BIO1PS 2012 Plant Science Lecture 4 Hormones Pt. I Dr. Michael Emmerling Department of Botany Room 410 m.emmerling@latrobe.edu.au
Hormones and Ghost gum Eucalyptus papuana Coordination ~3 Lectures Leaves ~4 Lectures Stems ~2 Lectures ~3 Lectures Ladiges et al. (2010), 4 th ed., Fig. 16.1 Soil and Roots
Recommended Reading Ladiges et al. (2010), "Biology", 4 th Edition Ch. 18, Plant hormones and growth responses, pp. 400-419 Knox et al. (2005), "Biology", 3 rd Edition Ch. 17, Plant hormones and growth responses, pp. 370-392 Useful website: http://www.plant-hormones.info/
Plant Hormones Introduction General principles Discovery of plant hormones: auxin Major groups of plant hormones
Learning Objectives Part 1 Define a plant hormone Describe how Went s curvature test lead to the discovery of auxin Describe the major roles of the plant hormone auxin Describe the role of conjugation in regulating plant hormone levels
Classes of Plant Hormones Auxin (IAA) Cytokinins Ethylene Gibberellins (GAs) Abscisic acid (ABA) Other hormones: Brassinosteroids Jasmonates
Plant Development and Physiology plants can t move (!!!!) external environment can change dramatically plants are very adaptable
growth and flowering leaf drop and abscission dormancy and germination light and shade heat and drought cold and frost Plant hormones influence almost every aspect of plant development and physiology
Plant Development Influenced by many factors internal genetics evolution, adaptation external light amount, quality, direction nutrients soil, CO 2 temperature water humidity, rainfall
Plant Hormones are small molecules are effective at very low concentrations regulate the development of plants through changes in the production changes in the sensitivity of the plant/tissue to the hormone
Plant Hormones are synthesised in a discrete organ or tissue are transported to a (specific) target tissue control a physiological response in a concentration-dependent manner regulate gene expression
The Study of Plant Hormones Traditional approach: purified from plants (or other organisms) and applied to an intact plant, or plant segment Modern approach: use of genetics and mutants has clarified many aspects of hormone action
Phototropism Canary grass seedlings Ladiges et al. (2010), 4 th ed., Fig. 18.2 Shoot apex perceives light, and induces bending below the tip, towards the light
Went's Avena Curvature Test Hypothesis/Idea: Frits Warmolt Went, 1956 The shoot apex is producing a diffusible factor that promotes growth Ladiges et al. (2010), 4 th ed., Fig. 18.2 J. R. Eyerman, Time/Life
Went's Avena Curvature Test Coleoptile http://extension.entm.purdue.edu/pestcrop/2010/issue4/graphic/popups/agron3.jpg and agron5.jpg
Went's Avena Curvature Test 1926 Ladiges et al. (2010), 4 th ed., Fig. 18.3
Auxin The substance inducing the curving of the coleoptile is auxin or indole-3-acetic acid (IAA) Indole acetic acid tryptophan Ladiges et al. (2010), 4 th ed., Fig. 18.4 Ladiges et al. (2010), 4 th ed., Fig. 18.3
Auxin-Induced Growth Cell elongation cell wall is acidified acidified cell wall is more "stretchable" turgor pressure expands cells Taiz and Zeiger (2010), 5 th ed., Fig. 19.2
IAA Conjugation IAA is often bound to sugars or amino acids within the plant 50-90% of auxin in maize is conjugated the level of active IAA within the plant is dependent on the formation and breakdown of these IAA conjugates the level of free IAA is tightly controlled
Auxin - Amino Acid Conjugates indole-3 acetic acid (IAA) IAA-aspartate Want to know more? Ludwig-Müller, J. (2011) Auxin conjugates: their role for plant development and in the evolution of land plants. J. Exp. Bot. 62(2), 1757-1773 IAA-glycine Bakllamaja, V. (2003), J Young Investigators 8 (1)
Plant Hormones Auxin GA Cytokinin Abscisic Acid Ethylene Dormancy Juvenility Extension Growth Root Development Flowering Fruit Development Senescence
Auxin Transport Shoot Transported in the phloem to the rest of the plant Roots Transported to the tip in the stele, then laterally in the epidermal/cortical cells
Gravitropism Tendency for shoots to grow away from gravity roots to grow towards gravity mediated by auxins
Gravitropism + auxin promoter GFP Confocal microscopy of live roots Green Fluorescent Protein (GFP): auxinresponsive gene expression Correlates with auxin concentration Auxin in the root? Transported via the Ottenschläger et al. (2003), Proc Nat Acad Sci USA 100, 2987 phloem
Redistribution of Auxin 1.5 hours + auxin gravitropism 3 hours promoter GFP Ottenschläger et al. (2003), Proc Nat Acad Sci USA 100, 2987 high auxin levels in root tissue reduce cell expansion
Apical Dominance The shoot apical meristem produces an "inhibitory substance" which prevents the outgrowth of other dormant buds (axillary meristems) low dominance ME ME high dominance
Apical Dominance regulates plant architecture ME ME
Ladiges et al. (2010), 4 th ed., Fig. 18.13 Trimming
Differential Responses to Auxin Observations: auxin in shoot inhibits expansion of lateral buds auxin in shoot promotes cell elongation (phototropism) auxin in root promotes growth of lateral roots auxin in roots reduces cell elongation (gravitropism) Different sensitivities to auxin Elongation, division, differentiation
Sensitivity to Auxin stimulates roots, no effect on shoots shoots Stimulation stimulates shoots inhibits roots auxin concentration Inhibition roots
Sensitivies to Hormones different tissues have different responses to hormones this is due (in part) to different sensitivities to hormone levels, and also due (in part) to interactions with other hormones
IAA and Auxins Involved in the regulation of many aspects of plant development: cell elongation (and division) tropisms differentiation apical dominance senescence abscission flowering
Synthetic Auxins similar in structure to IAA more "stable" than IAA transported less slowly than IAA 2,4-dichlorophenoxyacetic acid 2,4,5-trichlorophenoxyacetic acid results in excessive cell expansion Ladiges et al. (2010), 4 th ed., Fig. 18.4
"Rainbow Herbicides" Agent Pink Agent Green Agent Purple Agent Blue Agent White Agent Orange Deployed (1960s and 70s) in Korea and Vietnam
Agent Orange Mix of 2,4-D and 2,4,5-T Contaminated with the dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) Dioxins are fat-soluble and very stable carcinogenic lead to serious impairment of development, reproduction, immune function