By Lyudmila LUTOVA, Dr. Sc. (Biol.), St. Petersburg State University
Plants are known as the source of substances which have a "directional effect" upon phytotrophs. This makes it possible to regard what we call food dependence as the determining factor for a genetic analysis of interconnections of organisms in ecosystems, using the known regularities for breeding plant varieties which are resistant to certain pests.
The natural biochemical defense of plants from parasitic organisms rests on two principles: depriving the latter of the substances required for their development and synthesis of compounds which influence their physiological reactions. The best studied so far is the second type of relations. A sufficient number of phytometabolites have been described (including volatile organic compounds like alcohols, esters, acids) which attract or repel insects. For example wild potato Solarium bertaulthi, when damaged, secretes a substance similar to the pheromone of alarm of the plant lice (Aphididae). Canavanin synthesized by creeping beans Dioclea megacarpa is toxic for most insects whose vital functions it upsets by suppressing the effect of certain enzymes and putting out of action some proteins (incorporating into their molecules). In some cases, however, insects can overcome such biochemical defenses of plants. Thus the beetle Caryedes brasiliensis can easily infect the seeds of the aforesaid beans because it can disintegrate canavanin down to ammonia which it then uses for the synthesis of practically all of the aminoacids.
Less well studied are the requirement of phytotrophs in concrete veg-
Using methods of cell selection in vitro for obtaining nystatin-stable cell lines of tobacco. a -control medium-active growth of cell colonies; b-medium with lethal concentration of nystatin - no growth of cell colonies; c-medium with sublethal concentration of nystatin-growth of single cell colonies from which a cell strain of tobacco resistant to antibiotic is formed.
Formation of plants-regenerants of tobacco from cell lines resistant to nystatin.
etable metabolites. Usually mentioned as such compounds are irreplaceable aminoacids, in particular, some derivatives of tyrosine (incorporated in most proteins); they take part in the process of transfer of nervous excitation and in the biosynthesis of natural growth regulator.
The food value of plants depends not only on the presence of the necessary components, but also by their relative contents. Say, the resistance to larvae of Bryobia redikorzewi of some varieties of rice is associated with their low contents of asparagine. The larvae of the moth Pymusta nubibalis are sensitive to even the slightest changes in the levels of glucose. The difference in the levels of consumption of different varieties of wheat by the pest Eurygaster integriceps depends on the changing ratios of several compounds within starch (amylosa and amylopectin).
The most intriguing from the practical point of view in this range is what we call sterol dependence. It is characteristic of phytotrophs unrelated to one another: Arthropoda, nematodes and even some fungi. Sterols-poly-cyclic (solid alcohols-are incorporated into membranes enveloping the protoplasm of vegetable and animal
Fertility of drozophila in cultivation on homogenes obtained from nystatin- stable cell lines and plants-regenerants.
cells (their best known representative is cholesterol) providing them with optimal durability and plasticity. Lack of sterols causes gradual cell degeneration and death of an organism.
For insects, nematodes and mushrooms, which are incapable of synthesizing these substances all by themselves, the only source of them are plants. Most of the sterols they produce are transformed by phytophages into cholesterol and other compounds, including-in insects-the hormone of molting. And some of the reactions involved are highly specific for different kinds of arthropoda. The lack of most of the phytostyrenes in the rations of female locusts (Locusta migratoria) upset the development of the ovaries causing increased death rates of the embryos. And an addition into the food substances of the same substances stimulated the vegetative growth of some mushrooms and was absolutely indispensable for their proliferation.
Thus, by modifying the composition of styrenes one can obtain plants resistant to a wide range of phytostyrene-dependent pests. With this aim in view two varieties of selective agents are used. The first includes polyenic antibiotics, containing nystatin, philipin and amphotericin B; when these link up with one of the groups of styrenes, they precipitate its pathological changes. The second group includes inhibitors of biosynthesis of styrenes, causing an accumulation in membranes of passive compounds instead of biologically active ones.
Specialists at our Chair of Genetics and Selection conducted experiments on additional aspects of the influence of changes in the composition of vegetable styrenes on the phytotrophs dependent upon them.
With the help of ultraviolet irradiation the specialists obtained cellular lines and sprouts of wild tobacco which retained the signs of nystatin-resistance in cultivation in different environments. Usually larvae of Drosophila melanogaster successfully use plant styrenes and complete their own development using the homogenates of that plant. But in the above experiment that did not take place. While feeding on some of the nystatin-stable cell lines produced by ourselves, in some cases fruit flies had upsets in the development of larvae and anomalies in ovaries (60 percent of cases), while in other cases its multiplication was activated. And the total styrenes content in them was 1.3 times higher than in the wild analogue. And the only visible alteration of their phenotype was inability to blossom.
And there were other experiments with natural twin-species systems "plant- phytostyrenedependent organism". Our researcher L. Hodzhayeva, for example, studied the interaction of cultivated potato phytophtora. It was established as a result that there exists in this system a kind of a biochemical dialogue. Variations of styrene composition in potato and tomatoes suppressed phytophthora, and in tobacco they produced a multiple effect on the proliferation of the fruit flies and the development of its larvae. At the same time these changes have no effect on the food value of the product for man. And that means that the proposed method should be regarded as promising as a means of biological protection of plants.
Illustrations supplied by the author.
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