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by Victoria ROSHCHINA, Dr. Sc. (Biology), Leading Researcher, RAS Institute of Cell Biophysics
The present theory on molecular agents of irritability is predicated on our knowledge of acetylcholine and biogenic amines (dophamine, noradrenaline, adrenaline, serotonin, histamine). Discovered in animals, these substances have been found to be mediators (transmitters) in conducting an electric (nervous) impulse from one cell to another.
Further studies have shown such chemical mediating substances to be also present in organisms that have no nervous system, namely in plants, fungi and microorganisms, to perform signal and control functions. More than that, these compounds can play the role of mediators between any interacting cells, including those that belong to different organisms, which indicates their essential universality in living nature. This country's scientists-Ch. Koshtoyants( 1900 - 1961), Corresponding Member of the USSR Academy of Sciences, in the first place-have pioneered in studying this phenomenon.
Now let us look into the role of neurotransmitters in plants proceeding from what we know about them. In fact, our Institute of Cell Biophysics (Russian Academy of Sciences) has pioneering works in this field.
Neurotransmitters have been found in many plant species: acetylcholine - in as many as 78; histamine - in 48; serotonin - in 37; dophamine - in 18; noradrenaline (norepinephrine)-in 17; and adrenaline - in 5 plant species.
The content of these compounds varies. Their amount is high in secretory hairs (especially in nettle and insectivores), in fruit skin, seed and pollen. Within cells, the mediators are concentrated in secretory vesicles. They are also present in such organelles as mitochondria and
chloroplasts. Their content is lowest in cell nuclei.
All these substances have a strong effect on the energetic functions of cells, on fertilization, on motor and growth reactions in plants, and so forth. Possessing high biological activity, they act as chemosig-nalizers and as growth and development regulators, they can change cell membrane permeability, and so on.
Since neurotransmitters are implicated in recognition processes, the phenomenon of their presence in plants is important in many ways. We can see that in the interaction of sex cells in fertilization (mediators found in pollen, in the male gametophyte) as the secreting surfaces of the pollen and pistil contact each other. The same thing occurs in allelopathy, the chemical recognition of cells in contacts among plants of different species, between plants and animals or microorganisms. A particular role is assigned to choline esterases, the enzymes that catalyze acetylcho-line hydrolysis. These enzymes are identified in insectivorous plants (in mucus) that capture their "victims" and in secretes exuded on the surface of the pollen and pistil. If the choline esterase activity is low or absent, this will lead to male sterility or self-incompatibility (when only open, not spontaneous, pollination is possible). If before pollination the pistil's stigma in a flower is treated with tubocurarine, the acetylcholine antagonist (this alkaloid blocks a mediator's receptors), no seeds will be formed in the fruit.
Plant neurotransmitters enable a deeper insight into the evolutionary processes and, which is just as important, are of practical significance, in medicine and farming above all. Unfortunately pharmacologists pay but little attention to them in medication. These compounds meanwhile, either in a complex or singly, exert a potent action on the organism of humans and animals by regulating vascular tension and the condition of the nervous system, and also impact the reproductive function. Thus, the shortage of dophamine may cause Parkinson's disease and depressions. In such cases a banana-based diet or preparations from cactuses, if enriched by this compound, may help cope with such maladies without the use of chemical drugs, not always safe by the way.
Now histamine. It stimulates the secretion of saliva and gastric juice, and is involved in allergic reactions. Histamine is often present in medicinal and food plants, the factor that druggists should take into account. This substance is also accumulated in plant products cultivated on saline soils or fields contaminated by chemical weed- or pest-killers.
Insecticides, inhibiting insect choline esterases, are also contained in plants, and thus may cause poisoning in humans. Therefore new pesticides should first be tested for the sensitivity of plant choline esterases :o them.
These are but only a few examples of how plant neurotransmitters could be applied in practice. The future will bring us new data and new discoveries in this area.
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