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PLANT HORMONES, or phytohormones, organic substances produced by plants that are different from nutrients and are usually formed not where their effect is manifested, but in other parts of the plant. These substances in small concentrations regulate plant growth and their physiological responses to various influences. In recent years, a number of phytohormones have been synthesized, and now they are used in agricultural production. They are used, in particular, for weed control and for producing seedless fruits.

A plant organism is not just a mass of cells growing and multiplying randomly; Plants, both morphologically and functionally, are highly organized forms. Phytohormones coordinate plant growth processes. This ability of hormones to regulate growth is especially clearly demonstrated in experiments with plant tissue cultures. If you isolate living cells from a plant that have retained the ability to divide, then in the presence of the necessary nutrients and hormones they will begin to actively grow. But if the correct ratio of various hormones is not exactly observed, then growth will be uncontrolled and we will get a cell mass resembling tumor tissue, i.e. completely devoid of the ability to differentiate and form structures. At the same time, by properly changing the ratio and concentration of hormones in the culture medium, the experimenter can grow a whole plant with roots, stem and all other organs from a single cell.

The chemical basis of the action of phytohormones in plant cells has not yet been sufficiently studied. It is currently believed that one of the points of application of their action is close to the gene and hormones stimulate the formation of specific messenger RNA here. This RNA, in turn, participates as an intermediary in the synthesis of specific enzymes - protein compounds that control biochemical and physiological processes.

Plant hormones were only discovered in the 1920s, so all information about them is relatively recent. However, back in 1880, Yu. Sachs and Charles Darwin came to the idea of ​​the existence of such substances. Darwin, who studied the effect of light on plant growth, wrote in his book The ability to move in plants(The Power of Movement in Plants): “When the seedlings are freely exposed to side light, some influence is transmitted from the upper part to the lower part, causing the latter to bend.” Speaking about the effect of gravity on the roots of a plant, he concluded that "only the tip (of the root) is sensitive to this influence and transmits some influence or stimulus to the neighboring parts, causing them to bend."

During the 1920s and 1930s, the hormone responsible for the reactions Darwin observed was isolated and identified as indolyl-3-acetic acid (IAA). This work was carried out in Holland by F. Vent, F. Kogl and A. Hagen-Smith. Around the same time, Japanese researcher E. Kurosawa studied substances that cause hypertrophied growth of rice. Now these substances are known as phytohormones gibberellins. Later, other researchers working with plant tissue and organ cultures discovered that crop growth was significantly accelerated if small amounts of coconut milk were added to them. The search for the factor causing this increased growth led to the discovery of hormones that were called cytokinins.

MAIN CLASSES OF PLANT HORMONES

Plant hormones can be grouped into several main classes, depending either on their chemical nature or on the effect they exert.

Auxins.

Substances that stimulate plant cell elongation are collectively known as auxins. Auxins are produced and accumulated in high concentrations in the apical meristems (growth cones of the shoot and root), i.e. in those places where cells divide especially quickly. From here they move to other parts of the plant. Auxins applied to a cut stem accelerate the formation of roots in cuttings. However, in excessively large doses they suppress root formation. In general, the sensitivity to auxins in root tissues is much higher than in stem tissues, so the doses of these hormones that are most favorable for stem growth usually slow down root formation.

This difference in sensitivity explains why the tip of a horizontally lying shoot exhibits negative geotropism, i.e. bends upward, and the root tip has positive geotropism, i.e. bends towards the ground. When auxin accumulates on the underside of a stem under the influence of gravity, the cells on that underside stretch more than the cells on the upper side, and the growing tip of the stem bends upward. Auxin acts differently on the root. Accumulating on its lower side, it suppresses cell elongation here. In comparison, the cells on the upper side stretch more, and the root tip bends toward the ground.

Auxins are also responsible for phototropism - growth bending of organs in response to one-sided lighting. Since the breakdown of auxin in meristems appears to be somewhat accelerated by light, cells on the shaded side stretch more than those on the illuminated side, which causes the shoot tip to bend towards the light source.

The so-called apical dominance - a phenomenon in which the presence of an apical bud prevents the lateral buds from awakening - also depends on auxins. Research results suggest that auxins, in the concentration in which they accumulate in the apical bud, cause the tip of the stem to grow, and moving down the stem, they inhibit the growth of lateral buds. Trees in which apical dominance is sharply expressed, such as conifers, have a characteristic upward-pointing shape, unlike mature elm or maple trees.

After pollination has occurred, the ovary wall and receptacle grow rapidly; a large fleshy fruit is formed. The growth of the ovary is associated with cell elongation, a process in which auxins are involved. It is now known that some fruits can be obtained without pollination if auxin is applied at the appropriate time to some organ of the flower, for example on the stigma. This formation of fruits - without pollination - is called parthenocarpy. Parthenocarpic fruits are seedless.

Rows of specialized cells, the so-called, are formed on the stalk of ripened fruits or on the petiole of old leaves. separating layer. The connective tissue between the two rows of such cells gradually loosens, and the fruit or leaf is separated from the plant. This natural separation of fruits or leaves from the plant is called abscission; it is induced by changes in auxin concentration in the separating layer.

Of the natural auxins, indolyl-3-acetic acid (IAA) is the most widely distributed in plants. However, this natural auxin is used in agriculture much less frequently than synthetic auxins such as indolylbutyric acid, naphthylacetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D). The fact is that IAA is continuously destroyed by plant enzymes, while synthetic compounds are not subject to enzymatic destruction, and therefore small doses can cause a noticeable and long-lasting effect.

Synthetic auxins are widely used. They are used to enhance root formation in cuttings that otherwise do not take root well; for producing parthenocarpic fruits, for example, in tomatoes in greenhouses, where conditions make pollination difficult; in order to cause the fall of some flowers and ovaries in fruit trees (the preserved fruits with such “chemical thinning” turn out to be larger and better); to prevent pre-harvest fruit drop in citrus fruits and some pome trees, for example apple trees, i.e. to delay their natural fall. In high concentrations, synthetic auxins are used as herbicides to control certain weeds.

Gibberellins.

Gibberellins are widely distributed in plants and regulate a number of functions. By 1965, 13 molecular forms of gibberellins had been identified, very similar chemically, but very different in their biological activity. Among synthetic gibberellins, the most commonly used is gibberellic acid, produced by the microbiological industry.

An important physiological effect of gibberellins is the acceleration of plant growth. For example, genetic dwarfism in plants is known, in which the internodes (the sections of the stem between the nodes from which leaves arise) are sharply shortened; as it turned out, this is due to the fact that in such plants the formation of gibberellins during metabolism is genetically blocked. If, however, gibberellins are introduced into them from the outside, then the plants will grow and develop normally.

Many biennial plants require a period of time in either low temperatures or short days, or sometimes both, in order to shoot and bloom. By treating such plants with gibberellic acid, they can be forced to bloom under conditions in which only vegetative growth is possible.

Like auxins, gibberellins can cause parthenocarpy. In California, they are regularly used to treat vineyards. As a result of this processing, the clusters are larger and better formed.

During seed germination, the interaction of gibberellins and auxins plays a decisive role. After the seed swells, gibberellins are synthesized in the embryo, which induce the synthesis of enzymes responsible for the formation of auxin. Gibberellins also accelerate the growth of the primary root of the embryo at a time when, under the influence of auxin, the seed coat is loosened and the embryo grows. The root emerges from the seed first, followed by the plant itself. High concentrations of auxin cause rapid elongation of the embryo stalk, and eventually the tip of the seedling breaks through the soil.

Cytokinins.

Hormones known as cytokinins, or kinins, stimulate cell division rather than elongation. Cytokinins are formed in the roots and from here enter the shoots. Perhaps they are also synthesized in young leaves and buds. The first cytokinin discovered, kinetin, was obtained using DNA from herring sperm.

Cytokinins are “great organizers” that regulate plant growth and ensure the normal development of their shape and structures in higher plants. In sterile tissue cultures, the addition of cytokinins at the proper concentration induces differentiation; primordia appear - undivided rudiments of organs, i.e. groups of cells from which various parts of the plant develop over time. The discovery of this fact in 1940 served as the basis for subsequent successful experiments. In the early 1960s, they learned how to grow entire plants from a single undifferentiated cell placed in an artificial nutrient medium.

Another important property of cytokinins is their ability to slow down aging, which is especially valuable for green leafy vegetables. Cytokinins contribute to the retention of a number of substances in cells, in particular amino acids, which can be used for the resynthesis of proteins necessary for plant growth and renewal of its tissues. Thanks to this, aging and yellowing are slowed down, i.e. Leafy vegetables do not lose their presentation so quickly. Attempts are currently being made to use one of the synthetic cytokinins, namely benzyladenine, as a senescence inhibitor in many green vegetables, such as lettuce, broccoli and celery.

Flowering hormones.

Florigen and vernaline are considered flowering hormones. The assumption about the existence of a special flowering factor was made in 1937 by the Russian researcher M. Chailakhyan. Chailakhyan's later work led to the conclusion that florigen consists of two main components: gibberellins and another group of flowering factors called anthesins. Both of these components are necessary for plants to bloom.

It is assumed that gibberellins are necessary for long-day plants, i.e. those that require a fairly long daylight period to bloom. Anthesins stimulate the flowering of short-day plants, which bloom only when the day length does not exceed a certain permissible maximum. Apparently, anthesins are formed in the leaves.

The flowering hormone vernaline (discovered by I. Melchers in 1939) is believed to be necessary for biennial plants that need exposure to low temperatures, such as winter cold, for some time. It is formed in the embryos of germinating seeds or in the dividing cells of the apical meristems of adult plants.

Dormins.

Dormins are plant growth inhibitors: under their influence, actively growing vegetative buds return to a dormant state. This is one of the last discovered classes of phytohormones. They were discovered almost simultaneously, in 1963 and 1964, by English and American researchers. The latter named the main substance they isolated “abscisin II.” By its chemical nature, abscisin II turned out to be abscisic acid and is identical to dormin, discovered by F. Weiring. It may also regulate leaf and fruit abscission.

B vitamins.

Some B vitamins are sometimes classified as phytohormones, namely thiamine, niacin (nicotinic acid) and pyridoxine. These substances formed in the leaves regulate not so much the formation processes as the growth and nutrition of plants.

Synthetic retardants.

Under the influence of some synthetic phytohormones created in the last half century, plant internodes are shortened, stems become stiffer, and leaves acquire a dark green color. Plant resistance to drought, cold and air pollution increases. In some cultivated plants, such as apple trees or azaleas, these substances stimulate flowering and inhibit vegetative growth. In fruit growing and growing flowers in greenhouses, three such substances are widely used - phosphon, tsikocel and alar.

Chemical structure and classification

Unlike auxins, the criterion for classifying a substance as a gibberellin is its compliance with a certain chemical structure rather than the presence of biological activity. 136 different, similar in structure, substances belonging to the group of gibberellins were found in plants, fungi and bacteria. Thus, gibberellins are the most extensive class of phytohormones. Gibberellins are derivatives of ent-gibberellane and are diterpenoids, but the precursor for biosynthesis is ent-kauren. Gibberellins can have a tetra- or pentacyclic structure (an additional five-membered lactone ring) and accordingly contain 20 (C 20 -gibberellins, for example GK 12) or 19 (C 19 -gibberellins) carbon atoms. Most gibberellins are acids and therefore the designation GA (gibberellic acid) is accepted with an index indicating the order of discovery, for example GA 1, GA 3. The index does not in any way reflect the proximity of chemical structure or position in metabolic pathways. Despite the diversity of gibberellins, several compounds have significant biological activity (GK 4, GK 1, GK 7, GK 3), the rest are biosynthesis precursors or inactive forms. In experimental work, HA 3 is most often used. Gibberellins are unstable and quickly degrade in acidic or alkaline environments.

History of discovery

Gibberellins were discovered by the Japanese scientist E. Kurosawa () while studying rice disease (excessive growth) caused by a fungus Gibberella fujikuroi Sow. The Japanese scientist T. Yabuta isolated gibberellins from this mushroom in crystalline form and gave them their current name.

In higher plants, fast-growing tissues are richest in gibberellins; they are contained in unripe seeds and fruits, sprouts, developing cotyledons and leaves.

Biochemistry of gibberellins

Gibberellins are components of a system that regulates plant growth.

Application

Gibberellins are used in plant growing practice to increase the yield of hemp and flax fiber, to increase the size of berries in seedless grape varieties, to increase the yield of grass, to stimulate seed germination (treatment with gibberellins disrupts the state of tissue dormancy and has a stratifying effect on seeds; with the natural emergence of seeds from the state dormancy, the content of endogenous gibberellins increases), etc. Since gibberellins cause a sharp acceleration in the growth of green mass of plants, their use should be accompanied by increased plant nutrition.

To accelerate the ripening of tomatoes, cherries, apples, as well as to prevent the overgrowth of cereal crops, plants are treated with retardant substances that inhibit the action of gibberellins, for example, 2-chloroethylphosphonic acid (ethephon).

Receipt

Gibberellins are obtained mainly microbiologically from the waste products of fungi of the genus Fusarium.

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See what “Gibberellins” are in other dictionaries:

Substances that play the role of biolins in phytocenoses, stimulating the growth and development of plants. Contained in some lower plants (for example, in mushrooms of the genus Gibberella). Ecological encyclopedic dictionary. Chisinau: Main editorial office of the Moldavian... ... Ecological dictionary

GIBBERELLINS, substances of organic origin (HORMONES) contained in plants that stimulate cell division, stem elongation, overcoming dormancy (resting state) of plants and seeds (which begins the production of enzymes,... ... Scientific and technical encyclopedic dictionary

A group of plant hormones (phytohormones). Stimulates the growth and development of plants, promotes seed germination. By chemical nature, diterpene polycyclic acids; known to St. 60 gibberellins, base gibberellic acid. Formed... ... Big Encyclopedic Dictionary

Plant hormones from the group of diterpenoids are designated GA1, GA2, GA3 (in the sequence of isolation and establishment of structure). Possessing the same molecular skeleton, G. differ from each other in type, number, and location of functions. groups... ... Biological encyclopedic dictionary

Plant growth substances that stimulate seed germination, cell growth and elongation. More than 60 different G. (diterpene polycyclic acids) are known. In addition to plants, they are formed by many microorganisms. First G. (gibberellic acid)… … Dictionary of microbiology

A group of plant hormones (phytohormones). Stimulates the growth and development of plants, promotes seed germination. By chemical nature, diterpene polycyclic acids; Over 60 gibberellins are known, the main gibberellic acid.... ... encyclopedic Dictionary

Plant growth substances. Known 27 G.; they all belong to tetracyclic diterpenoids and are carboxylic acids. The main structural unit of g. is considered to be gibberellin GK9 (I); the rest of G. are considered as his... ... Great Soviet Encyclopedia

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GIBBERELLINS , plant hormones from the group of diterpenoid acids. They are designated GA 1, GA 2, GA 3 (in the sequence of isolation and establishment of the structure). Possessing the same molecular skeleton, gibberellins differ from each other in the type, number and arrangement of functional groups. In low concentrations, gibberellins are widely distributed among higher plants as endogenous growth regulators. In higher concentrations, gibberellins are produced by the fungi Fusarium moniliforme (the conidial stage of the ascomycete Gibberella fujikuroi, which causes hypertrophied growth of rice), Sphaceloma manihoticola and possibly other microorganisms. In total, over 40 gibberellins have been identified in plants, over 20 in mushrooms. One of the most active gibberellins, gibberellic acid (GA 3), is produced by the microbiological industry. In plants, gibberellins are synthesized in intensively growing organs - developing seeds, apical stem buds, and less often in roots. During ontogenesis, the assortment and content of gibberellins change: during seed germination or flowering, physiologically inactive gibberellins - precursors or associated forms of the most active gibberellins - are transformed into the latter (for example, into GA 3), and during fruit ripening and transition to dormancy, active gibberellins form inactive forms ( glucosides, glucose esters and other conjugates, as well as products of oxidative catabolism). The most characteristic physiological effect of gibberellins is the acceleration of organ growth (to a greater extent the stem, to a lesser extent the root) due to both cell division and cell elongation. In addition, gibberellins interrupt the dormant period in seeds, tubers and bulbs, induce flowering of long-day plants on short days, stimulate pollen germination, cause parthenocarpy of fruits, and eliminate physiological and genetic dwarfism; treatment of winter cereals with gibberellins replaces vernalization. Gibberellins are the only known phytohormones for which a direct effect on the biosynthesis of enzymes has been proven. For example, in germinating cereal seeds, gibberellins formed in the embryo pass into the endosperm, where they induce the formation of mRNA responsible for the biosynthesis of α-amylase and other hydrolytic enzymes; this effect ensures the mobilization of seed reserve substances. The primary gibberellin receptors in plant cells are cytoplasmic proteins. Gibberellins are used in agriculture to increase the yield of seedless grape varieties, the yield of flax and hemp fiber, stimulate the germination of seeds, bulbs and tubers, as well as in the production of malt.

Gibberellins are very effective as a stimulator of seed germination. It awakens the seeds from sleep. Under its action, reserve nutrients are mobilized during seed germination.

The seed consists of an embryo, endosperm and seed coat. The supply of nutrients is concentrated in the endosperm in the form of starch, fats, and proteins.

Gibberellin starts the process of starch destruction in seeds, the seed receives the nutrients necessary to begin active development.

It is enough to treat the seeds with a weak solution of gibberellins, and the number and rate of seed germination will increase sharply.

When water enters dry seeds, the embryo secretes gibberellin, which diffuses into the aleurone layer and stimulates the formation of a number of enzymes, including a-amylase. When seeds emerge from a dormant state, gibberellins accumulate in them, so treatment with gibberellin accelerates the germination process of plant seeds, activating the work of enzymes in them. The % germination rate of seeds increases.

When treated with gibberellin, the plant grows in height.

The most general and striking manifestation of the physiological effect of gibberellin is its ability to sharply enhance stem growth in dwarf forms of various plants. The causes of dwarfism are varied. Genetic dwarfism is caused by changes at the gene level and may be associated with disturbances in the synthesis of gibberellins. However, dwarfism may be due to the accumulation of inhibitors. In this case, the addition of gibberellin only neutralizes their effect. Typically, dwarfism is expressed in a decrease in the length of stem internodes while maintaining their number. Dwarf plants treated with gibberellin are equal in height to normal ones, but in subsequent generations dwarfism continues to persist. Molecular genetic studies have expanded our understanding of the features of growth regulation by this phytohormone. Many mutants are known that lack this hormone. As a rule, such gibberellin-defective mutants are dwarf plants that differ from normal ones in one gene, which encodes the formation of gibberellins.

On the left is the control plant. The right plant was treated with Gibberellin. Under its action, the plant stem lengthens and its height increases.

Gibberellins noticeably increase stem elongation and many normal plants. Thus, the stem height of many plants is influenced by spraying with gibberellin increases by about 30-50% . There is a certain relationship between the growth rate of plant stems and the content of gibberellins. Thus, the content of gibberellins and the course of growth of the hemp stem correlate well with each other.

This property allows some researchers to consider gibberellin stem growth hormone. An increase in stem growth occurs both due to increased cell division and due to their elongation. The effect of gibberellins on elongation is associated with the formation of the cell wall protein extensin and an increase in enzyme activity. It has already been noted that auxin also affects elongation growth, but its effect is mainly due to acidification of the cell wall. The influence of gibberellin on the flowering of long-day plants under short-day conditions is associated with the growth of the stem and the emergence of the plant from the rosette state (bolting). The importance of gibberellins for the formation of stolons in potatoes is shown.

Gibberellins, like auxins, are involved in the growth of the ovary and the formation of fruits. Gibberellins accumulate in the kidneys upon emerging from the dormant state. Accordingly, treatment with gibberellin causes interruption of rest in the kidneys.

In some cases under the influence of gibberellin, the total mass of the plant organism increases.

Pea plants (Meteor drarf pea) and the effect of treatment with gibberellins at different concentrations. There is a general increase in the green mass of the plant.

Thus, it does not contribute to the redistribution of nutrients, but to their general accumulation. There is evidence that gibberellins accumulate in chloroplasts. In the light, the influence of gibberellin introduced from outside is more pronounced. All this indicates the importance of gibberellin for the regulation of photosynthesis. Data on this issue are contradictory. However, it has been shown that gibberellin enhances the process of photosynthetic phosphorylation, primarily non-cyclic, and, as a consequence, the main products of this process - ATP and NADPH (N.I. Yakushkina, G.P. Pushkina). At the same time, a decrease in chlorophyll content is observed. Consequently, under the influence of gibberellin, the intensity of use of a chlorophyll unit increases and the assimilation number increases. In the dark, gibberellin only acts on cell stretching without causing

increasing the intensity of their division (K.Z. Hamburg). It can be assumed that in the dark, gibberellin has an indirect effect through a change in the level of auxin content. With different manifestations, gibberellin acts in different ways.

Another example of the attractive effect of gibberellins is the stimulation of the development of seedless fruits. This is especially important when growing seedless grape varieties. If you use gibberellin, the berries are larger and the yield increases.

Effect of gibberellin on grapes.

Gibberellin and plant flowering.

One of the striking effects of gibberellins is the stimulation of flowering in a number of plants. As a rule, the level of endogenous gibberellins increases with increasing day length. In many temperate latitude plants, flowering is controlled by photoperiod. Species that bloom on long days can be forced to bloom With with the help of gibberellins. In the experiments of M.Kh. Chailakhyan, rudbeckia, Kalanchoe, and carrots turned out to be sensitive to gibberellin. However, other species (for example, winter wheat) did not flower after treatment with gibberellins, although for all of these species long days are an important inducer of flowering. Thus, the participation of gibberellin in the regulation of flowering is obvious, although the result largely depends on the particular physiology of a particular plant.

Gibberellin and sex expression in plants

Gibberellin can be used to induce sex changes in plants. In the 1970s, under the leadership of M.Kh. Chailakhyan, research was carried out on cucumbers and hemp. Cucumbers produce both male and female flowers on the same plant, while cannabis is a typically dioecious plant (male and female flowers on different specimens). Treatment with gibberellins caused an increase in the percentage of male plants in hemp and increased the initiation of male flowers on cucumbers. The antagonist hormone in these experiments was cytokinin, which caused the formation of female flowers.

Sometimes the expression of sex depends not only on the species, but also on the genetic line to which the plant belongs. For example, treatment of wild-type tomatoes with gibberellins caused the formation of an excess number of nests in the ovaries (stimulated female development). In tomato mutants stamenless devoid of stamens, gibberellin caused the normalization of the androecium, i.e. stimulated the development of the male sphere in the flower.

Undoubtedly, the level of gibberellins affects the expression of sex in plants. However, the result depends on the type, line, and external circumstances under which the treatment is carried out.

Discovery of gibberellins

In the 1920s, a group of Japanese scientists working at the University of Tokyo investigated a worldwide widespread and very harmful disease of rice seedlings, which was caused by a fungus. Gibberella(now it is classified as a genus Fusarium). Infected seedlings grew elongated, became discolored, and eventually died or produced very poor yields. In 1926, an extract of a fungus was obtained that caused all these symptoms in rice plants. In 1935, the first active substance was obtained in crystalline form, and by 1938 two more were obtained. These substances were named gibberellins by the name of the mushroom. Language barriers, and then the Second World War, delayed the development of work in the West, but immediately after the end of the war, competition began between English and American scientists trying to isolate these substances. In 1954, English researchers isolated one of the active substances, which they called gibberellic acid. It turned out to be the third and most active gibberellin (GA 3) of those isolated in Japan. Gibberellins were soon isolated from a number of higher plants, but the chemical structure of GA 3 was finally established only in 1959 (Fig. 15.19). Currently, over 50 natural gibberellins are known, and all of them differ only slightly from GA 3.

Structure of gibberellins

All gibberellins belong to terpenes, a very complex group of plant substances close to lipids; they are all weak acids and all contain a “gibban” skeleton (Fig. 15.19).

Synthesis of gibberellins and their distribution in the plant

Most gibberellins are found in young, growing organs, and they are synthesized mainly in young apical leaves (possibly in chloroplasts), buds, seeds and root tips. From here they migrate up and down the plant - movement from the leaves is non-polar. They move along the phloem and xylem.

Action of gibberellins

Like auxins, gibberellins primarily cause stem elongation, mainly through cell elongation. With their help, you can restore normal growth in dwarf varieties of peas and corn or turn dwarf beans into a climbing vine (Fig. 15.20), as well as stimulate shoot growth in ordinary plants. The interaction of gibberellins with auxins will be discussed in section. 15.3.

One of the classic effects of gibberellins, which has been very intensively studied in an attempt to understand its mechanism, is the emergence of dormancy in the seeds of some plants, especially cereals. Seed germination is induced by soaking them in water. After the embryo absorbs moisture, it begins to secrete gibberellin, which diffuses into the aleurone layer and stimulates the formation of a number of enzymes, including α-amylase (Fig. 15.21). Enzymes begin to break down the food reserves of the endosperm, and the products of their digestion diffuse into the embryo, where they are used for its growth.