S2011 Hormones in Plant Development 1. Introduction to Phytohormones www.plantcell.org/ “........characterised 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 Dutch botanist In 1928, he isolated auxin from plants Known for the Cholodny-Went model Develop synthetic plant hormones at Caltech labs. Worked on the effects of air pollution on plant growth 2 What are phytohormones? English-American plant physiologist and microbiologist Determined the structure of auxin chemical messengers that coordinate the cellular functions of multicellular organisms. 3 Phytohormones Notholaena standleyi © 2008 Carl Rothfels Phytohormones regulate: - cellular activities (division, elongation and differentiation), - pattern formation, organogenesis, - reproduction, sex determination, - responses to abiotic and biotic stress. 4 Phytohormones - Names and structures Auxin Cytokinins Gibberellins Abscisic Acid Ethylene Brassinosteroids Salicylates Jasmonates Strigolactones 4 Phytohormones - Names and structures Auxin Cytokinins Gibberellins Abscisic Acid Ethylene Brassinosteroids Salicylates Jasmonates Strigolactones 5 Phytohormones regulate all stages of the plant life cycle Fertilisation Seed dormancy Embryogenesis Fruit Development Germination Flower development Growth and branching Fruit ripening Seed maturation 6 Phytohormones also help plants to cope with stress throughout their life Fertilisation Seed dormancy Embryogenesis Fruit Development Germination Flower development Growth and branching Fruit ripening Seed maturation 7 Phytohormones also help plants to cope with stress throughout their life During this course, we will look at each hormones within the context of • their role during development • their crosstalk • Their function during abiotic stress response 8 Arabidopsis thaliana life cycle Kramer, eLife, 2015 9 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 10 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 11 Hormones: synthesis, transport, perception, signalling and responses Binding to receptor Downstream effects Transport Downstream effects Production of active hormone Signal transduction 12 Hormones: synthesis Production of active hormone Many tightly regulated biochemical pathways contribute to active hormone accumulation. Conjugation can temporarily store a hormone in an inactive form, lead to catabolic breakdown, or be the means for producing the active hormone. H Synthesis Breakdown H Conjugation De-conjugation 13 Hormones: transport and perception Binding to receptor Transport H Several hormone receptors have been identified. They can be membrane-bound or soluble. They can be at the cell surface/organelles, cytosolic, nuclear. Hormones can move: through the xylem or phloem across cellular membranes, or through apoplast through regulated transport proteins Production of active hormone 14 Membrane-localized receptors Ethylene Brassinosteroids Cytokinins Hormone binding initiates an information relay (signalling) P P P H D H D P P H 15 Soluble receptors: Binding to the hormone increases affinity to the co-receptor Hormones can act like “molecular glue” ABA PYR1 PP2C SnRK2 P GID1 GA DELLAGID1 GA DELLA TIR1 Aux/IAA IAA Jasmonates JAZ COI1 JA- Ile 16 Signal transduction Signal transduction Following binding to its receptor, the hormone induces a signalling cascade, that can be of diverse nature, mainly: (1) reversible protein phosphorylation (2) targeted proteolysis H Protein phosphorylation Protein dephosphorylation Proteolysis P 17 Hormones: Responses Downstream effects Downstream effects Signal transduction Downstream effects can involve: (1) changes in gene transcription (2) changes in other cellular activities like ion transport Transcription Non-genomic effects (e.g. Ion channel regulation) 18 Signal transduction by targeted proteolysis The hormones bind to receptors, initiating proteolysis of repressors to activate a transcriptional regulator Gibberellin Auxin TIR1 Aux/IAA Aux/IAA ARF ARF IAA 26S proteasome PIF PIF DELLA GID1 GA PIF DELLA Jasmonate MYC2 MYC2 JAZ JAZ COI1 JA- Ile 19 Hormones: synthesis, transport, perception, signaling and responses Binding to receptor Downstream effects Transport Downstream effects Production of active hormone H Signal transduction Production of active hormone H Synthesis Breakdown H Conjugation De-conjugation Protein phosphorylation Protein dephosphorylation Proteolysis P Non-genomic effects (e.g. Ion channel regulation) Transcription 20 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 21 Hormones affect vegetative growth: elongation, branching and organogenesis Photo courtesy of Shawn Conley Growth by elongation Growth by branching Germinated seedling Elongation in the shoot and root of a germinating soybean Organogenesis Arabidopsis Wild type Wild type Auxin response mutant Brassinosteroids biosynthesis mutants GA Wild type Gibberellin biosynthesis mutant Pea Auxin Brassinosteroids Arabidopsis 22 Disrupting hormone synthesis or response interferes with elongation Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J.B. (1997) Mendel’s stem length gene (Le) encodes a gibberellin 3β-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 23 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 24 Auxin regulates plant development Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Inhibit branching in the shoot Promote branching in the root Lateral organ initiation at the shoot apical meristem Maintain stem cell fate at the root apical meristem Patterning and vascular development 25 N CH2 COOH H Indole-3-acetic acid (IAA) Cell elongation Cell division Embryogenesis Phyllotaxis Root development Vascular differentiation Branching Tropic growth Fruit development Auxin regulates plant development 26 Many of auxin’s effects are mediated by changes in gene expression Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Genes controlling cell growth Genes coordinating other hormone response pathways Genes involved in signalling 27 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 28 Cytokinins trans-zeatin, a cytokinin ✓ Cell division ✓ Control of leaf senescence ✓ Control of nutrient allocation ✓ Root nodule development ✓ Stem cell maintenance ✓ Regulate auxin action ✓ Vascular formation 29 Cytokinins act antagonistically to auxins Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Inhibit branching in the shoot Promote branching in the root Promote lateral organ initiation at the shoot apical meristem Maintain stem cell fate at the root apical meristem Promote stem cell fate at the shoot apical meristem Promote branching in the shoot Inhibit branching in the root Promote differentiation at the root apical meristem Cytokinin Auxin 30 Auxin and cytokinin regulate each other’s function at the root apex 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. Auxin Cytokinin Cytokinin biosynthesis Auxin transport and response Cell division Cell differentiation Auxin transport 31 Cytokinins affect grain production and drought tolerance Wild-type Elevated CK Rice plants that accumulate more CK can produce more grain per plant because of changes in inflorescence architecture. 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, Rivero, R. M. et al. (2007) PNAS 104: 19631-19636. 32 Cytokinins affect root formation and drought tolerance in barley Barley with less cytokinin, by overexpression of CKX in roots Pospíšilová, H.; Jiskrová, E.; Vojta, P.; Mrízová, K.; Kokáš, F.; Čudejková, M. M.; Bergougnoux, V.; Plíhal, O.; Klimešová, J.; Novak, O.; Dzurová, L.; Frébort, I.; Galuszka, P. Transgenic barley overexpressing a cytokinin dehydrogenase gene shows greater tolerance to drought stress. N Biotechnol 2016, 33, 692–705. Water content Dry weight 33 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 34 Gibberellins ✓ Growth ✓ Seed germination ✓ Promote flowering ✓ Promote sex determination in some species ✓ Promote fruit growth A Gibberellin (GA4 ) 35 Gibberellins regulate growth Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J.B. (1997) Mendel’s stem length gene (Le) encodes a gibberellin 3βhydroxylase. Plant Cell 9: 1435-1443. 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 GA Wild type Gibberellin biosynthesis mutant le Pea 36 Genes controlling GA synthesis are important “green revolution” genes Distinguished plant breeder and Nobel Laureate Norman Borlaug 1914-2009 Tremendous increases in crop yields (the Green Revolution) during the 20th century occurred because of increased use of fertiliser and the introduction of semi-dwarf varieties of grains. The semi-dwarf varieties put more energy into seed production than stem growth, and are sturdier and less likely to fall over. Photos courtesy of S. Harrison, LSU Ag center and The World Food Prize. 37 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. 38 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 39 Hormonal control of reproductive development In angiosperms: ✓ transition from vegetative to reproductive growth ✓ flower development, ✓ fruit development and ripening ✓ seed development, maturation and germination Kramer, eLife, 2015 Arabidopsis thaliana life cycle 40 Kramer, eLife, 2015 Arabidopsis thaliana life cycle 40 41 Transition to flowering The decision to reproduce is tightly controlled by environmental and hormonal factors. For many plants day length 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. Photo credit: Early Crocus (Crocus tommasinianus) by anemoneprojectors via Flickr. Photo credit: Sady Stary Liskovec, Brno Photo credit: A. Bielach 42 GA’s role in initiating flowering varies by species and growth-habit 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. Malus domestica Perennial No Arabidopsis thaliana Annual Lolium temulentum Annual temperate grass Yes Beta vulgaris Biennial Yes Short Days Yes Long Days No GA promotes flowering? 43 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 synchronise flowering. Images couresy of Dave McShaffrey, Marietta College, ©2009 44 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 45 Ethylene and gibberellins are involved in sex determination Image of Abdelhafid Bendahmane, URGV - Plant Genomics Research INRAE Hermaphrodite Male Female 46 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. 47 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 Auxin + GA Photo credits: Grape flowers by Bruce Reisch; Strawberry flower by Shizhao GA Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. Auxin Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. CytokininAuxin Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. GACytokininAuxin Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. GA GA perception mutant CytokininAuxin Auxin and GA promote cell division and growth of the fruit 48 Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. GA GA perception mutant CytokininAuxin UNP P UNP, GA3 [10] UNP, BA[1] UNP, NAA[10] Auxin and GA promote growth of the fruit without seeds 49 parthenocarpy Alabadí D, Blázquez MA, Carbonell J, Ferrándiz C & Perez-Amador MA (2008) Instructive roles for hormones in plant development. Int. J. Dev. Biol. 50 Fruit ripening is induced by ethylene Auxin GA Ethylene Ethylene is a gaseous hormone that promotes fruit softening and flavour and colour development 51 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 52 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 C C H HH H C2H4 C2H4 Ethylene induces the triple response: ✓ reduced elongation, ✓ hypocotyl swelling, ✓ apical hook exaggeration. Apical hook https://www.frontiersin.org/articles/10.3389/fpls.2013.00441/full Apical hook https://www.frontiersin.org/articles/10.3389/fpls.2013.00441/full 54 Aspidistra is ethyleneresistant and so became popular houseplant. Beyer, Jr., E.M. (1976) A potent inhibitor of ethylene action in plants. Plant Physiol. 58: 268-271. Cotton plants 7 days ethyleneAir (control) Ethylene promotes leaf and petal senescence. Ethylene promotes senescence of leaves and petals 55 Ethylene shortens the longevity of cut flowers and fruits 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 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 56 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 57 Abscisic acid ✓ Seed maturation and dormancy ✓ Desiccation tolerance ✓ Stress response ✓ Control of stomatal aperture 58 Auxin Cytokinin ABA accumulates in maturing seeds Seed maturation requires ABA synthesis and accumulation of reserve nutrients to confer desiccation tolerance to the seed. Braybrook SA & Harada JJ (2008) LECs go crazy in embryo development. Trends in Plant Science 13, 624–630. 59 Developmental control of seed maturation ABA promotes seed maturation. ABA deficient mutants do not complete processes associated with maturation & dormancy e.g. abi3 ABA insensitive mutant from Arabidopsis undergoes: ✓ Precocious germination ✓ Retain chlorophyll ✓ Differentiate shoot meristem and vasculature Seeds from Arabidopsis plants showing normal brown mature seeds (wild type Landsberg erecta ecotype, Ler) and mutant abi3-5 seeds which are still green. Precocious germination of maize seed in the maize viviparous1 mutant. McCarty et al (1989) The Plant Cell, Vol. 1, 523-532,. Roschzttardtz et al., (2009) Plant Physiology 150:84-95 60 Once dormant and dry, seeds can remain viable for very long times Sallon, S., et al. (2008). Germination, genetics, and growth of an ancient date seed. Science 320: 1464, Lotus picture by Peripitus 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. 61 GA is required for seed germination Reserve mobilisation Cell expansion Seed germination requires elimination of ABA and production of GA to promote growth and breakdown of seed storage products. Braybrook SA & Harada JJ (2008) LECs go crazy in embryo development. Trends in Plant Science 13, 624–630. Auxin Cytokinin 62 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. Summary – hormonal regulation of reproductive development 63 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 64 Hormonal responses to abiotic stress Photo-oxidative 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. 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 65 Hormonal responses to biotic stress Bacteria, fungi, viruses – Biotrophic organisms Herbivores – insects, other animals, fungi – Necrotrophic organisms Salicylic Acid Jasmonates Photo credits: A. Collmer, Cornell University; Salzbrot. 66 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects Salicylic acid ✓ Response to biotrophic pathogens ✓ Induction of defense responses ✓ Systemic acquired resistance Salicylic acid ✓ Response to biotrophic pathogens ✓ Induction of defense responses ✓ Systemic acquired resistance Acetylsalicylic Acid - aspirin Salicylates contribute to Systemic Acquired Resistance ✓ SA production is induced by pathogen attack ✓ A mobile signal is produced for SAR ✓ Activation of an immunity response ✓ Hypersensitive response involves cell death of infected cells to prevent pathogens from spreading 69 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects Jasmonates ✓ Response to necrotrophic pathogens ✓ Induction of anti-herbivory responses ✓ Production of herbivore-induced volatiles to prime other tissues and attract predatory insects 71 Jasmonate signaling contributes to defense against herbivory 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. 72 Jasmonates induce the expression of anti-herbivory chemicals Wound-induced signals Insect oral secretions production of protease inhibitors Feeding deterrents JA signaling R.J. Reynolds Tobacco Company,   Bugworld.org ➡ Contributes to systemic responses 73 Hormonal signaling is critical for plant defenses against biotic and abiotic stresses Auxin, cytokinin, 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 agriculture yields. Abiotic and biotic stresses are major causes of crop losses. Summary – hormonal regulation of the stress response 74 Lectures outline How hormones work Hormonal control of vegetative development Auxin Cytokinins Gibberellins Hormonal control of reproduction Ethylene Abscisic Acid Hormonal responses to stress   Salicylates   Jasmonates Cross-regulation of hormonal effects 75 Crosstalk between hormone signalling pathways Crosstalk (or cross-regulation) occurs when two pathways are not independent. It can be positive and additive or synergistic, or negative. Response Response H1 H1 H2 H1 H2 76 Crosstalk between hormone signaling pathways Crosstalk (or cross-regulation) occurs when two pathways are not independent. It can be positive and additive or synergistic, or negative. Response Response H1 H1 H2 H1 H2 Crosstalk can affect the synthesis, transport or signalling pathway of another hormone. H1 Response H2H3 77 S2011 course outline (2) 26/2 - AUXIN - production, transport, signalling; discovery of the hormone; tropism (physiology, genetics) (3) 5/3 - AUXIN - root meristem and root-derived organs; Shoot organogenesis (differences and similarity with root), correlation with local auxin gradients, transport and organ formation.  (4) 12/3 - AUXIN - Embryogenesis - pattern formation during embryogenesis, Arabidopsis mutants, gene identities. (5) 19/3 - CYTOKININ - production, degradation, perception, signal transduction, isolation and veri fi cation of the receptors and downstream components. (6) 26/3 - CYTOKININ - Function in plant development. (7) 2/4 - ETHYLENE - Genetic dissection of ethylene signalling; Molecular characterisation and arrangement of the pathway.  (8) 9/4 – ABSCISIC ACID - production and signalling; role in drought stress responses, root development and seed desiccation and dormancy (9) 16/4 – GIBBERELLIC ACID – production and signalling; role in development, stress responses and seed development (10) 23/4 — SALICYLIC ACID - an immunity response (11) 30/4 — JASMONATES - A systemic response (12) 7/5 – Illustration of HORMONAL CROSSTALK (13) 14/5 - Hormones and ABIOTIC STRESS (14) 21/5 - Round table, discussion, questions References • Abbas M, Alabadí D, Blázquez MA. Differential growth at the apical hook: all roads lead to auxin. Front Plant Sci. 2013 Nov 5;4:441. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3817370/. PMID: 24204373; PMCID: PMC3817370. • Alabadí D, Blázquez MA, Carbonell J, Ferrándiz C & Perez-Amador MA (2009) Instructive roles for hormones in plant development. Int. J. Dev. Biol 53: 1597-1608. • Ashikari, M. et al. (2005) Cytokinin oxidase regulates rice grain production. Science 309: 741 – 745. • Bentsink , L and Koornneef, M (2008) Seed Dormancy and Germination, The Arabidopsis Book 70, 1. • Beyer, Jr., E.M. (1976) A potent inhibitor of ethylene action in plants. Plant Physiol. 58: 268-271. • Braybrook SA & Harada JJ (2008) LECs go crazy in embryo development. Trends in Plant Science 13, 624–630. • Clouse SD (2002) Brassinosteroids: The Arabidopsis Book. Rockville, MD: American Society of Plant Biologists. doi: 10.1199/ tab.0009 • Gray WM (2004) Hormonal regulation of plant growth and development. PLoS Biol 2(9): e311 • Krämer, U. Planting molecular functions in an ecological context with Arabidopsis thaliana. eLife 2015. • Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J.B. (1997) Mendel’s stem length gene (Le) encodes a gibberellin 3β-hydroxylase. Plant Cell 9: 1435-1443. • McCarty et al (1989) The Plant Cell, Vol. 1, 523-532. • Pospíšilová, H.; Jiskrová, E.; Vojta, P.; Mrízová, K.; Kokáš, F.; Čudejková, M. M.; Bergougnoux, V.; Plíhal, O.; Klimešová, J.; Novak, O.; Dzurová, L.; Frébort, I.; Galuszka, P. Transgenic barley overexpressing a cytokinin dehydrogenase gene shows greater tolerance to drought stress. N Biotechnol 2016, 33, 692–705. • Rivero, R. M. et al. (2007) PNAS 104: 19631-19636. • Roschzttardtz et al., (2009) Plant Physiology 150:84-95 • Sallon, S., et al. (2008). Germination, genetics, and growth of an ancient date seed. Science 320: 1464, • 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 • 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 • Vivian-Smith A & Koltunow AM (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol 121, 437–451. • Wolters, H., and Jürgens, G. (2009). Survival of the flexible: Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317.