Plant Hormones
• Plant hormones
–
–
–
–
Organic compounds produced in low concentrations
Produced in one part of plant (i.e. source)
Transported to another part of plant (i.e. target)
Cause physiological or developmental responses
(stimulatory or inhibitory)
• Also called plant growth regulators or
phytohormones
Plant Hormones
• Major types of plant hormones:
–
–
–
–
–
Auxins- most common is IAA or indole –acetic acid
Gibberellins
Abscisic acid ABA
Cytokinins
Ethylene
Plant Hormones
• Auxins
– Types of auxins used in horticulture
• Natural auxin is indole-3-acetic acid (IAA)
– IAA is broken down in sunlight
• Synthetic auxins used in horticulture:
• 2, 4-dichlorophenoxyacetic acid (2, 4-D)
– Site of auxin production in plants:
• Shoot and root tips (apical meristems)
• Young, expanding leaves
• Young seeds
Plant Hormones
• Auxins
– Action of auxins in plants:
• Stimulate cell elongation
– Auxins increase the length of most plant cells and thereby contribute
to the growth and elongation of the plant. Also cause a bend toward
light - positive phototropism- because more auxin remains in cells
on opposite side of light source
• Promotes apical dominance
– Pinching bud removes source of auxin and releases axillary buds
from apical dominance
• Promotes growth of adventitious roots- roots in uncommon
–
Eg. geranium cuttings, transplanting.
place
Plant Hormones
• Auxins
– Actions of auxins in plants
• Promotes or inhibits abscission of leaves, flowers and fruit
– NAA (synthetic auxin) applied shortly after bloom is used to thin
apples
– NAA applied during fruit development inhibits abscission of apples
• Auxin concentration determines if action is stimulatory or
inhibitory
– Low concentration promotes adventitious root growth, but high
concentration inhibits root growth of cuttings
– 2, 4-D is used as herbicide by applying at high concentrations
Plant Hormones
• Gibberellins
• Several different gibberellins (GA) produced by plants
– Large, complicated molecules not synthesized
– Commercial gibberellins produced by fungus
– Site of gibberellin production in plants:
•
•
•
•
•
Shoot and root tips (apical meristems)
Young, expanding leaves
Embryos
Fruits
Tubers
Plant Hormones
• Gibberellins
– Action of gibberellins in plants:
• Stimulate cell elongation
– Dwarf plants treated with gibberellins produce normal growth
– Applied to grapes to elongate the peduncle (stem of flower cluster)
and pedicels (stem of single flower), making looser cluster
• Promotes cell division in vascular cambium
• Promotes seed germination
– causes production of amylase enzyme that breaks down starch into
energy needed for growth
– Used by beer brewers to stimulate sugar production in barley malt
(sugar is converted into alcohol during fermentation)
• Influences flower and fruit development
Plant Hormones
• Abscisic Acid (ABA)
– Similar structure as gibberellins
– Site of abscisic acid production in plants:
• All organs (e.g. roots, leaves, stems, fruits)
– Actions of abscisic acid in plants
• Counteracts effects of auxins and gibberellins
• Maintains dormancy in seeds and buds
• Stimulates guard cells to close stomates (to conserve water)
source of ABA are the spongy mesophyll cells
Plant Hormones
• Ethylene has been used in practice since the ancient
Egyptians, who would gas figs in order to stimulate
ripening.
– Site of ethylene production in plants:
• Throughout plants
– Actions of ethylene in plants
• Inhibits root and shoot elongation by blocking transport of
auxins from apical meristems
• In addition to causing fruit to ripen, it can cause plants to
die. It can be produced when plants are injured, either
mechanically or by disease. Ethylene will cause a wide
range of effects in plants, depending on the age of the plant
and how sensitive the plant is to ethylene.
Plant Hormones
• Cytokinins
– Types of cytokinins used in horticulture
• Several natural forms
• Synthetic cytokinins used in horticulture:
– Benzyladenine (BA)
– Site of cytokinin production in plants:
• Embryos
• Young leaves and fruit
• Apical meristems of roots
Plant Hormones
• Cytokinins
– Actions of cytokinins in plants
• Promotes cell division (cytokinesis)
• Contributes to cell enlargement (in leaves)
• Stimulates differentiation of cells (with auxins)
– High cytokinin and low auxin promotes shoot initiation in tissue
culture, whereas reverse combination promotes root formation
– Moderate levels of both hormones promotes callus growth
• Delays senescence =changing of colour in leaves (maintains
and promotes synthesis of chlorophyll)
Plant Hormones
• Ethylene
– Actions of ethylene in plants
• Induces adventitious root formation by blocking (and
accumulating) auxin at tip of stem cutting
• Enhances flow of latex in rubber trees
• Enhances flowering in pineapples
• Stimulates abscission of leaves and fruit
– Used as a harvest aid for cherries
• Promotes fruit ripening (apple, tomato, citrus, coffee)
• Promotes senescence (aging) of flowers