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Phytohormones The growth and development of a plant are influenced by: • Gene:c factors • External environmental factors • Chemical hormones inside the plant  Receptor  Ac:va:on or repression of sets of genes in the nucleus Secondary messengers mediate the phytohormone responses 1.  Involve in the transfer informa:on from sources to targets 2.  Amplify the signal produced by the phytohormone Hormone, from the Greek word horman, meaning "to s:mulate”. Hormones control • Growth • Development • Movement. Phytohormones Plant hormones are chemical messengers Hormones are organic compounds that are effec:ve at very low concentra:on (1g 20,000 tons-­‐1) Usually synthesized in one part of the plant and are transported to another loca:on. They interact with specific target :ssues to cause physiological responses • Growth • Fruit ripening Each response is oWen the result of two or more hormones ac:ng together. Phytohormones • Hormones s:mulate or inhibit plant growth • Growth regulators rather than s:mulators. 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 Phytohormones EARLY EXPERIMENTS ON PHOTROPISM SHOWED THAT A STIMULUS
(LIGHT) RELEASED CHEMICALS THAT INFLUENCED GROWTH
Results on growth of coleoptiles of
canary grass and oats suggested that the
reception of light in the tip of the shoot
stimulated a bending toward light
source. Auxins Auxin causes several responses in plants: * Bending toward a light source (phototropism) * Downward root growth in response to gravity (geotropism) * Promo:on of apical dominance * Flower forma:on * Fruit set and growth Precursors of hydathodes produced auxins * Forma:on of adven::ous roots * Differen:a:on of vascular :ssues (de novo or repairing existent vascular :ssue) Auxins Addi:on of auxins produce parthenocarpic fruit. Natural or ar:ficially induced produc:on of fruit without fer:liza:on of ovules. Stenospermocarpy Biological mechanism that produces seedlessness in some fruits (many table grapes, watermelon) diploid + tetraploid parent = triploid seeds vegeta6ve parthenocarpy Plants that do not require pollina:on or other s:mula:on to produce parthenocarpic fruit (cucumber) Auxins Acropetal transport Basipetal transport • Passive diffusion • Ac:ve transport polar auxin (IAA) transport 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 ra:o of 2,4-­‐D and 2,4,5-­‐T used to defoliate trees in Vietnam War. Dioxin usually contaminates 2,4,5-­‐T, which is linked to miscarriages, birth defects,leukemia, and other types of cancer. Cytokinins In 1964, zea:n was the first naturally occurring cytokinin isolated from corn. Zea:n and zea:n riboside are found in coconut milk. Natural All cytokinins (ar:ficial or natural) are chemically similar to adenine. Cytokinin from Cytokinesis Synthe:c Cytokinins Lateral bud development Move non polarly in xylem, phloem, and parenchyma cells. • shoot ini6a6on Callus development • roots <[auxin] = roots • 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 ) • Non polarly in xylem, phloem, and parenchyma cells. • Causes abscission of fruits and flowers Ethylene was first detected as flower buds opened and exhibited diurnal oscilla:ons with peak produc:on prior to petal abscission Ethylene Plants oWen increase ethylene produc:on in response to stress Is found in high concentra:ons within cells at the end of a plant's life. Methionine is the precursor of ethylene Most produced organic compound 107 million metric tons in 2005 Once fruit ini:ated red colora:on, ethylene release becomes elevated in a linear fashion (without diurnal fluctua:ons). 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 germina:ng High concentra:ons of ABA in guard cells during periods of drought stress play a role in stomatal closure 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 • Transport is non-­‐polar Thompson seedless grapes (Vi/s vinifera) Can be readily obtained in large quan::es from fermenta:ons of the fungus Gibberella fujikuroi • cell division and elonga:on • break seed dormancy • speed germina:on Gibberellins bol6ng 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 mesenger: Calcium Geotropism or gravitropism Phytphormones: cytokinin and auxin External factors and plant growth. Geotropism Starch -­‐ statolith hypothesis: Gravity has caused amyloplast to fall inside this statocyte, lodging them against other cell components. Protoplast pressure hypothesis: The weight of the en:re mass of the protoplast is involved in the percep:on. Tesengrity model Tension between structural components External factors and plant growth. Thigmotropism Thigmotropism is direc:onal growth response to contact with an object. Greek thigma = touch Rapid response Diferen:a:on in the elonga:on of the cells tendrils External factors and plant growth. Nas6c movements and heliotropism Nas:c movements are independent of the s:mulus' posi:on Touch, mechanical, electrical, thermal or chemical In Mimosa pudica. change in the osmo:c poten:al (turgor) of cells of the pulvinus due to the movement of K+ ions and contrac:on of the cytoskeleton 1cm/sec. Venus Flytrap, Dionaea muscipula. Acid growth (cells that to elongate or expand quickly at low pH) and turgor (> 1 second) 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 orienta:on 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 Endogenous and synchronized by the environment Biological clock Photosynthesis Movement Day Night External factors and plant growth. Photoperiodism According to their photoperiods, plants can be classified into three groups: a)  Short day plants. Examples: Nico7ana tabacum, soybean, strawberry, and chrysanthemum. b) Long day plants. Examples: Onion, carrot and spinach. c)  Day neutral plants. Examples: Tomatoes, cucumbers, sunflower, dandelions and coqon. Phytocromes are the photoreceptors for photoperiodism