Chapter 30 13-03-03 1:59 PM
Hormone: a signaling molecule secreted by a cell that can alter the activity
of any cell with receptor for it.
Dormancy: a period in the lifecycle in which biological activity is suspended
Photoperiodism: the response of plants to changes in the relative lengths
of light and dark periods in their environment during each 24-hour period
• There must be a certain period of darkness… if this period is
interrupted, the plant will cease to grow and will die
Circadian Rhythm: any biological activity that is repeated in cycles, each
about 24 hours long, independently of any shifts in environmental changes.
Thigmotropism: growth in response to contact with a solid object.
Nadistic Movement: in plants, a reversible response to non-directional
stimuli, such as mechanical pressure or humidity.
Phototropism: Growth in response to light.
Gravitropism: a directional growth response to earth’s gravitational pull
that is induced by mechanical or hormonal influences.
Statolith: a moveable starch- or carbonate-containing stone-like body
involved in sensing gravitational pull.
Second-Messenger system: in particular , signal transduction pathway, an
internal, nonprotein signal molecule that directly or indirectly activates
protein kinases, which elicit cellular response.
Abscission: in plants, the dropping of flowers, fruits, and leaves in response
to environmental signals.
⋅ plant cell walls grow much faster in an acid environment ⋅ this hypothesis suggests that that auxin causes cells to secrete acid (H+)
into the cell wall by stimulating the plasma membrane H+ pumps to
move hydrogen ions from the cell interior into the cell wall
30.1 Plant Hormones
- hormones in plants are less complex and fewer in number than animal
- in animals, hormones travel around via circulatory and nervous system but
plants don’t have that..
• hormones in plants can move via the vascular system, from cell to
cell, or through intercellular spaces.
In plants, a hormone is a signaling molecule that regulates or helps
coordinate some aspect of the plant’s growth, metabolism, or development.
Plant hormones act in response to two types of cues:
1. Internal chemical conditions related to growth and development
2. Conditions in the external environment that affect plant growth,
such as light or water availability.
Some hormones exert their effects where they are while others must be
transported to other parts of the plant.
• They are very small organic molecules and act in extremely low
• Hormone have specificity: each one effects a particular tissue in a
Hormone Where Tissues Effected Effects
Auxins Apical meristems, Growing tissues, Promote growth
developing leaves buds, roots, and elongation of
and embryos leaves, fruits, stems, formation of
vascular tissues lateral roots and
dormancy in lateral
buds; promote fruit
plants with respect to light, gravity
Gibberellins Root and shoot Stems and Promote cell
tips, young leaves developing divisions and
and developing seeds growth and
embryos elongation of stems
Cytokinins Mainly in root tips Shoot apical Promote cell
meristems, division; inhibit
leaves, buds senescence of
growth of roots and
shoots (with auxin)
Ethylene Shoot tips, roots, Seeds, buds, Regulates
leaf nodes, seedlings, elongation and
flowers, fruits mature leaves, division of cells in
fruits, flowers seedling stems,
roots; in mature
abscission of leaves
flowers and fruits
Brassinosteroids Young seeds, Mainly shoot Stimulate cell
shoots and tips, developing division and
leaves, pollen embryos elongation,
Abscisic Acid Leaves, roots Buds, seeds Promotes responses
stomata to environmental
to close in water- stressed plants
Jasmonates Roots, seed, Various tissues, In defense
probably other including responses, promote
tissues damaged ones transcription of
possible role in
plant response to
Oligosaccharins Cell walls Damages Promote synthesis
tissues, most of phytoalexins in
plant cells injured plants; may
Systemin Damaged tissues Damaged In tomatoes and
tissues closely related
species; roles in