Phytohormones The growth and development of a plant are influenced by: •Gene:c factors •External environmental factors •Chemical hormones inside the plant Secondary messengers 1. Involve in the transfer informaon from sources to targets 2. Amplify the signal produced by the phytohormone Phytohormones Plant hormones are organic compounds that are effec:ve at very low concentraon (1g 20,000 tons-1) They interact with specific target :ssues to cause physiological responses •Growth •Fruit ripening Phytohormones •Hormones s:mulate or inhibit plant growth Major groups of hormones: 1. Auxins 2. Gibberellins 3. Ethylene 4. Cytokinins 5. Abscisic acid 6. Brassinostereoids 7. Salicylic acid 8. Polyaminas 9. Jasmonates 10. Systemin 11. Nitric oxide Arabidopsis thaliana Phytohormones EARLY EXPERIMENTS ON PHOTROPISM SHOWED THAT A STIMULUS (LIGHT) RELEASED CHEMICALS THAT INFLUENCED GROWTH Auxins Auxin causes several responses in plants: * Phototropism * Geotropism * Promo:on of apical dominance * Flower formaon * Fruit set and growth * Formaon of adven::ous roots * Differen:aon of vascular :ssues (de novo or repairing existent vascular :ssue) Auxins Addi:on of auxins produce parthenocarpic fruit. Stenospermocarpy Biological mechanism that produces seedlessness in some fruits (many table grapes, watermelon) diploid + tetraploid parent = triploid seeds vegetave parthenocarpy Plants that do not require pollinaon or other s:mulaon to produce parthenocarpic fruit (cucumber) Auxins Synthe:c auxins Widely used in agriculture and hor:culture • prevent leaf abscission • prevent fruit drop • promote flowering and frui:ng • control weeds Agent Orange - 1:1 rao of 2,4-D and 2,4,5-T Dioxin usually contaminates 2,4,5-T, which is linked to miscarriages, birth defects, leukemia, and other types of cancer. Cytokinins In 1964, zean was the first naturally occurring cytokinin isolated from corn. Zean and zean riboside are found in coconut milk. Natural Synthe:c Cytokinins Lateral bud development Move non-polarly in •shoot iniaon Callus development xylem, phloem, and •roots <[auxin] = roots parenchyma cells. •seeds <[kine6n] = buds •fruits •young leaves •chloroplast development Ethylene •Is found only in the gaseous form •Induces ripening (e.g., green bananas). •Epinasty (causes leaves to droop in the fall ) •Causes abscission of fruits and flowers Ethylene Most produced organic compound 107 million metric tons in 2005 Once fruit ini:ated red coloraon, ethylene release becomes elevated in a linear fashion (without diurnal fluctuaons). Abscisic acid (ABA) Abscisin is made from carotenoids and moves non-polarly through plant :ssue. Abscisic acid (ABA) is a general plant-growth inhibitor. Induces dormancy Prevents seeds from germinang Viviparous mutants Reduced sensi:vity for ABA Gibberellins In 1930's, Ewi Kurosawa and colleagues were studying plants suffering from bakanae, or "foolish seedling" disease in rice. •Synthesized in apical por:ons of stems and roots Thompson seedless grapes (Vis vinifera) Can be readily obtained in large quan::es from fermentaons of the fungus Gibberella fujikuroi •cell division and elongaon •break seed dormancy •speed germinaon Gibberellins bolng External factors and plant growth. Tropic responses Tropic responses Direc:onal movements by growth in response to a direc:onal s:mulus Phototropism is a response to blue light Phytphormone: Auxin Second messenger: Calcium Geotropism or gravitropism Phytphormones: cytokinin and auxin External factors and plant growth. Thigmotropism Thigmotropism is direc:onal growth response to contact with an object. Greek thigma = touch Rapid response Diferen:aon in the elongaon of the cells tendrils External factors and plant growth. Nasc movements and heliotropism Nas:c movements are independent of the s:mulus' posion Touch, mechanical, electrical, thermal or chemical Venus Flytrap, Dionaea muscipula. Acid growth (cells that to elongate or expand quickly at low pH) and turgor External factors and plant growth. Heliotropism Heliotropism is the diurnal mo:on of plant parts (flowers or leaves) in response to the direc:on of the sun. During the night, the flowers may assume a random orientaon Heliotropism is a response to blue light. External factors and plant growth. Circadian rhythms Roughly 24-hour cycle in the biochemical, physiological, or behavioral organisms’ processes Hours of light Day Night Endogenous and synchronized by the environment Biological clock Photosynthesis Movement External factors and plant growth. Photoperiodism According to their photoperiods, plants can be classified into three groups: a) Short day plants. Examples: Nico8ana tabacum, soybean, strawberry, and chrysanthemum. b) Long day plants. Examples: Onion, carrot and spinach. c) Day neutral plants. Examples: Tomatoes, cucumbers, sunflower, dandelions and coon. .