction of parts of the plant, burns), and the system potential, which are often accompanied by other signals,” concludes Vladimir Sukhov.
Electrical signals propagating from the stressor action zone cause a variety of physiological responses in the cells, whose intensity may vary at different distances from the stimulated zone. The most prominent and well-studied plant responses are locomotor movements of plants when carnivorous plants close their traps or when mimosa folds its leaves.
It was recently discovered, in particular, that plants “know how to count to five”: insects touching the sensitive hairs cause the propagation of a series of electrical signals in the carnivorous plant Venus flytrap; when it happens, the trap closes during the propagation of the second signal and further signals cause increasingly intense secretion of digestive enzymes.
At the same time, the effects of electrical signals on the physiological state is also very significant in plants that are not capable of active movements. Numerous studies show that the electrical signals activate genes responsible for the protection of plants against insects feeding on them, and apparently protect plants from such insects. Among other responses, one can note an important role of the activation of stress hormones (abscisic and jasmonic acid) synthesis in plants, reduction of photosynthetic activity, increased respiration, reduced evaporation from leaves that slows down the flow of organic molecules from the leaf to the root through the phloem, the accumulation in plant leaves of ATP, which is the main “energy currency” in plants, and many other things.
It is clear that such changes in the physiological state of the plants must perform some important function. According to the hypothesis put forward in the review, the main role of the responses induced by electrical signals is to increase plant resistance to various environmental factors (for example, high or low temperature) and attacks of pests or pathogens. Indeed, an increased level of ATP, whose accumulation is associated with the activation of breathing and with a reduction of СО2 consumption during photosynthesis may be used to activate the defense mechanisms of the plant organism and to restore damage inflicted on it; decreased evaporation of water reduces the likelihood of plant wilting; slowing down the flow of organic compounds in the phloem hampers the movement of pathogenic bacteria, etc.
Thus, we can assume that many physiological responses caused by electrical signals are aimed to minimize the interaction of cells with each other and of the whole plant with the environment.
On the one hand, it saves the resources of cells and organs exposed to adverse factors, pathogens and pests; on the other hand, such processes isolate the damaged areas and promote the survival of a plant as a whole (for example, by stopping the spread of infection).
It is important to note that different electrical signals (action potential, variation potential and system potential) appear to have different functions in the implementation of the adaptation process. Thus, the variation potential may play a major role in higher plants, as it arises with a very high probability under the action of life-threatening factors on the plant organism.
The action potential, on the other hand, may occur in stable conditions of the environment under very weak effects that may precede the action of more hazardous factors. It can be assumed that its role is to “warn” plants of possible threats in the future. Finally, the system potential that accompanies the development of other electrical signals, rather plays the role of an additional signaling mechanism in plant areas not reached by the variation potential or action potential.
“On the whole, the survey shows that electrical signals play an important role in the life of plants, by warning them of the danger and triggering metabolic restructuring to adapt to stressors’ actions,” Vladimir Sukhov says in conclusion.
These research results are not only of fundamental but also of practical importance, as they provide the foundation for the development of new methods for improving the sustainability of agricultural plants by controlling their long-distance electric signals and offer new approaches to the assessment of stress conditions in plants in the course of crop monitoring using remote sensing data.