1901, Dimitry Neljubov, a graduate student at the Botanical Institute of St. Petersburg in Russia, observed that dark-grown pea seedlings growing in the laboratory exhibited symptoms that were later termed the triple response: reduced stem elongation, increased lateral growth (swelling), and abnormal, horizontal growth.However when these plants were allowed to grow in fresh air, they regained their normal morphology and rate of growth. Neljubov identified ethylene, which was present in the laboratory air from coal gas, as the molecule causing the response.
The very first indication that ethylene is a natural product of plant tissues was published by H.H Cousins in 1910. Their reports suggests that “emanations” from oranges stored in a chamber caused the premature ripening of bananas when these gases were passed through a chamber containing the fruit. It was concluded that oranges synthesize relatively little ethylene compared to other fruits, such as apples, it is likely that the oranges used by Cousins were infected with the fungus Penicillium, which produces copious amounts of ethylene.
In 1934, R.Gane and others identified ethylene chemically as a natural product of plant metabolism, and because of its dramatic effects on the plant it was classified as a hormone.
This hormone (Ethylene)can be produced by almost all parts of higher plants, although the rate of production depends on the type of tissue and the stage of development. Generally, meristematic regions and nodal regions are the most active in ethylene biosynthesis. The production of Ethylene also increases during leaf abscission and flower senescence, as well as during fruit ripening. Any type of injury /wounding can also induce ethylene biosynthesis, as can physiological stresses such as flooding, chilling, disease, and temperature or drought stress.
The precursor of ethylene is the amino acid methionine, and ACC (1-aminocyclopropane-1-carboxylic acid) serves as an intermediate in the conversion of methionine to ethylene.
Ethylene promotes the ripening of some fruits
Fruit ripening refers to the changes in fruit that make it ready to eat. Such changes typically include softening due to the enzymatic breakdown of the cell walls, starch hydrolysis, sugar accumulation, and the disappearance of organic acids and phenolic compounds, including tannins. From the perspective of the plant, fruit ripening means that the seeds are ready for dispersal.
Ethylene is recognized as the hormone that accelerates the ripening of edible fruits. Exposure of such fruits to ethylene hastens the processes associated with ripening, and a dramatic increase in ethylene production accompanies the initiation of ripening. However, surveys of a wide range of fruits have shown that not all of them respond to ethylene.
Many of the fruits that ripen in response to ethylene exhibit a characteristic respiratory rise before the ripening phase called a climacteric. These fruits also show a spike of ethylene production immediately before the respiratory rise. Inasmuch as treatment with ethylene induces the fruit to produce additional ethylene, its action can be described as autocatalytic. Apples, bananas, avocados, and tomatoes are examples of climacteric fruits.
In contrast to this , fruits such as citrus fruits and grapes do not exhibit the respiration and ethylene production rise and are called nonclimacteric fruits. For example, Bell pepper, Cherry, Citrus Grape, Pineapple, Snap bean, Strawberry and Watermelon.
Elucidation of the role of ethylene in the ripening of climacteric fruits has resulted in many practical applications aimed at either uniform ripening or the delay of ripening. Although the effects of exogenous ethylene on fruit ripening are straightforward and clear, establishing a causal relation between the level of endogenous ethylene and fruit ripening is more difficult. Inhibitors of ethylene biosynthesis (such as AVG) or of ethylene action (such as CO2, MCP, or Ag+) have been shown to delay or even prevent ripening.
Ethylene has important commercial use
Ethylene also regulates many physiological processes in plant development, it is one of the most widely used plant hormones in agriculture. The production of Auxins and ACC can trigger the natural biosynthesis of ethylene.
The most widely used such compound is ethephon, or 2- chloroethylphosphonic acid, which was discovered in the 1960s and is known by various trade names, such as Ethrel.The compound Ethephon is usually sprayed in aqueous solution and is readily absorbed and transported within the plant. It releases ethylene slowly by a chemical reaction, allowing the hormone to exert its effects: Ethephon hastens fruit ripening of apple and tomato and degreening of citrus, synchronizes flowering and fruit set in pineapple, and accelerates abscission of flowers and fruits.
It can be used to induce fruit thinning or fruit drop in cotton, cherry, and walnut. It is also used to promote female sex expression in cucumber, to prevent self-pollination and increase yield, and to inhibit terminal growth of some plants in order to promote lateral growth and compact flowering stems. The facilities of storage has developed to inhibit ethylene production and promote preservation of fruits have a controlled atmosphere of low O2 concentration and low temperature that inhibits ethylene biosynthesis. A relatively high concentration of CO2 (3 to 5%) prevents ethylene’s action as a ripening promoter. Also low pressure (vacuum) is used to remove ethylene and oxygen from the storage chambers, reducing the rate of ripening and preventing overripening.
Specific inhibitors of ethylene biosynthesis and action are also useful in postharvest preservation. Silver (Ag+) is used extensively to increase the longevity of cut carnations and several other flowers. The potent inhibitor AVG retards fruit ripening and flower fading, but its commercial use has not yet been approved by regulatory agencies. The strong, offensive odor of trans-cyclooctene precludes its use in agriculture. Currently, 1-methylcyclopropene (MCP) is beingdeveloped for use in a variety of postharvest applications.
Gibberllins stimulate stem growth in Dwarf and Rossette plants
The Gibberellic acid was first discovered in Japan under unusual circumstances. Japanese farmers, before the World War, found that their paddy crop plants were afflicted with a strange disease called Bakane. The plants with disease showed unusual growth where plants were very tall, weak and sterile. Investigation into the cause, Hori a Japanese plant pathologist discovered that this disease was due to a fungus called Gibberella Fujikoroi , but it is now identified as Fusarium moniliforme. Later Sawada and Kurusowa found that the extracts of this fungus simulated what the fungus could cause on infection. After that a group of Japanese workers led by Yabuta and Sumuki obtained an active principle from the extracts of the causative fungus, and later Yabuta and Hayashi identified the active principle as Gibberllic acid, which is new popularly called as GIBBERLLINS(GA).
The Gibberellins constitute a large family of diterpene acids and are synthesized by a branch of the terpenoid pathway, Gibberellins are tetracyclic diterpenoids made up of four isoprenoid units. The compound Terpenoids is made up of five-carbon (isoprene) building blocks, joined head to tail. The first compound in the isoprenoid pathway committed to gibberellin biosynthesis is ent-kaurene. The biosynthesis up to ent-kaurene occurs in plastids. ent-Kaurene is converted to GA12—the precursor of all the other gibberellins—on the plastid envelope and then on the endoplasmic reticulum via cytochrome P450 monooxygenases.
Commonly a hydroxylation at C-13 also takes place to give GA53.The highest levels of gibberellins are found in immature seeds and developing fruits. Gibberellins that are synthesized in the shoot can be transported to the rest of the plant via the phloem.
Applied gibberellin promotes internodal elongation in a wide range of species. However, the most dramatic stimulations are seen in dwarf and rosette species, as well as members of the grass family. Exogenous GA3 causes such extreme stem elongation in dwarf plants that they resemble
the tallest varieties of the same species.
Accompanying this effect are a decrease in stem thickness, a decrease in leaf size, and a pale green color of the leaves. Some plants assume a rosette form in short days and undergo shoot elongation and flowering only in long days. Gibberellin application results in bolting (stem growth) in plants kept in short days, and normal bolting is regulated by endogenous gibberellin. In addition, as noted earlier, many long-day rosette plants have a cold requirement for stem elongation and flowering, and this requirement is overcome by applied gibberellin. GA also promotes internodal elongation in members of the grass family. The target of gibberellin action is the intercalary meristem—a meristem near the base of the internode that produces derivatives above and below.
The Gibberellin’s effect on genetic dwarfs in its specific action is on internodal growth. The internodes respond favorably than any other parts. The elongation of internodes becomes possible by two simultaneous processes i.e. one by meristematic divisions of intercalary meristems and the second is by the elongation of meristematic cells. The exact mechanism by which cell elongation is achieved is not clear, nonetheless, GA induced early growth of internodes can be inhibited by colchicine, but not by the inhibitors of transcription and translation. This indicates that the cytoskeleton structures like microtubules are involved in GA mediated growth similar to that of auxin induced cell elongation.
The dwarf rice plants also respond very well to GA treatment. GA induced internode elongation in rice plants is due to the activation of inter-calary meristems. After going under few cell divisions, the cell derivatives elongated 80-1000 times the original size. Elongation, in this case, is due to the excretion of protons and labializing the cell wall to be plastic. Meanwhile,the intracellular turgidity also builds up due to the action of GA on plasma membranes. The combined effect of loosening the cell wall, the increased turgour pressure results in the cell elongation.
Cytokinins delay Leaf senescence
Mature plant cells generally do not divide in the intact plant, but they can be stimulated to divide by wounding, by infection with certain bacteria, and by plant hormones, including cytokinins. Cytokinins are N6-substituted aminopurines that will initiate cell proliferation in many plant cells when they are cultured on a medium that also contains an auxin. The principal cytokinin of higher plants—zeatin, or trans-6-(4-hydroxy-3-methylbut-2-enylamino) purine—is also present in plants as a riboside or ribotide and as glycosides. These forms are generally also active as cytokinins in bioassays through their enzymatic conversion to the free zeatin base by plant tissue.
The first committed step in cytokinin biosynthesis—the transfer of the isopentenyl group from DMAPP to the 6 nitrogen of adenosine tri- and diphosphate—is catalyzed by isopentenyl transferase (IPT). The product of this reaction is readily converted to zeatin and other cytokinins.
The Cytokinins are synthesized in the roots, in developing embryos, young leaves, fruits, and crown gall tissues. They are also synthesized by plant-associated bacteria, insects, and nematodes.
Cytokinin oxidases degrade cytokinin irreversibly and may play a role in regulation of the levels of this hormone. Conjugation of both the side chain and the adenosine moiety to sugars (mostly glucose) also may play a role in the regulation of cytokinin levels and may target subpools of
the hormone for distinct roles, such as transport. Cytokinins are also interconverted among the free base and the nucleoside and nucleotide forms.
Leaves detached from the plant slowly lose chlorophyll, RNA, lipids, and protein, even if they are kept moist and provided with minerals. This phenomenen of programmed aging leading to death is termed senescence. The Leaf senescence is more rapid in the dark than in the light. Treatment of isolated leaves of many species with cytokinins will delay their senescence.
However applied cytokinins do not prevent senescence completely, their effects can be dramatic, particularly when the cytokinin is sprayed directly on the intact plant. If only one leaf is treated, it remains green after other leaves of similar developmental age have yellowed and dropped off the plant. Even a small spot on a leaf will remain green if treated with a cytokinin, after the surrounding tissues on the same leaf begin to senesce. In contrast to young leaves, mature leaves produce little if any cytokinin.They may depend on root-derived cytokinins to postpone their senescence.
Senescence is initiated in soybean leaves by seed maturation—a phenomenon known as monocarpic senescence—and can be delayed by seed removal. Although the seedpods control the onset of senescence, they do so by controlling the delivery of root-derived cytokinins to the leaves.
Auxin responsible for promoting formation of lateral and adventitious roots
The most common naturally occurring form of auxin is indole-3-acetic acid (IAA). Another most important roles of auxin in higher plants is the regulation of elongation growth in young stems and coleoptiles. Low levels of auxin are also required for root elongation, although at higher concentrations auxin acts as a root growth inhibitor.
IAA is synthesized primarily in the apical bud and is transported polarly to the root. Polar transport is thought to occur mainly in the parenchyma cells associated with the vascular tissue.
However elongation of the primary root is inhibited by auxin concentrations greater than 10–8 M, initiation of lateral (branch) roots and adventitious roots is stimulated by high auxin levels. The Lateral roots are commonly found above the elongation and root hair zone and originate from small groups of cells in the pericycle. Auxin stimulates these pericycle cells to divide. The dividing cells gradually form into a root apex, and the lateral root grows through the root cortex and epidermis.
The Adventitious roots (roots originating from non-root tissue) can arise in a variety of tissue locations from clusters of mature cells that renew their cell division activity. These dividing cells develop into a root apical meristem in a manner somewhat analogous to the formation of lateral roots. It plays a important role in horticulture, the stimulatory effect of auxin on the formation of adventitious roots has been very useful for the vegetative propagation of plants by cuttings.
Auxins have been used commercially in agriculture and horticulture for more than 50 years. The early commercial uses included prevention of fruit and leaf drop, promotion of flowering in pineapple, induction of parthenocarpic fruit, thinning of fruit, and rooting of cuttings for plant propagation. Rooting is enhanced if the excised leaf or stem cutting is dipped in an auxin solution, which increases the initiation of adventitious roots at the cut end. This is the basis of commercial rooting compounds, which consist mainly of a synthetic auxin mixed with talcum powder.
