Lecture 1-4 (1).pptx

Principles of Plant Physiology Dr Claudia Meisrimler Rm 426 claudia.meisrimler@canterbury.ac.nz Lecture 1-4 Introduction to Phytohormones & Signal Transduction Taiz et al. (2015) 6th Edition Plant Physiology and Development (or 5th) Chapter 15 pp 408-409; 414-419 Chapter 18 pp 526-543 www.plantcell.org/cgi/doi/10.1105/tpc.110.tt0310 The plant life cycle – developmental decisions Figure 17.2 Major phases of sporophyte development Figure 17.1 Two contrasting examples of plant form arising from indeterminate growth processes Plants are sessile • Respond to their environment through their physiology, growth and development • use environmental cues to orient in space – light (quantity and quality) – gravity • and in time – daylength – temperature Light • Phototropism – growth towards or away from light • Photomorphogenesis – development in response to light • Photoperiodism – ability of an organism to detect daylength Gravity • Growth in response to gravity - gravitropism Temperature • Response to cold temperatures – dormancy – vernalisation – acclimation to very low temperatures Figure 15.1 Timing of plant responses to the environment ranges from very rapid to extremely slow 1. Internal co-ordination - orientation of growth (polarity) 2. Co-ordination with the external environment Flowering: – leaves perceive daylength – meristems respond by flowering Bud dormancy: – leaves perceive shortening daylength – buds respond Tuberisation: – wild potato – leaves perceive the daylength – underground stolon responds by forming a tuber Tropisms: E.g. gravitropism in roots – root cap perceives gravity – cells in elongation zone respond E.g. phototropism in shoots - shoot tip perceives light - cells in elongation zone respond How are these signals perceived? What are the transmissible stimuli? Response to an abiotic stimulus requires a response to a physical stimulus i.e. requires transduction of a signal • There are three key steps in any signal transduction pathway – Perception/reception of the physical/chemical stimulus – Transduction (from a physical to a chemical stimulus) – Transmittance of stimulus, resulting in a biological response Figure 15.2 General scheme for signal transduction In many instances, the role of the transmissible stimulus taken by the plant hormones What are phytohormones? “........characterized by the property of serving as chemical messengers, by which the activity of certain organs is coordinated with that of others”. -Frits Went and Kenneth Thimann, 1937 Frits Went, 1903-1990 Kenneth Thimann, 1904-1997 Frits Went image courtesy of Missouri Botanical Garden ©2010 Kenneth Thimann photo courtesy of UC Santa Cruz Phytohormones Phytohormones regulate cellular activities (division, elongation and differentiation), pattern formation, organogenesis, reproduction, sex determination, and responses to abiotic and biotic stress. Notholaena standleyi © 2008 Carl Rothfels Phytohormones – old timers and newcomers Cytokinins Gibberellins Auxin Abscisic Acid Ethylene Brassinosteroids Strigolactones Salicylates Jasmonates Phytohormones regulate all stages of the plant life cycle Germination Fruit ripening Embryogenesis Fertilization and fruit formation Seed dormancy Flower development Growth and branching Hormones also help plants cope with stress throughout their life Germination Fruit ripening Embryogenesis Seed dormancy STRESS Fertilization and fruit formation Flower development Growth and branching Most hormones affect most stages of the plant life cycle We will examine each hormones within the context of one of its roles. Remember that these are merely examples; most hormones affect most processes in one way or another. Lecture outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Strigolactones Gibberellins Brassinosteroids Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress Salicylates Jasmonates Cross-regulation of hormonal effects Hormones: Synthesis, transport, perception, signaling and responses Downstream effects Production of active hormone Transport Downstream effects H Binding to receptor Signal transduction Synthesis Conjugation H H De-conjugation Synthesis Production of active hormone Breakdown Many tightly regulated biochemical pathways contribute to active hormone accumulation. Conjugation can temporarily store a hormone in an inert form, lead to catabolic breakdown, or be the means for producing the active hormone. Transport and perception Production of active hormone Transport H Binding to receptor Hormones can move: • through the xylem or phloem • across cellular membranes • through regulated transport proteins Several hormone receptors have recently been identified. They can be membrane bound or soluble Signal transduction Hormonal signals are transduced in diverse ways. Common methods are reversible protein phosphorylation and targeted proteolysis Protein phosphorylation P H Protein dephosphorylation Proteolysis Signal transduction Responses Downstream effects can involve changes in gene transcription and changes in other cellular activities like ion transport Downstream effects Transcription Downstream effects Non-genomic effects (e.g. Ion channel regulation) Hormones: Synthesis, transport, perception, signaling and responses Conjugation H De-conjugation H Synthesis Production of active hormone Transport Downstream effects Breakdown Transcription Protein phosphorylation P H Binding to receptor Protein dephosphorylation Proteolysis Signal transduction Downstream effects Non-genomic effects (e.g. Ion channel regulation) Receptors can be membrane-bound Brassinosteroids Cytokinins Ethylene P ( ) (H P P Hormone binding initiates an information relay Soluble receptors can facilitate interactions between proteins Hormones can act like “molecular glue” TIR1 Auxin PYR1 ABA GA GID1 COI1 JAIle Some receptors initiate protein proteolysis The hormones (red) bind to receptors (green), initiating proteolysis of repressors (yellow) to activate a transcriptional regulator (blue) Auxin Gibberellin Jasmonate Reprinted by permission from Macmillan Publishers, Ltd: NATURE Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Copyright 2009. Proteolytic targets are covalently linked to ubiquitin Ubiquitin Target Ubiquitin is a small (76 aa) protein that targets proteins for proteolytic cleavage. Ubiquitin by Rogerdodd Ubiquitin ligase complexes ubiquitinate target proteins TIR1 Auxin The auxin and jasomonate receptors are F-box proteins, part of an SCF ubiquitin ligase complex Ubiquitin is ligated to the target Ubiquitinated proteins are targeted for proteolysis 26S proteasome Hormones affect vegetative growth: elongation, branching and organogenesis Elongation in the shoot and root of a germinating soybean Organogenesis Germinated seedling Growth by branching Growth by elongation Photo courtesy of Shawn Conley Disrupting hormone synthesis or response interferes with elongation GA Auxin Pea Wild type Gibberellin biosynthesis mutant Brassinosteroid Arabidopsis Wild type Auxin response mutant Arabidopsis Wild type Brassinosteroid biosynthesis mutants Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J.B. (1997) Mendel’s stem length gene (Le) encodes a gibberellin 3 -hydroxylase. Plant Cell 9: 1435-1443.;Gray WM (2004) Hormonal regulation of plant growth and development. PLoS Biol 2(9): e311; Clouse SD (2002) Brassinosteroids: The Arabidopsis Book. Rockville, MD: American Society of Plant Biologists. doi: 10.1199/tab.0009 Auxin • Growth • Phototropism and gravitropism • Branching • Embryonic patterning • Stem cell maintenance • Organ initiation Indole-3-acetic acid (IAA), the most abundant natural auxin Auxin controls growth Charles Darwin studied the way seedlings bend towards light, a direct effect of auxin action Site of signal perception Site of response Coleoptile drawing from Darwin, C., and Darwin, F. (1881) The power of movement in plants. Available online. Darwin concluded that a signal moves through the plant controlling growth “We must therefore conclude that when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend.” Coleoptile drawing from Darwin, C., and Darwin, F. (1881) The power of movement in plants. Available online. Photograph of Darin statue by Patche99z Differential cell growth is a result of auxin movement to the shaded side Cell length Auxin concentration Auxin accumulation on shaded side stimulates elongation and bending. Esmon, C.A. et al. (2006) A gradient of auxin and auxin-dependent transcription precedes tropic growth responses. Proc. Natl. Acad. Sci. USA 103: 236–241. Friml, J., et al. (2002) Lateral relocation of auxin efflux regulator PIN3 mediates tropism in Arabidopsis. Nature 415: 806-809. Auxin moves in part by a chemiosmotic mechanism Cell wall pH 5.5 Cytoplasm pH 7 IAA- IAAH IAA- + H+ IAAH Auxin is a charged anion (IAA-) in the cytoplasm (pH 7). In the more acidic cell wall (pH 5.5) some is uncharged (IAAH). The uncharged form crosses the plasma membrane into the cell where it is deprotonated and unable to exit other than through specific transporters. IAA- + H+ Redrawn from Robert, H.S., and Friml, J. (2009) Auxin and other signals on the move in plants. Nat. Chem. Biol. 5: 325-332. Polar auxin transport Cell wall pH 5.5 Cytoplasm pH 7 IAA- IAAH IAA- + H+ IAAH Auxin transport out of cells is controlled by three families of transport proteins that collectively control the directionality of auxin movement. Asymmetric distribution of the transporters controls polar auxin transport. Net flow of auxin IAA- + H+ Redrawn from Robert, H.S., and Friml, J. (2009) Auxin and other signals on the move in plants. Nat. Chem. Biol. 5: 325-332. Auxin biosynthesis Indole IAA is produced from tryptophan (Trp) via several semi-independent pathways and one Trp-independent pathway. Environmental and developmental control of the genes controlling auxin biosynthesis, conjugation and degradation maintain auxin homeostasis. Tryptophan Indole-3pyruvic acid (IPA) Tryptamine Indole-3acetamide (IAM) Indole-3acetaldoximine (IAOx) Indole-3acetaldehyde IAA Adapted from Quittenden, L.J., Davies, N.W., Smith, J.A., Molesworth, P.P., Tivendale, N.D., and Ross, J.J. (2009). Auxin biosynthesis in pea: Characterization of the tryptamine pathway. Plant Physiol. 151: 1130-1138.. Auxin regulates plant development Lateral organ initiation at the shoot apical meristem Inhibit branching in the shoot Patterning and vascular development Maintain stem cell fate at the root apical meristem Promote branching in the root Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Many of auxin’s effects are mediated by changes in gene expression Genes controlling cell growth Genes involved in signaling Genes coordinating other hormone response pathways Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. The auxin signaling pathway Aux/IAA ARF SCFTIR1 IAA IAA Aux/IAA 1. Auxin binds to SCFTIR1 and Aux/IAA The auxin signaling pathway 2. Aux/IAA ubiquitinated and degraded by 26S proteasome Aux/IAA ARF SCFTIR1 IAA IAA 1. Auxin binds to SCFTIR1 and Aux/IAA The auxin signaling pathway 2. Aux/IAA ubiquitinated and degraded by 26S proteasome Aux/IAA ARF SCFTIR1 IAA IAA 1. Auxin binds to SCF and Aux/IAA TIR1 ARF 3. Degradation of repressor permits transcriptional activation by ARF transcription factors Cytokinins • Cell division • Control of leaf senescence • Control of nutrient allocation • Root nodule development • Stem cell maintenance • Regulate auxin action trans-zeatin, a cytokinin Cytokinins are a family of related adenine-like compounds Isopentenyl adenine trans-zeatin dihydrozeatin cis-zeatin Hirose, N., Takei, K., Kuroha, T., Kamada-Nobusada, T., Hayashi, H., and Sakakibara, H. (2008). Regulation of cytokinin biosynthesis, compartmentalization and translocation. J. Exp. Bot. 59: 75–83. Cytokinin (CK) biosynthesis Regulated by CK and nitrogen Tissue specific; auxin, CK and ABA sensitive Meristem specific CK biosynthesis and inactivation are strongly regulated by CK, other hormones and exogenous factors. Inactive form CKX Upregulated by CK and ABA Hirose, N., Takei, K., Kuroha, T., Kamada-Nobusada, T., Hayashi, H., and Sakakibara, H. (2008). Regulation of cytokinin biosynthesis, compartmentalization and translocation. J. Exp. Bot. 59: 75–83. Cytokinins act antagonistically to auxins CK Auxin Promote stem cell fate at the shoot apical meristem Promote lateral organ initiation at the shoot apical meristem Promote differentiation at the root apical meristem Maintain stem cell fate at the root apical meristem Promote branching in the shoot Inhibit branching in the shoot Inhibit Promote branching in branching the shoot in the root Reprinted by permission from Macmillan Publishers, Ltd: NATURE Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Copyright 2009. Auxin and cytokinin regulate each other’s function at the root apex Cell differentiation Cell division Auxin transport Through effects on each other’s synthesis, transport and response, auxin and cytokinin establish two mutually exclusive domains that coordinate cellular activities at the root apex. Cytokinin Auxin transport and response Cytokinin biosynthesis Auxin Auxin, cytokinin and strigolactones control branching Root branches, called lateral roots, are promoted by auxin and inhibited by CK Shoot branches are promoted by CK and inhibited by auxin and strigolactones Branching controls every aspect of plant productivity from nutrient uptake to crop yields. Coleus shoot image by Judy Jernstedt, BSA ; lateral root image from Casimiro, I., et al. (2001) Auxin transport promotes Arabidopsis lateral root initiation. Plant Cell 13: 843-852. Cytokinin signaling is mediated by a two-component system Input domain Transmitter domain Receiver domain H D Histidine Kinase Output domain Response Regulator A two-component system is a short signaling pathway that moves information form an input to an output. In bacteria it usually consists of two proteins, a histidine kinase (HK) with a conserved histidine residue (H) and a response regulator (RR) with a conserved aspartate residue (D). Information and phosphoryl groups are relayed between the components ATP ADP P H Histidine Kinase P D Response Regulator Stimulation of the input domain activates the kinase activity, phosphorylating the transmitter domain. The phosphoryl group is subsequently relayed to the response regulator. Two component signaling in plants H D Hybrid histidine kinase Most plant HKs are hybrid histidine kinase, which transfer the phosphoryl group to a histidine phosphotransferase. The HPt transfers it to a response regulator. H Histidine phosphotransferase (HPt) D Response regulator Cytokinin control of gene expression in Arabidopsis Receptor H H D D HPt H Type B Arabidopsis response regulators (ARRs) are transcription factors. Type A or C ARRs interfere with CK signaling through as yet unknown means. D A-ARR or C-ARR D B-ARR Transcription Cytokinins affect grain production and drought tolerance Rice plants that accumulate more CK can produce more grain per plant because of changes in inflorescence architecture. Wild-type Elevated CK Tobacco plants that produce more CK are more drought tolerant because of the delay in leaf senescence conferred by CK. Ashikari, M. et al. (2005) Cytokinin oxidase regulates rice grain production. Science 309: 741 – 745, with permission from AAAS; Rivero, R. M. et al. (2007) PNAS 104: 19631-19636. Strigolactones Strigolactones, synthesized from carotenoids, are produced in plant roots. They attract mycorrhizal fungi and promote the germination of parasitic plants of the genus Striga. Image source USDA APHIS PPQ Archive ; Reprinted from Tsuchiya, Y., and McCourt, P. (2009). Strigolactones: A new hormone with a past. Curr. Opin. Plant Biol. 12: 556–561 with permission from Elsevier. Strigolactones inhibit branch outgrowth WT Mutant Apex Bud Auxin Strigolactone Auxin transported from the shoot to the root induces strigolactone synthesis, which indirectly inhibits bud outgrowth. In a rice mutant that does not produce strigolactones, tillers (lateral branches) grow out as shown. Lin, H., et al. (2009) DWARF27, an iron-containing protein required for the biosynthesis of strigolactones, regulates rice tiller bud outgrowth. Plant Cell 21: 1512-1525. Gibberellins • • • • Growth Seed germination Promote flowering Promote sex determination in some species • Promote fruit growth A Gibberellin (GA4) Gibberellins are a family of compounds GA4 is the major active GA in Arabidopsis Only some GAs are biologically active. The major bioactive gibberellins are shown here. Sun T (2008) Gibberellin metabolism, perception and signaling pathways in Arabidopsis: September 24, 2008. The Arabidopsis Book. Rockville, MD: American Society of Plant Biologists. doi: 10.1199/tab.0103 Major GA biosynthetic and catabolic pathways in higher plants Active Inactivation Olszewski, N., Sun, T.p., and Gubler, F. (2002) Gibberellin Signaling: Biosynthesis, Catabolism, and Response Pathways Plant Cell 14: S61-S80 Gibberellins regulate growth The pea mutant le, studied by Mendel, encodes GA3 oxidase, which produces active GA. Loss of function of le reduces active GA levels and makes plants dwarfed. Active Inactivation Wild type Gibberellin biosynthesis mutant le Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J.B. (1997) Mendel’s stem length gene (Le) encodes a gibberellin 3 -hydroxylase. Plant Cell 9: 1435-1443. Genes controlling GA synthesis are important “green revolution” genes Tremendous increases in crop yields (the Green Revolution) during the 20th century occurred because of increased use of fertilizer and the introduction of semidwarf varieties of grains. The semidwarf varieties put more energy into seed production than stem growth, and are sturdier and less likely to fall over. Distinguished plant breeder and Nobel Laureate Norman Borlaug 1914-2009 Photos courtesy of S. Harrison, LSU Ag center and The World Food Prize. Several of the green revolution genes affect GA biosynthesis Wild-type Semidwarf Active Semidwarf rice varieties underproduce GA because of a mutation in a the GA20 oxidase biosynthetic gene. Reprinted by permission from Macmillan Publishers, Ltd. (Nature) Sasaki, A., et al. (2002) Green revolution: A mutant gibberellin-synthesis gene in rice Nature 416: 701-702, copyright 2002.; Olszewski, N., Sun, T.p., and Gubler, F. (2002) Gibberellin sgnaling: Biosynthesis, catabolism, and response pathways Plant Cell 14: S61-S80. GA signaling pathway Low GA High GA Growth promotion GA-responsive transcription factor GA DELLA protein DELLA Transcription GA receptor No transcription DELLA proteins inhibit growth, in part through blocking transcription. GA triggers DELLA protein proteolysis. Proteolysis Reprinted from Davière, J.-M., de Lucas, M., and Prat, S. (2008) Transcription factor interaction: a central step in DELLA function. Curr. Opin. Genet. Devel. 18: 295–303.with permission from Elsevier A gene affecting DELLA protein stability is a “green revolution” gene Wild-type The wheat Rht1 locus encodes a DELLA protein. The dwarf allele lacks the DELLA domain and resists proteolysis. GA Dwarf rht1 Rht1 DELLA Transcription GA rht1 rht1 Reprinted by permission of Macmillan Publishers, Ltd. Peng, J., et al. (1999) 'Green revolution' genes encode mutant gibberellin response modulators. Nature 400: 256-261. Brassinosteroids • • • • Cell elongation Pollen tube growth Seed germination Differentiation of vascular tissues and root hairs • Stress tolerance Brassinolide, the most active brassinosteroid Brassinosteroid (BR) mutants are dwarfed Arabidopsis BRs promote cell elongation in part by loosening cell walls Tomato Pea Cell wall loosening Lowered resistance to internal turgor pressure; cell expansion Bishop, G. J., and Koncz, C. Brassinosteroids and plant steroid hormone signaling. (2002) Plant Cell14: S97-S110. Reducing BR signaling produces dwarf barley H H Wild-type uzu Cell elongation Less cell elongation The uzu plants have a missense mutation in the BR receptor, making them less sensitive to BR. This is the first dwarf grain produced through modification of BR signaling. Chono, M., et al., (2003) A semidwarf phenotype of barley uzu results from a nucleotide substitution in the gene encoding a putative brassinosteroid receptor Plant Physiology 133:1209-1219. BR signaling Low BR Receptor (BRI1) Inhibitor (BKI1) BIN2 BIN2 P P P Without BR, the receptor (BRI1) is bound to an inhibitor (BKI1). The active BIN2 kinase phosphorylates and inactivates transcription factors. BR signaling Low BR High BR Receptor (BRI1) BAK1 Inhibitor (BKI1) BIN2 BIN2 P P BSKs BSKs BSU1 BSU1 P P P P P BIN2 BIN2 Transcription BR-binding causes BAK1 and the BRI1 receptor to phosphorylate each other and BSKs. BSKs phosphorylate and active BSU1 phosphatase, which inactivates BIN2. When BIN2 is inactive, its target transcription factors are dephosphorylated and active. Summary – hormonal control of vegetative growth Plant hormones have diverse effects on plant growth. Auxin, gibberellins and brassinosteroids contribute to elongation growth. Auxin, cytokinins and strigolactones regulate branching patterns. Growth and branching profoundly affect crop yields. Hormonal control of reproductive development In angiosperms: • transition from vegetative to reproductive growth • flower development, • fruit development and ripening • seed development, maturation and germination Photo courtesy of Tom Donald Transition to flowering The decision to reproduce is tightly controlled by environmental and hormonal factors. For many plants daylength is critical in this transition, but other plants are day-length neutral. Similarly, some plants absolutely require specific hormonal signals which have little or no effects on other plants. Lithops flowering GA’s role in initiating flowering varies by species and growth-habit Lolium temulentum Annual temperate grass Yes Beta vulgaris Biennial Yes Malus domestica Perennial No Arabidopsis thaliana Annual Short Days Long Days Yes No Photos courtesy of Plate 271 from Anne Pratt's Flowering Plants, Grasses, Sedges and Ferns of Great Britain c.1878, by permission of Shrewsbury Museums Service; David Kuykendall ARS; Vincent Martinez; Takato Imaizumi. Ethylene promotes flowering in pineapples and other bromeliads A pineapple is a fruit produced from pineapple flowers. Commercial growers treat the plants with ethylene to synchronize flowering. Images couresy of Dave McShaffrey, Marietta College, ©2009, used by permission. Flower development Hormones contribute to flower development in many ways: • Patterning of the floral meristem • Outgrowth of organs • Development of the male and female gametophytes • Cell elongation Ethylene and gibberellins are involved in sex determination Hermaphrodite Male Female Image courtesy of Abdelhafid Bendahmane, URGV - Plant Genomics Research INRA Fruit development and ripening are under hormonal control Pollination initiates petal senescence, cell division and expansion in the ovary to produce a fruit, and fruit ripening. Auxin and GA promote cell division and growth of the fruit Seedless varieties of grapes and other fruits require exogenous application of GA for fruit development. Strawberry receptacles respond to auxin. Auxin + GA GA Auxin Photo credits: Grape flowers by Bruce Reisch; Strawberry flower by Shizhao Fruit ripening is induced by ethylene Auxin GA Ethylene is a gaseous hormone that promotes fruit softening and flavor and color development Ethylene Ethylene • Control of fruit ripening • Control of leaf and petal senescence • Control of cell division and cell elongation • Sex determination in some plants • Control of root growth • Stress responses H H C C H H C2H4 C2H4 Ethylene induces the triple response: • reduced elongation, • hypocotyl swelling, • apical hook exaggeration. Ethylene promotes senescence of leaves and petals Air (control) 7 days ethylene Cotton plants Ethylene promotes leaf and petal senescence. In gas-lit houses, plants were harmed by the ethylene produced from burning gas. Aspidistra is ethylene- resistant and so became popular houseplant. Beyer, Jr., E.M. (1976) A potent inhibitor of ethylene action in plants. Plant Physiol. 58: 268-271. Ethylene shortens the longevity of cut flowers and fruits Ethylene levels can be managed to maintain fruit freshness, commercially and at home. Strategies to limit ethylene effects Limit production - high CO2 or low O2 Removal from the air -KMnO4 reaction, zeolite absorption Interfere with ethylene binding to receptor - sodium thiosulfate (STS), diazocyclopentadiene (DACP), others Reprinted from Serek, M., Woltering, E.J., Sisler, E.C., Frello, S., and Sriskandarajah, S. (2006) Controlling ethylene responses in flowers at the receptor level. Biotech. Adv. 24: 368-381 with permission from Elsevier. Molecular genetic approaches can limit ethylene synthesis ACC synthase ACC oxidase H H S-adenosyl methionine ACC (1-aminocyclopropane-1carboxylic acid) Antisense ACC synthase Control C C H H Ethylene Introduction of antisense constructs to interfere with expression of biosynthesis enzymes is an effective way to control ethylene production. Theologis, A., Zarembinski, T.I., Oeller, P.W., Liang, X., and Abel, S. (1992) Modification of fruit ripening by suppressing gene expression. Plant Phys. 100: 549-551. Ethylene-regulated gene expression is negatively regulated Air Ethylene Air Receptor CTR In the absence of ethylene, CTR binds the receptor and prevents transcription. Ethylene binding to the receptor releases CTR, permitting transcription. Benavente, L.M., and Alonso, J.M. (2006) Molecular mechanisms of ethylene signaling in Arabidopsis. Mol. BioSyst. 2: 165–173. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC. Diagram adapted from Cuo, H., and Ecker, J.R. (2004) The ethylene signaling pathway: new insights. Curr. Opin. Plant Biol. 7: 40-49. Air or Ethylene Ethylene perception mutants interfere with ripening Wild type Receptor CTR Several mutations that affect ethylene perception and signaling interfere with fruit ripening. Green-ripe Never-ripe2 Never-ripe Barry, C. S., et al. (2005) Ethylene insensitivity conferred by the Green-ripe and Never-ripe 2 ripening mutants of tomato. Plant Physiol. 138: 267-275. Abscisic acid • Seed maturation and dormancy • Desiccation tolerance • Stress response • Control of stomatal aperture ABA accumulates in maturing seeds Embryonic patterning Reserve accumulation Desiccation tolerance Seed maturation requires ABA synthesis and accumulation of specific proteins to confer desiccation tolerance to the seed. ABA synthesis and signaling is required for seed dormancy ABA Protein Kinase Transcription Factor Loss of function of ABA signaling (protein kinase or transcription factor function) interferes with ABA-induced dormancy and causes precocious germination. Transcription Nakashima, K., et al. (2009) Three Arabidopsis SnRK2 protein kinases, SRK2D/SnRK2.2, SRK2E/SnRK2.6/OST1 and SRK2I/SnRK2.3, involved in ABA signaling are essential for the control of seed development and Dormancy. Plant Cell Physiol. 50: 1345–1363. Copyright (c) 2009 by the the Japanese Society of Plant Physiologists with permission from Oxford University Press. McCarty, D.R., Carson, C.B., Stinard, P.S., and Robertson, D.S. (1989) Molecular analysis of viviparous-1: An abscisic acid-insensitive mutant of maize. Plant Cell 1: 523-532. Once dormant and dry, seeds can remain viable for very long times These date palm seeds are nearly 2000 years old, but still viable and capable of germination. Five -hundred year old lotus seeds have also been successfully germinated. Having a thick seed coat may help these super seeds retain viability. Date palm growing from 2000 year old seed. From Sallon, S., et al. (2008). Germination, genetics, and growth of an ancient date seed. Science 320: 1464, with permission from AAAS Lotus picture by Peripitus GA is required for seed germination Seed germination requires elimination of ABA and production of GA to promote growth and breakdown of seed storage products. ABA Reserve mobilization Cell expansion GA GA is used by brewers to promote barley germination Breakdown of starch in the endosperm is initiated by GA produced by the endosperm or added during the malting process. GA GA sugars amylase starch Embryo Endosperm Aleurone Images by Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885 and Chrisdesign. Summary – hormonal regulation of reproductive development GA and ethylene promote flowering in some plants. Fruit growth, maturation and ripening are regulated by auxin, GA and ethylene. Seed maturation and germination are regulated by ABA and GA. Understanding the roles of hormones in plant reproduction is important for food production, because most of our caloric intake is derived from seeds. Hormonal responses to abiotic stress Photooxidative stress High temperature stress Water deficit, drought Soil salinity Air pollution Wounding and mechanical damage Cold and freezing stress Plants’ lives are very stressful..... ABA and ethylene help plants respond to stress. Reprinted by permission from Macmillan Publishers, Ltd. Nature Chemical Biology. Vickers, C.E., Gershenzon, J., Lerdau, M.T., and Loreto, F. (2009) A unified mechanism of action for volatile isoprenoids in plant abiotic stress Nature Chemical Biology 5: 283 - 291 Copyright 2009. ABA biosynthesis is strongly regulated ABA levels are tightly controlled. Critical steps in ABA biosynthesis (circled in red) are encoded by multiple tightly regulated genes to ensure rapid and precise control. Reprinted from Nambara, E., and Marion-Pol, A. (2003) ABA action and interactions in seeds. Trends Plant Sci. 8: 213-217 with permission from Elsevier. ABA synthesis is strongly induced in response to stress [ABA] µg/g dry weight Leaf water potential (atm) Hours of drought stress ABA levels rise during drought stress due in part to increased biosynthesis R.L. Croissant, , Bugwood.org www.forestryimages.org . Zabadel, T. J. (1974) A water potential threshold for the increase of abscisic acid in leaves. Plant Physiol. (1974) 53: 125-127. ABA induces stress-responsive genes Osmoprotectants (sugars, proline, glycine betaine) H2O2 Oxidative stress responses – peroxidase, superoxide dismutase O22¯¯ Membrane and protein stabilization (HSPs, LEAs) Movement of water and ions (aquaporins, ion channels) ABA binding to an intracellular receptor initiates transcriptional responses PYL1 is an ABA receptor. When PYL1 binds ABA, it also binds the protein phosphatase PP2C, inhibiting its function. Reprinted by permission of Macmillan Publishers Ltd. Miyazono, K., et al. (2009) Structural basis of abscisic acid signalling. Nature 462: 609-614. ABA signal transduction affects gene expression High ABA Low ABA ABA PYL1 PP2C TF SnRK When ABA is present, inactivation of the PP2C phosphatase permits a protein kinase (e.g. SnRK) to phosphorylate and activate ABA-inducible TFs, promoting transcription of ABA-inducible genes. PYL1 PP2C P P Transcription ABA regulates stomatal aperture by changing the volume of guard cels Pairs of guard cells surround the openings of plant pores called stomata. Image buYizhou Wang, University of Glasgow Guard cells control the opening and closing of stomata to regulate gas exchange: a fine balance is required to allow CO2 in for photosynthesis and prevent excessive water loss. Guard cell image © John Adds, obtained through the SAPS Plant Science Image Database . ABA controls stomatal aperture by changing the volume of guard cels When stomata are open, plants lose water through transpiration. ABA induced by drought causes the guard cells to close and prevents their reopening, conserving water. Sirichandra, C., Wasilewska, A., Vlad, F., Valon, C., and Leung, J. (2009)The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action. Journal of Experimental Botany 2009 60: 1439-1463. © The Author [2009]. Published by Oxford University Press on behalf of the Society for Experimental Biology. ABA-induced stomatal closure is extremely rapid and involves changes in ion channel activities ClA- channel K+in channel H2O K+ ABA triggers an increase in cytosolic calcium (Ca2+), which activates anion channels (A-) allowing Cl- to leave the cell. ABA activates channels that move potassium out of the cell (K+out) and inhibits channels that move potassium into the cell (K+in). The net result is a large movement of ions out of the cell. As ions leave the cell, so does water (by osmosis), causing the cells to lose volume and close over the pore. Adapted from Kwak JM, Mäser P, Schroeder JI (2008) The clickable guard cell, version II: Interactive model of guard cell signal transduction mechanisms and pathways. The Arabidopsis Book, ASPB. doi: 10.1199/tab.0114. Hormonal responses to biotic stress Bacteria, fungi, viruses – Biotrophic organisms Jasmonates Salicylic Acid Herbivores – insects, other animals, fungi – Necrotrophic organisms Photo credits: A. Collmer, Cornell University; Salzbrot. Jasmonates • Response to necrotrophic pathogens • Induction of antiherbivory responses • Production of herbivore-induced volatiles to prime other tissues and attract predatory insects JA biosynthesis Cytoplasm JAR1 conjugation JA-ILE JA conjugated to isoleucine (JAILE) is the active compound. From Acosta, I., et al. (2009) tasselseed1 is a lipoxygenase affecting jasmonic acid signaling in sex determination of maize. Science 323: 262 – 265. Reprinted with permission from AAAS. Jasmonate signaling contributes to defense against herbivory WT Mutant without JA When exposed to hungry fly larvae, plants unable to produce JA have low rates of survival. McConn, M., et al. (1997) Jasmonate is essential for insect defense in Arabidopsis. Proc. Natl. Acad. Sci. USA 94: 5473-5477. Jasmonates induce the expression of anti-herbivory chemicals Wound-induced signals Insect oral secretions Protease inhibitors Feeding deterants R.J. Reynolds Tobacco Company Slide Set and R.J. Reynolds Tobacco Company, Bugworld.org Jasmonates contribute to systemic defense responses Defense responses are activated in distant tissues Jasomonates stimulate production of volatile signaling compounds Herbivore-induced volatiles prime other tissues (and other plants) for attack making them unpalatable (indicated in red). Reprinted from Matsui, K. (2006) Green leaf volatiles: hydroperoxide lyase pathway of oxylipin metabolism. Curr. Opin. Plant Biol. 9: 274-280, with permission from Elsevier. Herbivore-induced volatiles are recognized by carnivorous and parasitoid insects Tim Haye, Universität Kiel, Germany Bugwood.org; R.J. Reynolds Tobacco Company Slide Set and R.J. Reynolds Tobacco Company, Bugworld.org JA-induced changes in gene expression Low JA-Ile High JA-Ile JAresponsive transcription factor F-box protein receptor (COI1) Defense genes JAZ protein No transcription JA-Ile Transcription Proteolysis Salicylic Acid – plant hormone and painkiller • Response to biotrophic pathogens • Induced defense response • Systemic acquired resistance Salicylic Acid Salicylic acid is named for the willow Salix whose analgesic properties were known long before the chemical was isolated. Acetylsalicylic Acid - aspirin Photo credit: Geaugagrrl Salicylate synthesis is induced upon pathogen attack Control +Pathogen Isochorismate synthase (ICS) is induced by pathogen infection SA accumulation induces PATHOGENESIS RELATED (PR) and other defense genes Reprinted from Métraux, J.P. (2002) Recent breakthroughs in the study of salicylic acid biosynthesis. Trends Plant Sci. 7: 332-334, with permission from Elsevier. Reprinted by permission from Macmillan Publishers Ltd. Wildermuth, M.C., Dewdney, J., Wu, G., and Ausubel, F.M. (2001). Isochorismate synthase is required to synthesize salicylic acid for plant defence. Nature 414: 562-565 copyright 2001. Salicylates contribute to systemic acquired resistance SA is necessary in systemic tissue for SAR, but the nature of the mobile signal(s) is still up in the air MeSA SAR SA MeSA It is likely that multiple signals contribute to SAR MeSA SA Plants recognize PAMPS (pathogenassociated molecular patterns) PAMP-triggered immunity SA Immune Responses Flagellin is a conserved bacterial protein recognized by plants, also known as a pathogen-associated molecular pattern (PAMP). Recognition of PAMPs by a plant cell triggers a set of immune responses that are mediated by salicylic acid. Redrawn from Pieterse, C.M.J, Leon-Reyes, A., Van der Ent, S., and Saskia C M Van Wees, S.C.M. (2009) Nat. Chem. Biol. 5: 308 – 316 . Some pathogens elicit a stronger defense response Many plants express resistance genes that recognize the effects of bacterial proteins effector proteins. The interaction of an R protein with an effector protein promotes a stronger immune response, including the hypersensitive response. Effector-triggered immunity R SA Immune Responses Redrawn from Pieterse, C.M.J, Leon-Reyes, A., Van der Ent, S., and Saskia C M Van Wees, S.C.M. (2009) Nat. Chem. Biol. 5: 308 – 316 . The hypersensitive response involves cell death Effector-triggered immunity R Pathogen Response (PR) genes Antimicrobial compounds Strengthening of plant cell walls Programmed cell death Hypersensitive response (HR) SA Immune Responses From Cawly, J., Cole, A.B., Király, L., Qiu, W., and Schoelz, J.E. (2005) The plant gene CCD1 selectively blocks cell death during the hypersensitive response to cauliflower mosaic virus infection. MPMI 18: 212-219; Redrawn from Pieterse, C.M.J, Leon-Reyes, A., Van der Ent, S., and Saskia C M Van Wees, S.C.M. (2009) Nat. Chem. Biol. 5: 308 – 316. The hypersensitive response seals the pathogen in a tomb of dead cells HR No HR The HR kills the infected cells and cells surrounding them and prevents the pathogen from spreading. Without a hypersensitive response, the pathogen can multiply. Drawing credit Credit: Nicolle Rager Fuller, National Science Foundation; Photo reprinted by permission of Macmillan Publishers Ltd. Pruitt, R.E., Bowman, J.L., and Grossniklaus, U. (2003) Plant genetics: a decade of integration. Nat. Genet. 33: 294 – 304. Other hormones affect defense response signaling As part of their immune responses, plants modulate synthesis and response to other hormones. Some pathogens exploit the connections between growth hormones and pathogen-response hormones to their own advantage, by producing “phytohormones” or interfering with hormone signaling. Reprinted from Robert-Seilaniantz, A., Navarro, L., Bari, R., and Jones, J.D.G. (2007). Pathological hormone imbalances. Curr. Opin. Plant Biol. 10: 372–379. with permission from Elsevier. Summary – stress responses Hormonal signaling is critical for plant defenses against abiotic and biotic stresses. ABA and ethylene are produced in stressed plants and critical for activating their defense pathways. JA and SA contribute to local and systemic defenses against pathogens. Understanding plant hormonal responses to stress is needed to improving agricultural yields. Abiotic and biotic stresses are major causes of crop losses and reduced yields and which must be minimized. Crosstalk between hormone signaling pathways H1 Response H1 H2 H1 H2 Response Crosstalk (or cross-regulation) occurs when two pathways are not independent. It can be positive and additive or synergistic, or negative. Crosstalk between hormone signaling pathways H1 Response H1 H2 H1 H2 Response Crosstalk (or cross-regulation) occurs when two pathways are not independent. It can be positive and additive or synergistic, or negative. H2 H1 H2 Response Crosstalk can affect the synthesis, transport or signaling pathway of another hormone. Synergistic requirement for JA and ET signaling in defense response NO JA response NO ET response JA and ET signaling are both required for highlevel expression of ERF1, a TF that induces defense gene expression JA and ET ERF1 ERF1 Defense genes Lorenzo, O., Piqueras, R., Sánchez-Serrano, J.J., and Solano, R. (2003) ETHYLENE RESPONSE FACTOR1 integrates signals from ethylene and jasmonate pathways in plant defense. Plant Cell 15: 165-178. Negative interaction between JA and SA in defense responses In defense signaling, the JA and SA pathways are mutually antagonistic (locally), and both are antagonized by ABA. Why does ABA reduce SA and JA signaling? Perhaps a plant that is already stressed and producing high levels of ABA may be better off temporarily restricting its responses to pathogens. Reprinted from Spoel, S.H., and Dong, X. (2008) Making sense of hormone crosstalk during plant immune responses. Cell Host Microbe 3: 348-351 with permission from Elsevier. GA and DELLAs interact extensively with other signaling pathways Just about every stage of the plants life is coordinated by GA and its effects on the other hormone signaling pathways. Although it is clear that GA’s effects on DELLA proteins are very important, we don’t yet understand what these proteins do. Weiss, D., and Ori, N. (2007) Mechanisms of cross talk between gibberellin and other hormones. Plant Physiol. 144: 1240–1246. GA and DELLAs interact extensively with other signaling pathways ABA AUXIN CK ET ABA Weiss, D., and Ori, N. (2007) Mechanisms of cross talk between gibberellin and other hormones. Plant Physiol. 144: 1240–1246. Ongoing research • Hormones coordinate plant growth and defense • Many aspects of hormone synthesis, homeostasis and signaling are still being discovered • Knowledge of these processes provides tremendous opportunities for agricultural improvements including the development of stress-resistant and pathogen-resistant plants, plants with greater abilities to take up nutrients, foods that stay fresh longer, and increased crop yields

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