Introduction to Cytokinins:

Discovery:

Gottlie Haberlandt has reported in 1913 that unknown compound in vascular tissue stimulated cellular division. In 1940s, botanists Johannes Van Overbeek seen that plant embryos grew faster when provided coconut milk. Later in 1950s, Folke Skoog and Carlos Miller began another study, after the series of attempts; they were able to isolate development factor liable for cellular division from DNA preparation. They termed the substance Kinetin. And they termed class to which Kinetin belong cytoinins as these substance stimulated cytokinesis, or cellular division.

First naturally happening cytokinins was isolated in 1964 from com (Zean mays) and was termed zeatin. Soon after that, influence of coconut milk on cellular division was described when it was demostrated to have zeatin and zeatin riboside, another cytoinins. Botanists have isolated other naturally occurring cytoinns (like kinetina and 6-benzylamino purine). All of the cytoinins have structures like adenine: i.e., they have side chain rich in carbon and hydrogen joined to nitrogen protruding from top of purine ring. Cytokinins are frequently slight components of RNA.

Synthesis and Transport:

In contrast to what was thought, cytokinins are not breakdown products of DNA rather, they are prepared through mevalonate pathway, same pathway utilized to make gibberellins. Cytokinins are widespread, if not universal, in plants: they have been isolated from Gymnosperms, angiospers, Mosses, and ferns. In angiosperms mot cytoinins are prepared in rots, and also happen in seed, fruit and young leaves. Cytokinins move non-polarly in phloem, xylem, and parenchyma cells.

Effects of Cytokinins:

Though cytokinins have comparatively some uses in agriculture, they do strongly influence plant growth and development.

a) Cellular Division:

Cytokinins stimulate cellular division by speeding up transition of cells from G2 stage (growth stage following DNA replication) to M phase (mitosis) of cell cycle. This effect relies on presence of auxin. For instance, cytokinins alone have no effect on cultured tobacco cells; cellular division starts only when auxin is added to culture medium. Discovery that cytokinins promote cellular division raised another interesting query: could cytokinins be liable for animal cancers that result from uncontrolled cellular division. Research has shown repeatedly that cytokinins have no effects on animal cells and that here is not direct connection between animal cancers and cytokinins.

b) Effect on Cotyledons:

Cytokinins promote cellular division and expansion in cotyledons. Cellular expansion results from cytokinin-induced increase in wall plasticity that doesn't involve wall acidification. Cytokinins also increase amount of sugars (particularly glucose and fructose) in cells that may account for osmotic influx of water and resulting expansion of cytokinins-treated cells in cotyledons.

c) Organogenesis:

Cytokinis and IAA influence organogenesis that is formation of organs illustrates influence of changing amounts of cytokinins and IAA on formation of shoots and roots. Cultured cells grow only in presence of cytokinin and IAA. High cytokinin/auxin ratio favors formation of shoots while low ratios favor formation of roots. Therefore, plant can be entirely regenerated from single cells by varying the amounts of cytokines and IAA. This hormonally controlled means of pant regeneration utilized to propagate plants which are resistant to pathogens, drought and other stresses.

d) Senescence:

Cytokines delay breakdown of chlorophyll in detached leaves, apparently by preventing genes which stimulate chlorophyll formation form being turned off. Cytokines treated areas of leaves stay healthy as remaining plants of leaf senesce. This effect of cytokines may be because of its skill to establish sink to which nutrients move. Though, cytokine induced delay in leaf senescence happen only in detached leaves. Leaf senescence is also delayed by formation of adventitious roots. Roots you will recall are rich in cytokines; and transport of the cytokines to leaves could account for delayed senescence.

Cytokinins and Calcium:

Little amounts of cytokines don't stimulate cellular division in cultured cells. Which just enlarge because of presence of auxin. Though, addition calcium switches growth pattern from cellular growth to cellular division. Thus, presence of calcium increases cell's sensitivity to cytokinin. Cytokinins also stimulate bud formation in mosses; substance which increases cells permeability to Ca2+ mimic effect of cytokinins, and cytokinin-induced formation of buds can be inhibited by applying inhibitors of clamodulin. Likewise, addition of calcium improves cytokinin- induced delay of leaf senescence. These effects may be mediated by cytokinin-induced changes in concentration of Ca2+

Ethylene:

Discovery:

Soviet plant physiologist Dimitry Neljubow recognized ethylene as combustion product of "illumination gas" which was liable for defoliating and inhibiting elongation of plants growing near lamps. R. Gane illustrated that ethylene is prepred by plants and that it causes faster ripening of several fruits, comprising bananas. Substances research illustrated that ethylene fulfill needs of plant hormone; it is prepared in one part of plant and transported to another, where it induces physiological response. Therefore, was discovered first gaseous plant hormone; ethylene.

Synthesis and Transport:

Enthylene is prepared from methionine, and amino acid. Its synthesis is inhibited by CO₂ and needs oxygen. When plants are placed in pure CO₂ or O₂ free air, ethylene synthesis reduces dramatically. Every part of angiospenns prepare ethylene but particularly large amounts are released into air by senescing flowers, roots, nodes, shoot apical meristems and ripening fruits. As most ethylene induced effects result from ethylene in air, effects of ethylene can be contagious. Ethylene prepared by one bad (i.e, overripe) apple can "spoil" (that is induce rapid ripening of) entire bushel of apples.

Effects of Ethylene:

a) Fruit Ripening:

The stimulation of fruit ripening through ethylene is multifaceted and comprises breakdown of chlorophyll and synthesis of other pigments. (that is apples changing from green to red in ripening). Fruit softening because of breakdown of cell walls by cellulose and pectinase, production of volatile compounds related with scent of fruit, and conversion of starches and acids to sugar. Ethylene stimulates each of the features of fruit ripening.

In fruits like tomatoes and apples, there is conspicuous increase in respiration instantly before fruit ripening. This increase in respiration is known as climacteric, and fruits which show it are referred to as climacteric fruits (like Apples). Climacteric starts after huge increase in ethylene production. Therefore climacteric and fruit ripening are triggered by ethylene.

b) Flowering:

Ethylene inhibits flowering in many specier but helps it in few plants comprising pineapples, mangoes and few ornamental plants. This consequence was known by Puerto Rican pineapple growers and Filipino, mango growers who place bonfire near crops. Fires produced ethylene that initiated and synchronized flowering of the plants. Ethylene also helps senescence of flowers. When pollens germinate, stigmas of flowers make large amounts of ethylene which trigger senescence of floral part.

c) Abscission:

Increased production of ethylene at abscission zone triggers breakdown of middle lamella, and thus starts abscission. Fruits like, thus permitting growers to harvest the crops in shorter periods of time.

d) Sex Expression:

Sex of flowers on monoecious plant that is plants which have male and female flowers on same individual is found by ethylene and gibberellins. For instance, cucumber buds treated with ethylene turn out to be carpellate flowers while those treated with gibberellins turn out to be staminate flowers. Likewise, buds which finally become carpellate flowers produced more ethylene than do buds which become staminate flowers.

e) Stem Elongation:

Mechanical disturbances like shaking decrease elongation. This effect known as thigmomorphogenesis is mediated by ethylene. Mechanical disturbances raise ethylene production many fold that causes cells to arrange the cellulose micro-fibrils in longitudinal hoops. The lengthwise reinforcement inhibits cellular elongation, causing cells to enlarge radically and form short, thick stems. This effect is opposite of that of auxins that causes cells to orient the microfibrils more transversely, thus accounting for cellular elongation.

f) Water Logging:

Ethylene synthesis is really reduced in water logged plants as such plants don't have access to sufficient oxygen that is needed in ethylene production. Small amount of ethylene which is made in the roots is trapped where it collects and finally stimulates activity of cellulose and pectirose. These enzyme break down, cell wall and in so doing, form several intercellular spaces characteristic of hydrophytes. In the meantime ethylene precursors in shoot are converted to ethylene that causes parenchyma cells on upper side of petiole to develop and point the leaf down, response known as episnasty.

Ethylene and Auxin:

Ethylene doesn't account for all effect elicited by applying IAA. Many responses of plants to IAA are unrelated to ethylene. For instance, IAA's stimulation of cellular elongation happens independently of ethylene. On the other hand, leaf epinasty and decreased elongation of roots are responses to ethylene rather than IAA.

Abscisic Acid:

Discovery:

Botanists believed that few aspects of plant growth and development resulted from inhibition rather than stimulation of growth. Then Torsten Hemberg of Sweden verified that dormant buds of ash and potato contained inhibitors which blocked effects of IAA. When binds germinated amount of the inhibitors decreased. Hemberg termed the inhibitors Dormins. In early 1960's, Philip Wareing verified Hemberg's finding and reported that applying dormin to bud induced dormancy. Then F. T. Addicott found the compound which stimulated abscission of cotton fruit. He termed this substance abscisin.

Synthesis and Transport:

ABA in Plants is prepared from caratenoids. It happens in angiosperms gymnosperm, and mosses, but in fact not in liveworts. Once synthesized ABA moves all through plant in phaloem, xylem and parenthyma. Like gibberellin and cytokinin ABA moves non polarly. There is no synthetic abscisis acid.

Effects of Abscisic Acid:

1. Closure of Stomata:

In drought, leaves prepare large amounts of ABA that causes stomata to close. Therefore ABA functions as messenger which allows plants to conserve water in drought. As ABA closes stomata within 1-2 minutes, this effect perhaps happens independently of protein synthesis.

2. Bud Dormancy:

ABA was originally thought to manage bud dormancy, but recent proof questions this conclusion, for instance.

• Affect leaves are treated with radioactive ABA, no radioactivity can be detected in buds.
• In many plants, induction and breaking of dormancy don't correlate with changes in endogenous amounts of ABA
• Treatments which induce dormancy don't modify amounts of ABA in buds.

Such results recommend that dormancy is not managed by ABA alone, but is maybe influenced by cytokinins and IAA induced synthesis of ethylene.

3. Seed Dormancy:

Applying ABA delays seed germination in several species. Likewise, amount of ABA in seeds of several plants decreases when seeds germinate. Therefore, ABA may handle seed dormancy in certain species. This situation may not apply to all plans, though, as germination of several seeds happens without any changes in amount of ABA.

4. ABA Counteracts Stimulatory Effects of Other Hormones:

ABA generally inhibits stimulatory effects of other hormones. For instance ABA

i. Inhibits amylase made by seeds treated with gibberellins

ii. Promotes chlorosis which is inhibited by cytoinins

iii. Inhibits wall elasticity and cell development promoted by IAA.

Other Plant Hormones:

Investigations have recognized other compounds which function as hormones in different groups of plants. These comprise:

(a) Oligosaccharins:

Oligosccharins manage plant differentiation, growth, reproduction and defense against disease. To this amount they play role of plant hormones. But unlike hormones they bring out specific effects from different species. For instance oligosaccharins which inhibit flowering and help vegetative development in one species have same effects another species.

Also they are released from cell walls by enzymes. Various oligosaccharins are released by different enzymes and each one conveys the message which regulates specific function. This specificity has prompted many botanists to propose that several effect of hormones such as IAA and gibberellins may be because of activation of enzymes which release specific oligosaccharins.

(b) Batasins:

This is enclosed in yam and induces dormancy of bulbils (vegetative reproductive structures) which form from lateral buds.

(c) Brassosteroids:

Are plant hormones in tea, bean and rice plants which stimulate development of stems.

Hormonal Interactions:

Hormones seldom, if function alone, rather, plant development generally result from interactions of plant hormones. Hormones are controlled in 2 ways.

i. Regulation of rate of synthesis many factors influence rate of hormone production. For instance, daylight can trigger synthesis of IAA and synthesis of gibberellins in biennials is stimulated by cold temperatures.

ii. Regulation of rate of breakdown or inactivation. Inactivation of hormones generally happens either by oxidizing hormone or by conjugating (i.e. combining) it with another compound. Coordinated synthesis and inactivation could control amount of hormone present and thus manage growth response.

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