The reaction of plants to external stimuli
Scientists from the United States and Japan found that plants have a kind of nervous system. Biologists conducted experiments on a plant called Arabidopsis (rockcress). When leaves were cut off, they noticed that the plant changes the level of calcium.
Thereby, a plant injured on one leaf by a nibbling insect can alert its other leaves to begin anticipatory defense responses.
This process involves neurotransmitters – the substances by which this impulse is transmitted from one neuron to another. There are plans to learn from specialists to control and properly interpret plant signals.
Although plants have nothing in common with the central nervous system, they are capable of generating electrical signals propagating throughout the body in response to various stimuli, resembling animal nerve impulses. These signals do not carry specific information about the nature of the stimulus and serve, apparently, for the general mobilization of the body’s defenses.
It is well known that in animals the coordination of the work of parts of the body and purposeful behavior are ensured primarily by the activity of the nervous system. In response to certain stimuli, nerve cells generate an electrical signal – action potential (action potential), rapidly spreading along the processes of neurons and transmitted from one neuron to another with the help of special signaling molecules – neurotransmitters – in the places of interneuronal contacts (synapses).
General characteristics of AP in animals and plants.
The force of AP, that is, the amplitude of the pulses, in animals and plants is similar: from several tens to hundreds of millivolts.
The generation of AP in animals and plants occurs according to the “all or nothing” principle. When excited below the threshold pulse does not occur at all, and when the excitation threshold is reached, the maximum power pulse is generated immediately. The general principle of the propagation of an impulse is also similar: in both animals and plants, the spread of AP is based on the “electroton” phenomenon (the occurrence of AP on a given section of the cell membrane leads to the self-generation of an impulse in neighboring resting regions).
Compared with animals, in plants the AP is “slowed down” by 3-4 orders of magnitude. In animals, the duration of the PD itself and the refractory period is measured in milliseconds, in plants – seconds and tens of seconds (the refractory period is the time during which the cell that generated the AP remains immune to new stimuli and cannot in response to them generate a new AP). Accordingly, the rate of propagation of AP in plants is several orders of magnitude lower than in animals.
In animals, the principle of “all or nothing” does not prevent neurons from transmitting information about the strength of the stimulus. Although by themselves all PDs are the same in strength, in response to a more powerful stimulus, a neuron can issue a series of multiple APs quickly following one after another — something like a “machine-gun burst”. In plants, on the contrary, APs are usually solitary; only a series of several impulses stretched in time are observed only occasionally. Apparently, AP in plants usually do not carry almost any information about the specificity of the stimulus. Many researchers consider plant APs to be non-specific electrical signals.
True, recently there have been reports in the literature about the discovery of rhythmically repetitive ultra-fast APs in some plants, whose propagation velocity is comparable to the propagation speed of nerve impulses (Volkov AG, Collins DJ, Mwesigwa J. 2000. Plant Sci. V. 153. P. 185-190 ). Some scientists note, that plants lacking a nervous system (as a coding and decoding organ) simply do not need to generate a series of such ultra-fast APs, and therefore this discovery is likely to be another artifact (mistake).
The role of the “nerves” in plants is played by conductive beams, which by their structure and “cable properties” remotely resemble the nerves of animals. It was believed that the main role in the conduction of nerve impulses is played by the cells of the bundle parenchyma, which are connected with each other by means of cytoplasmic “bridges” – plasmodesmata. Recently, information has appeared on the participation in the conduction of impulses of phloem cells, sieve tubes as well.
Mechanisms of AP generation in animals and plants
The basis of AP generation in plants and animals is the occurrence of passive ion flows through the ion channels of the cell membrane. In the nerve cell in a “calm” state, the furniture is polarized: the outer side is positively charged, the inner side is negative. When AP occurs in the membrane, ion channels open that allow sodium ions to pass through. Positively charged sodium ions rush inside the cell, which leads to membrane depolarization. A depolarized membrane region stimulates the same depolarization of neighboring regions – this is how the AP spreads.
In plants, AP is also a membrane depolarization, but, unlike animals, it occurs not due to the input current of sodium ions, but due to the outgoing current of chlorine ions. In addition, in the generation of AP in plants (unlike animals), an active role is played by the active work of the “proton pump” pumping protons (H +) from the cell.
After depolarization, the membrane is polarized again due to the outgoing current of potassium ions – this is true for both animals and plants.
The functional role of AP in animals and plants.
With animals, everything is more or less clear. In response to these or other stimuli, neurons generate a series of AP entering the central nervous system. These APs carry information about the strength of the stimulus, as well as its nature since different neurons respond to different stimuli. In the central nervous system, this information is to some extent interpreted and analyzed, and eventually, new APs are generated – control commands to the organs of the body that provide an adequate response of the body to the received signal. Interpretation and analysis can be extremely simple, but in general the whole circuit should consist of at least two parts – “centripetal” (carries a signal from receptors to the central nervous system) and centrifugal (transmits a signal from the central nervous system to the organ that should carry out a functional response to the stimulus) .
In plants, the chain of events must be radically shorter, since they do not have any structures that even remotely correspond in their purpose to the central nervous system. There are no plants and specialized receptor cells responsible for distinguishing external stimuli.
In plants, the stage of reception (perception) and the effector response (the body’s response to a signal) are linked together. A single impulse arises which cannot carry any information about the specific features of the stimulus. This impulse has a non-specific modulating effect on a number of physiological processes along the entire path of propagation through the plant. Given the long duration of AP in plants, as well as significant electrical, ionic and metabolic shifts during its generation and propagation, we can say that AP itself is part of the nonspecific “response” of the plant to the stimulus initiated. Plant AP can be an effective signal of a stress factor if, in the process of propagation through the plant, it itself behaves like a stressor, that is, it imitates its influence.
The development of stress in plants, if it does not lead to the depletion of reliability resources, ends with the transition of the organism to a state of increased resistance to stress factors. This condition in plants can develop not only under the direct influence of a stressor but also under the influence of AP. This phenomenon is called “AP-induced pre-adaptation.”
In general, despite some similarities with a nerve impulse, AP in higher plants is in many ways a unique system signal. Its main role, apparently, consists in “imitating the nonspecific influence of factors potentially important for vital activity outside the zone of irritation”.