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 informa on 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 concentra on
(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:
* Geotropism
* Promo on of apical dominance
* Flower forma on
* Fruit set and growth
* 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.
Stenospermocarpy Biological mechanism that produces seedlessness in some fruits (many table grapes, watermelon)
diploid + tetraploid parent = triploid seeds vegeta ve parthenocarpy Plants that do not require pollina on or other s mula on 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 ra o 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, zea n was the first naturally occurring cytokinin isolated from corn.
Zea n and zea n riboside are found in coconut milk. Natural
Synthe c
Cytokinins
Lateral bud development
Move non-polarly in •shoot ini a on Callus development xylem, phloem, and •roots <[auxin] = roots parenchyma cells. •seeds <[kine n] = 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 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
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 (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
bol ng
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 a on in the elonga on of the cells
tendrils External factors and plant growth. Nas c movements and heliotropism
Nas c movements are independent of the s mulus' posi on
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 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 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: Nico ana tabacum, soybean, strawberry, and chrysanthemum. b) Long day plants. Examples: Onion, carrot and spinach. c) Day neutral plants. Examples: Tomatoes, cucumbers, sunflower, dandelions and co on.