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Transcript
Chapter 19 Principal means of intercellular communication What are they? Chemical messengers produced in one place and modulate processes elsewhere Biosynthesis & Metabolism What is it? What does it do How does it do it? 2 3 Endocrine hormone – transported to sites of action distant from the site of synthesis Paracrine hormone – act on cells adjacent to source of synthesis Animals – endocrine only Plants – both endocrine and paracrine hormones Hormones (animal context) Naturally occurring molecules with profound influence on physiology Synthesized in discrete organs or tissues Transported from the site of synthesis to the site of action Control physiological processes in a concentration-dependent manner 4 Plant hormones are different ….. Synthesis -- more diffuse, occurring in many cell types simultaneously May act in the same cells or tissues where produced Relationship between concentration and the degree/ type of bioactivity is not always linear Occur in much smaller concentrations Groups of structurally similar compounds that can elicit the same biological effect 5 Not all stimulators – inhibitory effects Response depends on how “read” by target tissue 6 Same hormone different responses depending on when or where! Rarely act alone -- crosstalk Amount available depends on rates molecules enter and leave the “hormone pool” Enter – de novo synthesis Retrieval of hormone from storage Transport from somewhere else Leave Oxidation/ degredation Synthesis of deactivated conjugate 8 Hormones -- initially discovered as glandular secretions that influenced physiology but …. The search for plant compounds began as early as the mid 1700’s first real research by Darwin 1880 pub – The Power of Movement in Plants 1910 -- shown to be Chemical by Boysen-Jensen First isolated in 1928 by Went 9 6 generally recognized plant hormone families Auxins Giberellins Cytokinins Ethylene Abscisic Acid Brassinosteroids 10 Other signaling molecules (hormones?) Jasmonic acid Salicylic acid Systemin (short [ca. 18 AA] polypeptide) Florigen, the elusive [Raven says it’s been discovered!] Strigolactone Flavonoids ….. 11 Pineapple shoot – 6 μg of IAA/kg Comparable to a needle in 20 Metric Tons of Hay! Prefix 12 Abbreviation centi c 10-2 0.01 milli m 10-3 0.001 micro μ 10-6 0.000001 nano n 10−9 0.000000001 pico p 10−12 0.000000000001 femto f 10−15 0.000000000000001 atto a 10−18 0.000000000000000001 zepto z 10−21 0.000000000000000000001 yocto y 10−24 0.000000000000000000000001 0.000001M 1 μM 0.00001M 0.0001M 0.001M 0.01M 0.1M 1M Potassium Permanganate Dilution ergo, it’s a hormone! 14 Auxins – Most common -- indole-3-acetic acid (IAA) IAA concentrations in plants are generally 15 1 to 100 mg/kg (fresh weight) 5.7-570 nanomoles (0.0057-0.57 μM) in leaves, but even higher in seeds. Single corn grain 308 pM 4 days after germination Shoot 27 pM – required 10 pM/hr to support growth Defined – activity similar to IAA Promote cell elongation in coleoptile and stem sections Cell division in callus cultures in the presence of cytokinins Formation of adventitious roots on detached stems and leaves Other developmental phenomena 16 Natural & Synthetic auxins Principal function -- promotion of cell elongation. Activity of auxins -- manipulated to create herbicides …HOW? WHY? 17 2,4-D 2,4,5-T Dicamba Biosynthesis associated with rapidly dividing and growing tissues … aka ….. SAM and young leaves 1° sites RAM – especially as roots elongate Fruits and seeds …. 18 Dependant on shoot for most of the auxin Synthesized or transported? Structurally related to tryptophan Synthesis occurs by two primary pathways in plants. Tryptophan-dependent pathway Tryptophan-independent pathway Can be bound (conjugated) or free 19 Bound – hormonally inactive Effective signals must be transient and not accumulate over time Concentration controlled by de novo synthesis Inactivation by conjugation with Sugars Amino acids Inactivation by oxidation via IAA oxidase Transport away from target cells 20 Plant axes exhibit apex-base polarity. Structural polarity of plant dependant of polarity of auxin transport Unidirectional transport -from apex to base Only hormone with polar transport! Terms relative! 21 Transport independent of the orientation of the plant tissue Basipetal transport Shoots vascular Roots non-vascular Polar transport Requires energy Gravity independent 22 Polar transport primarily cell-to-cell via symplast Export – auxin efflux Import – auxin uptake of auxin influx 23 Requires metabolic energy – depressed in absence of O2, sucrose or metabolic inhibitors Velocity 3mm/hr – faster than diffusion; slower than phloem translocation Specific protein carriers 24 Chemiosmotic potential Can be chemically inhibited Auxins regulate gene-expression of auxin transport proteins Ethylene affects auxin transport 25 Activity and abundance of uptake and efflux transporters lateral root development Other hormones regulate expression of auxin transporter genes Cell Elongation Promotes growth in stems & coleoptiles Outer tissues the target 10-5 or 10-6 M (or less) is optimum In the absence of auxin, central tissues elongate faster! Inhibits growth in roots Low concentrations (10-10 to 10-9) promote growth Higher concentrations (10-6) inhibit growth Minimum lag time is 10 minutes ….. Remember 10-6 is micromolar ….. 26 Increases the extensibility of the cell wall Osmotic uptake of water Turgor pressure builds Biochemical wall loosening Cell expands in response to turgor pressure 27 Acid-growth hypothesis H+ ions Acid-growth hypothesis H+ ions Predictions ….. 28 Acid buffers should promote short-term growth Auxins should increase rate of acidification Neutral buffers inhibit auxin-induced growth Anything that promotes acidification should stimulate Something (aka The Wall Loosening Factor) has an acidic pH optimum Plant Tropisms – Growth responses Phototropism instrumental in discovery Gravitropism Thigmotropism Hydrotropism Chemotropism Thigmomorphogenesis – irrespective of direction 29 Nastic Movements – Movement responses Mediated by lateral redistribution Darwins -- sites of perception and response were different Coleoptiles sites of high auxin concentration Ability to perceive a directional light source Ability to transport auxin laterally in response 30 Phototropins (flavoproteins) are the photoreceptors (blue light) Lateral Transport Differential Growth Acidification of apoplast 31 Increases auxin transport Lateral redistribution Tip – produces auxin; tissues below tip can sense! No gravity gradient! 32 Amyloplasts (contain starch) sense gravity Statoliths & Statocytes How? Gravitropism Shoots Shoots Orthogravitropic +, Orth = Straight Plagiogravitropic 0-900 Plagio = oblique Diagravitropic 900 Dia = through Plant Physiology By Hopkins & Huner 2004 Sites of gravity perception Shoots & coleoptile – starch sheath around vascular tissues Roots – root cap Mechanism poorly understood Role of ER? 35 Nodal ER – 5-7 rough sheets like flower petals Starch-statolith hypothesis Amyloplasts only organelles that sediment Sed-rate closely correlated with time to perceive gravity Starchless mutants – slower response 36 Starch independent mechanisms may also exist! Response may involve changes in pH 37 Gravitropism Roots http://www.youtube.com/watch?v=I7TmX0rrNRM Notice lateral roots Radish Thigmotropism – touch http://www.youtube.com/watch?v=_SIJ4ov_FxA Rapid response – encircling in 5-10 minutes Long-lasting response -- brief contact induces response that lasts for days Response occurs in light; plants touched in the dark remember. 39 40 Hydrotropism – water Chemotropism – chemical signals Hydrotropism Hydrotropism: soil equally saturated with water Roots – Triticum sp. Wheat subterranean roots Moist Dry Chemotropism Videos? Nastic movements -- Movement unrelated to direction of stimuli. Directional movements 44 Nyctinasty Thigmonasty Heliotropism Nearly every stage of life cycle Morphology depends on directed movement maintains root-shoot polarity Loss of components severe embryonic defects Polarity established in early embryogenesis Effect depends on the identity of target tissue 45 Apical Dominance Floral bud development Phyllotaxy Lateral root formation Vascular development 46 Inhibits primary root formation – stimulates lateral and adventitious roots Below developing buds and young leaves Leaf abscission Fruit formation Apical dominance Apical bud inhibits growth of lateral buds But … how? diminished concentration? Nope – post decapitation concentration increases! 47 Radioisotope labeled apical auxin not present Source: xylem and interfasicular schlerenchyma 48 Vascular differentiation activated in the Spring by IAA production by young, developing leaves Xylem/Phloem ratio depends upon relative amounts of IAA to GA IAA > GA Xylem IAA < GA Phloem 49 Leaf abscission Auxin concentrations highest in young developing leaves (q.v., vascular differentiation) 50 Removal of the leaf blade promotes abscission Ethylene plays a role 51 Fruit Development – produced by developing seeds 52 The principal function of auxins is the promotion of cell elongation. This cell elongation contributes not only to typical vertical growth, but to the directional growth associated with phototropism and gravitropism (see Ch. 23). The cell elongation response to auxin increase with concentration but is saturable. 53 54 Figure 18.5 (individual panels in four slides) Auxins promote the cellular differentiation of vascular tissue, particularly in shoots. The regeneration of vascular tissue after wounding is also mediated by auxin. The differentiation of vascular tissue is concentration dependent: 55 Phloem sieve tubes differentiate under low auxin concentrations. Xylem differentiation occurs with higher auxin concentrations. 56 Figure 18.6 57 Auxins establish apical dominance over the axillary buds on the stem. The auxins produced by the apical meristem are transported down the stem, where they slow or suppress mitosis and cell expansion in the buds. Removing the apex of the plant, such as when a plant is pruned, releases the axillary buds from inhibition, allowing their growth until one of the buds establishes itself as the new apical meristem. 58 Auxins promote cell expansion. The effect of auxins is related to the pH decrease observed during cell enlargement. The role of auxins in cell wall extensibility is currently described by the acid-growth hypothesis. A docking protein anchors a membrane-bound auxin binding protein (or ABP1) receptor to the plasma membrane. When auxin binds to ABP1, a signal transduction cascade is activated which activates enzymes, including phospholipase A2 (PLA2). 59 The role of auxins in cell wall extensibility is currently described by the acid-growth hypothesis. The products of PLA2, lysophospholipids and fatty acids, trigger a protein kinase cascade. The protein kinase cascade activates the proton ATPase, which releases protons into the cell wall space. 60 The role of auxins in cell wall extensibility is currently described by the acid-growth hypothesis. The decreased pH is optimal for the function of cell wall-loosening enzymes such as expansins. When expansins loosen the walls, the turgor pressure within the cell causes the cell to expand. 61 62 Figure 18.8 63 The acid-induced growth of plants is transient, ceasing with 30 to 60 minutes. The maintenance of growth is also mediate by auxins, but through different mechanisms. A component of these mechanisms is the induction of gene expression by auxins. Figure 18.9 Auxins induce the transcription of a set of genes called the primary auxin responsive genes, which include: 64 Small upregulated auxin RNAs (SAUR) AUX/IAA SAUR genes are produced rapidly (<3 min.) in tissues in response to auxin, sometimes asymmetrically in association with gravitropic responses. 65 AUX/IAA genes are induced over longer time frames by auxin (up to 30 min.) The members of this gene family act as transcriptional regulators, influencing transcription through interaction with auxin response factors (ARFs). A repressor protein called Transport Inhibitor Response 1 (TIR1) is also involved and acts to derepress transcription of auxin responsive genes. The activation of a auxin responsive gene occurs via the following steps: TIR1 has a binding site that accepts both auxin and the AUX/IAA protein. When auxin and AUX/IAA bind, the TIR1 complex binds to the SCF complex. The AUX/IAA protein is ubiquinated, tagging it for degradation by the ubiquitin-26S proteaosome pathway. 66 The activation of a auxin responsive gene occurs via the following steps: The degradation of the AUX/IAA protein derepresses the gene. The gene is transcribed, with the resulting mRNA translated into an auxin-responsive protein. The ability of a molecule to bind with TIR1 and derepress gene expression is one of the defining factors that makes a molecule an auxin. 67 68 Figure 18.10 The transport of auxin from sites of synthesis contributes to the control of plant growth and development. The transport of auxin in the shoot is basipetal and occurs by two mechanisms: Some auxin moves in the phloem. Most auxin is transport be a complex cell-to-cell polar transport mechanism. 69 70 Figure 18.11 Transport of auxins in roots occurs in both directions: 71 An acropetal stream of auxin travels through the xylem parenchyma in the stele to the root tip. A basipetal stream reverses the direction of auxin flow upwards through the cortical region. The gradients of auxin created by polar transport contribute to numerous developmental processes. 72 The mechanism of polar auxin transport involves a carrier-mediated, secondary active transport mechanism. Evidence for this transport mechanism comes in part from inhibitor studies, including those with phytotropins such as TIBA, morphactin, and NPA. 73 Figure 18.12 The current chemiosmotic model for polar auxin transport has the following features: A proton motive force serving as the driving force for secondary active transport of IAA. An IAA influx carrier at the top of the auxintransporting cell. An efflux carrier at the base of the auxintransporting cell. 74 The steps involved in the transport of IAA into and out of the cell are as follows: In the cell wall space at the top of the transporting cell, the slightly acidic pH causes ~80% of the IAA to deprotonate to form IAA-. The IAA enters the cell via the influx carrier (encoded by the AUX1 gene), with a smaller amount of the protonated IAAH entering by simple diffusion. 75 The steps involved in the transport of IAA into and out of the cell are as follows: As the cell cytoplasm is near pH 7.0, all of the IAA will deprotonate to form IAA-. The efflux carriers that mediates the transport of IAA- from the cell (encoded by genes of the PIN family) are located only at the cell’s base. The specific positioning of the efflux carrier insures that IAA moves in a polar direction. 76 77 Figure 18.13 78 The PIN genes contribute not only to polar auxin transport, but contribute to other aspects of plant development as well. During embryogenesis, the localization of PIN proteins in the cell membrane help establish the apical-basal axis. Patterns of PIN protein distribution also contribute to lateral root initiation and tropic growth responses (Ch. 23). 79 Figure 18.4 80 Understand the concept of hormones, particularly with respect to plant hormones Recognize the range of chemical compounds considered to be “auxins” Know the tryptophan dependent and independent pathways for auxin synthesis 81 Know the roles for auxins, particularly with respect to plant growth and development Understand how auxins activate auxin responsive genes Know how auxins are transported in plants and the relationship of auxin transport to auxin function and to plant development 82 Figure 18.1 Other natural auxins include: Indole-3-ethanol Indole-3-acetaldehyde Indole-3-acetonitrile Indole-3-butyric acid (IBA) 4-chloroIAA Phenyl acetic acid (PAA) 83 As auxin activity is based upon structure, several synthetic auxins have been made, including: The auxinic activity of synthetic compounds can also be manipulated to create herbicides: 84 Naphthalene acetic acid (NAA) 2,4-D 2,4,5-T Dicamba 85 86 87 88 89 Chemical Nature: Indole-3-Acetic Acid (IAA) – principal naturally occuring auxin. Synthesized via tryptophandependent and tryptophan independent pathways Sites of Biosynthesis: primarily in leaf primordia and young leaves and in developing seeds Transport: both polarly (unidirectionally) and nonpolarly Effects: Apical dominance; tropic responses; vascular tissue differentiation; promotion of cambial activity; induction of adventitious roots on cuttings; inhibitions of leaf and fruit abscission; stimulation of ethylene synthesis; inhibition or promotion (in pineapples) of flowering; stimulation of fruit development First found: coleoptiles 90 Chemical Nature: Gibberellic acid, a fungal produce, is the most widely studied. Synthesized via the terpenoid pathway Sites of Biosynthesis: in young tissues of the shoot and developing seeds. It is uncertain whether synthesis also occurs in roots Transport: probably transported in the xylem and phloem Effects: hyperelongation of shoots by stimulating both cell division and cell elongation, producing tall, as opposed to dwarf plants; induction of seed germination; stimulation of flowering in long-day plants and biennials; regulation of production of seed enzymes in cereals. 91 Chemical Nature: N6-adenine derivatives, phenyl urea compounds. Zeatin is the most common cytokinin Sites of Biosynthesis: primarily in root tips Transport: transported in the xylem from roots to shoots Effects: promotion of cell division; promotion of shoot formation in tissue culture; delay of leaf senescence; application of cytokinin can cause release of lateral buds from apical dominance and can increase root development in arid conditions 92 Chemical Nature: synthesized from a carotenoid intermediate. Name is a minomer – has little to do with abscission. Sites of Biosynthesis: in mature leaves and roots, especially in response to water stress. May be synthesized in seeds Transport: exported from leaves in phloem; from roots in the xylem Effects: stomatal closure; induction of photosynthate transport from leaves to developing seeds; induction of storage-protein synthesis in seeds; embryogenesis; may affect induction and maintenance of dormancy in seeds and buds of certain species 93 Chemical Nature: gas; synthesized from methionine. Only hydrocarbon with a pronounced effect on plants. Sites of Biosynthesis: in most tissues in response to stress, especially in tissues undergoing senescence or ripening Transport: diffusion from its site of synthesis Effects: fruit ripening (especially in climacteric fruits, such as apples, bananas, and avocados); leaf and flower senescence; leaf and fruit abscission 94 Chemical Nature: polyhydroxylated steroid compounds, synthesized as a branch of the terpenoid pathway Sites of Biosynthesis: throughout the plant especially in young growing tissues Transport: endogenous brassinosteroids act locally, at or near their sites of synthesis Effects: a wide range of developmental and physiological processes, including cell division and cell expansion; branching; vascular tissue differentiation; development of lateral roots; seed germination; leaf senescence 95 96 97 Salicylic acid – implicated in the activation of disease resistance following pathogen invasion. Key in regulating thermogenesis in members of Araceae (i.e., skunk cabbage) 98 Jasmonic acid -- derivative of linolenic acid, activates plant defenses against insect herbivores and microbial pathogens. In addition, mediates response to drought, ozone, UV radiation, and other abiotic stresses. Transported via phloem 99 Systemin -- 18 amino acids! Mediates response to wounding by insects (involves protease inhibitors [interferes with protein digestion in atticking insects, retarding growth & development]) Protease inhibitors accumulate not only in wounded tissue, but also in undamaged leaves far from site of attack. Apparently signal jasmonic acid; biosynthesis activated by systemin 100 Florigen -- produced in leaves; causes flowering in the shoot apical meristem Strigolactone -- regulates outgrowth of lateral buds Flavonoids -- function as both intracellular and localized modulators of signal transduction pathways List of signaling agents and growth regulators continues to expand! 101 Auxins – first to be discovered. Master Hormone Primarily synthesized in meristems or actively growing organs 102 Coleoptile apices, root tips, germinating seeds, apical buds Young leaves, developing inflorescences, embryos Actively distributed throughout the plant body Involved in virtually every step of plant growth and development Cell elongation Vascular differentiation Apical dominance -- growth of axillary buds Secondary root initiation Development of axilary buds, flowers & fruits Gravitational responses 103 1. Cellular Elongation 2. Apical Dominance 3. Abscission 4. Differentiation of Vascular Tissues 5. Fruit Development 6. Formation of Adventitious Roots 104 The pathway for tryptophan-independent synthesis has not been fully described, but some details have been determined: Indole-3-acetonitrile is the precursor of IAA. The source of the indole-3-acteonitrile is not known, but is derived from the glucosinolate glucobrassicin in some plants. 105
