Plant development and growth result from complex highly organized events. It involves more than forming masses of new cells or increase in size of the organism. Plant growth and development are handled by internal signals. Leaves of corn plant for instance, always have same basic shape and seeds germinate in specific and predictable ways. Plant development is also affected by external signals from environment. External and internal signals which regulate plant growth are mediated, at least in parts, by plant development regulating substances, or hormones. Plant hormones are organic compounds which are prepared in small amounts in one part of plant and transported to another part where they begin physiological responses. Though, these responses are not always excitation or stimulation, regulation of bud dormancy, for instance, results from inhibition of growth. 5 main classes of plants hormones have been recognized and they are auxins gibberellins, abscisic acid, cytokinins, and ethylene. Such hormones have many characteristic features: they are prepared by plant, are active in small quantities, are transported to other parts of plant.
First hint that there may be a something, which influences growth came from work of Charles Darwin and his son in late 1870s. From the series of manipulations of coleoptiles of canary grass and oat, they concluded that development of coleoptiles toward light was someway controlled by tip of coleoptiles. In 1881, Dawins published the result, in book where they recommended that phototropism (i.e., movement of plant toward light), was because of influence produced in tip of coleoptiles which move to growing region where it caused coleoptiles to grow toward light. In 1913, a new Danish plant physiologist, Peter Boysen-Jensen took up study after cutting tip of coleoptiles, replacing it alone, replacing it on agar, and replacing it on butter, he accomplished that tips of coleoptiles don't have to be in normal position to influence growth, and influence which controls phototropism can move by agar. Boysen-Jensen reasoned that this influence was probably water soluble chemical as it could move through agar. Later he investigated it with butter and it didn't work, he also investigated it with pieces of platinum foil to observe if it contain any electrical signal, and again it didn't work. In 1918, Hungarian plant physiologist Arpad Paal took up effort Paal's study involved replacing cut off coleoptiles on other side and also influencing light conditions. His results recommended to him that tip of coleoptiles generates the substance which moves down and kindle growth, and more significantly, that light should cause it to collect on shaded side of coleoptiles. It was in 1926, that puzzle was lastly solved through studies of Frits Went. Went separated "influence" for phototropism from plant which produced it. He first cut off tips of oat coleoptiles and placed them onto agar so that their cut surfaces touched agar. After about an hour, tips were discarded, and pieces of agar that they had touched were placed on cuts lips of decapitated coleoptiles grown in dark. His findings were convincing.
Went concluded that phototropic curvature was not because of simple presence of coleoptiles tip, but rather because of chemical coming from coleoptiles tip which stimulated growth. He named this chemical AUXIN. Auxin does, infact, influence phototropism; light striking one side of coleoptiles causes auxin to migrate to shaded side of coleoptile, where it stimulates development and causes growth toward light. According to Went's description, auxin fit definition of hormone, it was prepared in one part of plant (tip of coleoptile) and transported to another part (growing region of coleoptile),m where it caused response (increased growth).
Most active naturally happening auxin in plants is Indole-3-acetic acid, or IAA. The most active areas of IAA synthesis are pollen, embryos, young leaves flowers, shoot tips and fruits.
There are 2 other naturally happening auxins 4-chloro-IAA and phenylacetic acid. Precise roles of the auxins in plant growth are unknown, and they are usually less active than IAA.
Though IAA is most naturally occurring auxin, many synthetic compound contain auxin-like effects. Synthetic auxins like 2, 4-D (2,4- dichlorophen-oxyacetic acid) and NAA (Naphthalene acetic acid) contain structures which resemble IAA. Synthetic auxins like 2,4-D are utilized extensively as herbicides as they are inexpensive, comparatively non-toxic to humans and contain elective effect, they kill broadleaf dicots but not monocots. Exact mechanism for the electivity is unknown. Synthetic auxins are also utilized to prevent preharvest dropping of fruit, to generate roots on cuttings, and to inhibit sprouting of lateral buds ("eyes") on Irish potatoes. Unluckily, effects of synthetic auxins on human physiology haven't all been positive. Agent Orange (a1:1 mixture of 2,4,5-T and 2, 4- D) which was utilized during Vietnam war destroyed forests.
IAA moves mainly through parenchyma cells of cortex, pith and vascular tissues it moves slowly. It moves polarly in roots and stems Polar transport needs energy; therefore inhibitors of ATP synthesis block transport of IAA. In stems, IAA is transported basicpetally, meaning that it moves towards base. Basipetal transport in stem continues even if stem is inverted so that apex is pointing down wards in roots, IAA is transported acropetally, meaning that it moves towards tip. IAA made in mature leaves moves non-polarly in the phoem.
Effects of Auxin:
Auxins influence plants in several methods; some of them are given below.
1. Cellular Elongation in Grass Seedlings and Herbs. This will be looked at from two perspectives.
(i) Short-term Effects:
There are two needs for cellular elongation
a. Positive Tugor Pressure:
b. Increased plasticity (stretch-ability) of cell wall. Tugor pressure in cell result mainly from presence of dissolved solutes and is not considerably influenced by IAA. Though, IAA does increase plasticity of, cell wall. IAA does this in many ways:
(ii) Long-Term Effects:
Growth induced by acid stops after 1-3 hours, whereas growth induced by IAA continues much longer. Thus, acid-growth may account for only early stages of IAA induced growth, while, long term growth continues. This recommends that IAA may act at gene level possibly by activating gene needed for making protein necessary for growth.
2. Apical Dominance:
IAA stimulates production of ethylene, another plant hormone. IAA coming from shoot tip stimulates cells around lateral buds to create ethylene and the ethylene made in respond to promptings of IAA inhibits bud growth. Cytokinins (another plant hormone) coming from rot also affect apical dominance Cytokinins.
These observation stress another generalization we can make about plant hormones namely, single aspect of growth and development can be affected by many hormones, the particular response probably results from changing ratios of hormones rather than from presence or absence of individual hormone.
Another example of interaction of IAA and other plant hormones is abscission that is shedding of leaves or fruits by plant. Abscission happens like this
i. Actively growing leaves and fruit produce large amount of IAA that is transported to stem. This IAA along with cytokinin and gibberellins from roots retard onset of senescence and abscission.
ii. Environmental stimuli drought, wounds or nutrient deficiency may cause decreased production of IAA. This starts senescence (aging).
iii. Few signals stimulated cells in abscission zone to expand, suberize and produce cellulose and pectinase.
iv. Cellulose and pectinase digest middle lamella that generally cements cells of abscission zone together.
v. Therefore the wall digestion and concurrent cellular expansion, cells.
4. Differentiation of Vascular Tissues:
Vascular cambium is activated in spring by IAA made by young developing leaves. Gibberellin is also involved. The high auxin/gibberellin ratio promotes differentiation of xylem, while low ratio favors differentiation of phloem. Non-hormonal factors like sugars produced in leaves also influence effect of IAA on cellular differentiation for instance:
These observations show the next generalization about plant hormones. Physiological responses elicited by hormones are strongly influenced by non hormonal factors.
5. Fruit Development:
Sources of auxin which stimulate fruit development are seeds in fruit. For instance fruits of strawberry are dispersed across the swollen receptacle, IAA from seed in every fruit triggers development of adjacent portion of receptacle. Therefore strawberries don't develop when fruits (i.e. sources of IAA are removed. (Similarly when fruits are removed from half of strawberry, only remaining half develops normally.
6. Formation of Adventitious Roots:
Nureries and amateur gardeners exploit skill of auxin to stimulate formation of adventitious roots to propagate plants. Procedure is simple cut surfaces of pieces of parent plant are dipped in the solution of synthetic auxin. Auxin stimulates formation of adventitious roots at cut surface.
Auxins and Calcium:
The Responses elicited by IAA rely on presence of another non-hormonal factor calcium ion (Ca2+). For instance, Ca2+ -is needed for polar transport of IAA, and Ca2+ - deficient plants are generally not responsive to IAA. Most recent model accounting for interaction of IAA and Ca2+ is as follows:
a) IAA stimulates release of Ca2+ from vacuole and endoplasmic reticulum of cell.
b) This release of Ca2+ increases concentration of Ca2+ in cytoplasm
c) This increase in internal Ca2+ activates Calmodulin a Ca2+ - activated protein which regulates several processes in animals, plants and microbes.
d) H+ pumps, and thus produces physiological response.
Ewiti Kurosawa, a Japanese, scientist pioneered efforts in this regard when he isolated hormone gibberllins from rice plant infected with Foolish seedling disease - Gibberella fujikuroi.
Synthesis and Transport:
Gibberelin is made by mevalonic acid pathway. More than 80 gibberellins have been isolated from different fungi and plants. Each gibberellins has inter locking ring structure and one or more carboxyl groups which impart acidic properties to molecule.
Gibberellins are abbreviated GA (for gibberellic acid) and assigned subscripts; which differentiated them from each other. For instance, GA3 is isolated from Gibberella fujikuroi and is most intensively studied gibberellin. Numerous commercial compounds inhibit synthesis of gibberellins. Such inhibitors are known as growth retardants and comprise phosphon D, Cycocel (CCC) and Ancymidol. Growth retardants inhibit stem elongation. Many gibberellins are active and are precursors of more active ones GA, is probably only gibberellin in which controls stem elongation in angiosperms. Gibberellins happen in mosses, gymnosperms, ferns, angiosperms, algae and fungi but are unknown in bacteria in angiosperms, they happen in immature seeds, spices of root and polar, it moves in all direction in xylem and phoem.
Effects of Gibberellins:
1) Extensive growth of intact plants:
Several dwarf mutants usually grow if given GA, and their sensitivity to GA is striking. For instance, dwarf pea seedling, reply to as little as one-billionth of gram of gibberellin. Dwarfism doesn't result from absence of GA, but rather from absence of active GA. Few dwarf plants have lot of GA but of inactive variety.
Therefore we recognize another generalization about plant hormones. Various hormones can elicit similar effects through different mechanisms.
2) Seed Germination:
GAS plays the vital role in seed germination in cereal grasses like barley in this way.
i. Sequence of events leading to seed germination starts with imbibition that is absorption of water by seed. Imbibition causes embryo to release GA.
ii. These GA stimulate transcription of genes of hydrolytic enzymes in seeds' aleurone layer, the specialized layer of endosperm 2-4 cells thick situated just inside seed coat. GA enhances the transcription of amylase mRNA.
iii. Hydrolytic enzymes are secreted by dictyosomes into seeds endosperm. One of the hydrolytic enzymes produced by aleurone layer amylase catalyses conversion of starch to sugar, that is used as energy source for growing seeding.
Many plants have a juvenite stage and an adult stage of growth. For example juvenile stages of eucalyptus have leaves that are shaped differently then those of adult stages. Gibberllins may help determine whether a particular part of a plat is juvenile or adult. For example, the buds of adult branches, usually develop only into adult branches but treating them with GA causes them to grow into juvenile branches.
During the first year of growth, biennial plants like cabbage have short internodes. Such plants are known as rosettes as their tightly paced leaves are arranged like petals of a rose. Rapid expansion of internodes and formation of flower by rosette plants in response to cold is referred to as bolting. Applying GA to rosette plants also induces bolting, recommending that cold temperatures someway stimulates synthesis of GA during following season.
5) Fruit Formation:
Most significant commercial use of gibberellin engages their ability to increase size of seedless grapes. Indeed, almost all vines of "Thompson seedless grape grown in California are sprayed with GA every year. Consequently grapes increase I size almost three fold and are more loosely packed making them less vulnerable to fungal infections.
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