Auxin: The Plant Growth Hormone PDF

Auxin: The Plant Growth Hormone  WHAT IS HORMONE ? Hormones are chemical messengers that are produced in one cell and modulate cellular processes in another cell by interacting with specific proteins that function as receptors linked to cellular transduction pathway. ...

Auxin: The Plant Growth Hormone  WHAT IS HORMONE ? Hormones are chemical messengers that are produced in one cell and modulate cellular processes in another cell by interacting with specific proteins that function as receptors linked to cellular transduction pathway.  Types of hormones Endocrine hormones Paracrine hormones Hormones that are Hormones that act on cells transported to sites of adjacent to the source of action in tissues distant synthesis are referred to as from their site of synthesis paracrine hormones. are referred to as endocrine hormones  Plant hormones The plant development is regulated by six major hormones: Auxins Gibberellins Cytokinins Ethylene Abscisic acid Brassinosteroids Table 39.1  Plant hormones There are some signaling molecules (hormones) like jasmonic acid,salicylic acid and small polypeptides that play roles in resistance to pathogens and defence against herbivores. Strigolactone regulates the outgrowth of letral buds.  Introduction to Auxin First growth hormone to be studied in plants Play important role in growth and development of plant Developmental processes like stem elongation, apical dominance, root initiation, fruit development, meristem development is controlled by auxin  Introduction to Auxin Auxin and cytokinin are required for viability of the plant embryo Whereas other hormones act as regulators of discrete developmental processes  The Emergence of the Auxin concept Charles Darwin and his son Francis studied plant growth phenomena involving tropisms Their interest of study was the bending of plant towards light The phenomenon, which is caused by differential growth, is called phototropism Seedlings of canary grass ( Phalaris canariensis) Young leaves are in protective organ called coleptile  Coleoptiles are highly sensitive to light The power of movement in plants 1881 1926 Frits Went’s studied growth promoting chemicals in tips of Avena sativa  The Principle Auxin: Indole-3-Acetic acid In the mid 1930,it was determined that natural auxin is indole-3-acetic acid Various other auxins are found in plant Auxins are the compound with biological activities similar to those IAA Cell elongation in coleptile and stem section Cell division in callus cultures Formation of adventitious roots on detached leaves and stem  Synthetic Auxins Naphthalene acetic acid (NAA) and indolebutyric acid (IBA) have many uses in agriculture and horticulture -Prevent abscission in apples and berries -Promote flowering & fruiting in pineapples 2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide commonly used to kill weeds 15 Physiological effect of Auxin  CELL ELONGATION Auxin was discovered as the hormone involved in the bending of coleoptiles toward light Auxins Promote Growth in Stems and Coleoptiles,While Inhibiting Growth in Roots Auxin transport basipetally from shoot apex to the tissue below Auxin causes continued elongation of these cells CELL ELONGATION  Action of auxin: Plant tropism 3 main guidance systems control the orientation of the plant axis Phototropism Gravitropism Thigmotropism  Phototropism is mediated by the lateral redistribution of auxin Charles and Francis Darwin provided the first clue concerning the mechanism of phototropism. When a shoot is growing vertically, auxin is transported polarly Auxin can also be transported vertically Cholodny- went model of phototropism Perception of a unilateral light stimulus A decrease in basipetal IAA transport and diversion to lateral transport in response to the phototropic stimulus  Phototropism is mediated by the lateral redistribution of auxin Precise sites of auxin production, light perception and lateral transport have been difficult to define. In maize coleoptiles auxin accumulation zone is upper 1-2mm of tip, the zones of photosensing and lateral transport are within the upper 5mm of the tip. Similar zones are observed in true shoots of all monocots and dicots. Two flavoproteins, Phototropin 1 and 2 are photoreceptors for the blue light signaling pathway. The action spectrum for blue light activation of kinase activity matches the action spectrum for phototropism.  Phototropism is mediated by the lateral redistribution of auxin Phototropin 1 displays a lateral gradient in phosphorylation. Phototropin phosphorylation results in dissociation of plasma membrane proteins In coleoptiles, phototropin phosphorylation gradient induce the lateral movement of auxin.  Phototropism is mediated by the lateral redistribution of auxin Agar block\ coleoptiles curvature bioassay supported the colodny-went models prediction. Acidification of the apoplast plays a role in phototropic growth. Phototropic auxin movement involves inhibition of ABCB 19, destabilization of PIN1 and inhibition\ or relocalization of laterally localized PIN3 proteins Phototropism is mediated by the lateral redistribution of auxin  When dark grown Gravitropism Avena seedlings involves are lateral redistribution oriented horizontally, the coleoptiles of auxin bend upwards in response to gravity. Tissues below the tip are able to respond to gravity as well. Lateral distribution of auxin is more difficult to demonstrate in shoot apical meristem than in coleoptiles. Gravitropism involves lateral redistribution of auxin Dense plastids serve as gravity sensors Gravity does not form gradient. The only way of sensing gravity is through the motion of a sedimenting body. In plants intracellular gravity sensors are large, dense amyloplast. Statoliths –amyloplasts that function as gravity sensors. Statocytes-specialized gravity sensing cells in which statoliths are present.  Gravity sensing may involve pH and calcium ions as second messengers Gravistimulation – when n gravity sensing mechanisms detect that the root or shoot axis is out of alignment with the gravity vector, signal transduction mechanisms transmit this information to initiate corrective differential growth. Localized changes in Ph and Ca2 + gradients are part of of that signaling. Changes in intracellular pH can be detected early in root columella cells responding to gravity.  Gravity sensing may involve pH and calcium ions as second messengers Rapid changes in Ph were observed in Arabidopsis roots, after roots were roated to a horizontal position. Activation of the plasma membrane H+-ATPase is one of the initial event that mediates root gravity perception. Ca2+ release from storage pools might be involved in root gravitropic signal transduction. Treatment of maize roots with EGTA, prevents ca2+ uptake by cells and inhibits root gravitropism. Localized changes in internal ca2+ pools have been observed in root thigmotropism responses. Gravity sensing may involve pH and calcium ions as second messengers  Developmental effects of auxins Influences every stage of plants life cycle from germination to senescence Polar auxin streams directed by PIN proteins which helps in plant organogenesis Tissue elongation and maturity Loss of PIN proteins can cause severe embryonic defect Auxin may act conjugation with other hormones, Ca and reactive oxide for plant development Auxin Regulates Apical Dominance Growing apical bud inhibits the lateral bud growth In 1920, Skog and Thimann performed an experiment on bean and result showed that outgrowth of auxillary bud is inhibited by auxin that is transported basipetally The auxin content of bud increases by decapitation of shoot apex Auxin control auxillary bud growth by acting in xylem and interfasicular sclerechyma of shoot Auxin Regulates Apical Dominance Strigolactones as signal that interact with auxin in regulating apical dominance Strigolactone is a group of trepenoids that produced in both shoots and roots and is transported through xylem Auxin Regulates Apical Dominance  Auxin transport regulates floral bud and phyllotaxy In absence of PIN1, auxin movement to meristem and phyllotaxy is disturbed If lanolin paste is applying on side of apical meristem then leaf perinmodia can be induced Auxin with combine action of AUX1,PIN1 & ABCB19 can predict phyllotactic pattern  Auxin Promotes the Formation of Lateral and Adventitious Roots primary root is inhibited by auxin concentrations greater than 10–8 M High concentration initiate lateral and adventitious roots Cell division in pericyclic is initiated by IAA transport acropetally PIN2, AUX1, ABCB19 & ABCB1 m,ediate uptake auxin into root LAX3 promote cell expension & cell wall modification by uptake auxin into cortical & epidermal cell of roots  Auxin Promotes the Formation of Lateral and Adventitious Roots Cell division and growth is maintained by root and shoot derived auxin  Auxin induce vascular differentiation Vascular differentiation involves interaction of PIN1, ethylene and other hormones When apical bud is grafted onto a clamp of undifferentiated cells forms Auxins controls the regeneration of vascular tissues Auxin induce vascular differentiation  Auxins delays onset of leaf abscission Auxins level high inn young leaves and low in senescing leaves when abscission started Auxin is transported from leaf blade normally prevent abscissions  Auxins promotes fruits development Regulation of fruit development After fruit set, fruit growth depends on auxins Auxins promotes fruits development  Synthetic auxins have a variety of commercial uses Prevention of fruit & leaf drop, induction of parthenocarpic fruit development, thinnning of fruit & rooting of cutting for plant propagation If excised leaf is dipped in auxins then rooting enhanced Parthenocarpic fruit can be induced by treatment of unpollinated flowers with auxins  Synthetic auxins have a variety of commercial uses 2,4-D and dicamba are synthetic auxins which used in herbicides 2,4-D also used as weed control while monocotsare able to inactivate synthetic auxins rapidly than dicots