BIO 250

PLANTS HORMONES AND RESPONSE TO INTERNAL AND EXTERNAL SIGNALS

NAME : MUHAMMAD HASIF BIN MOHAMAD SUZAINIID NUMBER : 2011890782CLASS : AS1204G2LECTURES NAME : AHMAD ZAIMI BIN MOHD ZAWAWI

PLANTS HORMONES In general, plant hormones take control in plant growth and development. Plants cells use hormones to communicate with one another. Plants hormonesare signaling molecules that can stimulate or inhibit plant development, including growth. Environmental cues such as the availability of water,temperature and gravity influence plants by triggering the production and dispersal of hormones. When a plants hormone binds to a target cell, it may modify gene expression, solute concentration, enzyme activity or activity another molecule in the cytoplasm. Some major classes of plant hormones are auxin (IAA), cytokinins, gibberellins, abscisic acid (ABA) and ethylene.

Auxin (Indoleacetic acid, IAA)The term auxin is derived from the Greek word, auxein which mean to grow. Therefore, any chemical substance that have ability and promotes cell elongation can be considered as auxin. The natural auxin in plants is indoleacetic acid, or IAA. Auxins are plants hormones that promote or inhibit cell division and elongation, depending on the target tissue.Auxins that are produced in apical meristems result in elongation of shoots. They also induce cell division and differentiation in vascular cambium, fruit development in ovaries and lateral root formation in roots. Auxins also have inhibitory effect. For example, auxins produced in shoot tip prevents the growth of lateral buds along a lengthening stem, an effect called apical dominance.

Chemical structure of Auxins.

CytokininsGenerally, cytokinins take control in the cell division and differentiation. Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kinetin. Kinetin was the first cytokinin discovered and so named because of the compounds ability to promote cytokinesis (cell division). Cytokinins have been found in almost all higher plants as well as mosses, fungi, bacteria, and also in tRNA of many prokaryotes and eukaryotes. Cytokinins are produced in actively growing tissues, particularly in roots. Cytokinins produced in the root reach their target tissues by moving up the plant in the xylem sap.Cytokinins interact with auxins to stimulate cell division and differentiation. In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not divide. In the presence of cytokinins and auxins, the cells divide, while cytokinins alone have no effect.If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a callus, remains undifferentiated. If cytokinin levels are raised, shoot buds form from the callus. If auxin levels are raised, roots form.Cytokinins, auxins, and other factors interact in the control of apical dominance, the ability of the terminal bud to suppress the development of axillary buds.Cytokinins retard the aging of some plant organs. They inhibit protein breakdown by stimulating RNA and protein synthesis and by mobilizing nutrients from surrounding tissues. Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.

Chemical structure of cytokinins.

Gibberellins (GA)Growth and other processes of development in all flowering plants, gymnosperms, mosses, ferns, and some fungi are regulated in part by gibberellins. These hormones induced cell division and elongation in stem tissue, so they cause stem lengthen between the nodes.The major sites of gibberellins production is at root and leaves. The effects of gibberellins in enhancing stem elongation are evident when certain dwarf varieties of plants are treated with gibberellins. After treatment with gibberellins, dwarf pea plants grow to normal height. However, if gibberellins are applied to normal plants, there is often no response, perhaps because these plants are already producing the optimal dose of the hormone.The embryo of a seed is a rich source of gibberellins. After hydration of the seed, the release of gibberellins from the embryo signals the seed to break dormancy and germinate. Gibberellins support the growth of cereal seedlings by stimulating the synthesis of digestive enzymes that mobilize stored nutrients.

Chemical structure of gibberellins

EthyleneEthylene the only gaseous hormone. It produced by damaged cells in response such as drought, flooding, injury, and infection. It also produced in autumn in deciduous plants or near the end of the life cycle as part of a plants normal process of aging.Ethylenes initiate a seedling to perform a growth called the triple response that enables a seedling to avoid an obstacle as it grows through soil. In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally. It is ethylene and not the physical obstruction that induces the stem to grow horizontally. In simple words, the response of ethylene is mediates fruit ripening.The uses of ethylene in commercial is allows shipping of green, still-hard fruit. Carbon dioxide application stops ripening of fruit in transit to market, then ethylene is applied to ripen distributed fruit quickly.

Chemical structure of ethylene

Abscisic acid (ABA)Abscisic acid is a hormone that was inhibits growth, and has little to do with abscission. ABA is part of a stress response that causes stomata to close. It also diverts photosynthetic products from leaves to seeds, an effect that overrides growth-stimulating effects of other hormones as the growing season ends. ABA inhibits seed germination in some species, such as apple. Such seeds do not germinate before most of the ABA they contain has been broken down, for example by a long periods of cold and wet conditions.Some cormmercial uses of ABA is induces nursery stock to enter dormancy before shipment to minimize damage during handling.

Chemical structure of Abscisic acid

HormonesPrimary souceEffectSite of effects

AuxinsStem tip, young leavesStimulates cell elongationGrowing tissues

Initiate formation of lateral rootsRoots

Inhibits growth (apical dominance)Axillary buds

Stimulate differentiation of xylemCambium

Inhibit abscissionLeaves, fruits

Developing embryosStimulates fruit developmentovary

GibberellinsStem tip, young leavesStimulates cell division,elongationStem inertnode

EmbryoStimulates germinationSeed

Embryo (grass)Stimulates starch hydrolysisEndosperms

Abscisis acidleavesCloses stomataGuard cells

Stimulate formation of dormant budsStem tip

ovuleInhibits germinationSeed coat

CytokininsRoot tipStimulate cell divisionStem tip, axillary buds

Inhibit senescence (aging)Leaves

EthyleneDamaged or aged tissueInhibits cell elongationStem

Stimulate senescence (aging)Leaves

Stimulate ripeningFruits

Major plants hormones and some of their effects.

PLANTS RESPONSE TO INTERNAL AND EXTERNAL SIGNALS

Plants respond to environment stimuli by adjusting the growth of roots and shoots. These response are called tropisms and they are mediated bythe plants hormones. For example, a root or shoot bends because of differences in auxin concentration. Auxin that accumulates in cells on side of a shoot causes the cells to elongate more than the cells on the other side. The results is that the shoot bends away from the side with more auxin. Auxin has the opposite effect in roots. It inhibits elongation of root cells. Thus, a root will bend towards the side with more auxin.

PhototropismLight streaming in from one direction causes stem to curve towards its surce. This response is called phototropism. In this response, phototropism, orients certain parts of the plants in the direction that will maximize the amount of light intercepted by its photosynthetic cells.Phototropism in plants occurs in response to blue light. Nonphotosynthetic pigment called phototropins absorb blue light and translate its energy into a cascade of intercellular signals. The ultimate effect of this cascade is that auxin is redistributed to the shaded side elongate of a shoot or coleoptile. As a result, cell on the shaded side elongate faster than cells on the illuminated side. Differences in growth rates between cells on opposite sides of a shoot or coleoptiles cause the entire structure to bend toward the light.

Auxin is transported to the shaded side, where it cause cells to lengthen.Sunlight strike only one side of a coleoptiles.GravitropismNo matter how aseed is positioned in the soil when it germinates, the radical always grows down, and the primary shoot always grows up. Even if a seedling is turned upside down just after germination, the primary root and shot will curve so the root grows down and the shoot grows up. A growth response to gravity is called gravitropism.The plant know which direction up by gravity-sensing mechanismsof many organisms are based on statoliths. In plants, statoliths are starch-grainstuffed amyloplasts that occur in root cap cells, and also in specialized cells at the periphery of vascular tissue in the stem.Starch grains are heavier than cytoplasm, so statoliths tend to sink to the lowest region of the cell, wherever that is. When statoliths move, they put tension on actin microfilamens of the cell,s cytoskeleton. The filaments are connected to the cell,s membrane, and the change in tension is thought to stimulate certain ion channels in the membranes. The result is that the cell,s auxin efflux carriers move to the new bottom of the cell within minutes of a change in orientation. Thus, auxin is always transported to down-facing side of roots and shoots.

ThigmotropismA plants contact with a solid object may result in a change in the direction of its growth, a response called thigmotropism. The mechanism that gives rise to the response is not well understood, but it involves the products of calcium ions and at least five genes called touch.We can see thigmotropism after a plants tendril touches an object. The cells near the area of contact stop elongating, and the cells on the opposite sideof the shoot keep elongating. The result of unequal growth rates of cells on opposite sides of the shoot is cause to curl around the object.A similar mechanismcauses roots togrow away from contact, which allows them to feel their way around rock and other impassable objects in the soil.

PhototaxisPhototaxis is the ability of organisms to move directionally in response to a light source. Many cyanobacteria exhibit phototaxis, both towards and away from a light source. In the environment, the ability to move into optimal light conditions for photosynthesis is likely to be an advantage. We are particularly interested in how cells perceive light of different wavelengths; the photoreceptors involved and the signal transduction cascade involved in this process.

ChemotaxisChemotaxis, movement toward or away from chemicals, is a universal attribute of motile cells and organisms. The cells swim toward amino acids (serine and aspartic acid), sugars (maltose, ribose, galactose, glucose), dipeptides, pyrimidines and electron acceptors (oxygen, nitrate, fumarate). Figure 1 shows two simple methods for assessing attractant responses. The capillary assay relies on diffusion-generated gradients and is more quantitative, but also more laborious. Soft agar assays involve metabolism-generated chemoeffector gradients and provide a more qualitative, but expedient, measure of chemotactic ability. Cell also swims away from potentially noxious chemicals, such as alcohols and fatty acids, but repellent responses haven't been as extensively studied.

Figure 1: The test chemical diffuses from the capillary mouth, establishing a steep gradient that attracts bacteria to the entrance. The cells enter the capillary and are subsequently documented by colony counts.

Refrences The unity and Diversity of life, 12th edition. Campbell Biology, 9th edition. http://chemotaxis.biology.utah.edu/Parkinson_Lab/projects/ecolichemotaxis/ecolichemotaxis.html http://dpb.carnegiescience.edu/labs/bhaya-lab/projects/phototaxis http://plantsinaction.science.uq.edu.au/edition1/?q=content/8-2-1-gravitropism http://dpb.carnegiescience.edu/article/lighting-plant-hormone-%E2%80%9Ccommand-system%E2%80%9D