36
Plant Growth and Development
Lecture Outline
I.
Germination and early growth
A. Seed germination requires favorable environmental conditions
1. Seeds absorb water by imbibition by osmosis and adsorption of water to
cellulose, pectin, and starches
2. Respiratory processes convert energy-rich molecules in the endosperm or
cotyledons to ATP
a) Oxygen is required for aerobic respiration in most plants
b) Rice seeds respire anaerobically, since they are planted in flooded
fields
3. Temperature must be optimum
a) Some seeds require a period of cold temperatures, which prevents
seeds from germinating in the winter
4. A requirement for light prevents some seeds from germinating unless they
are close to the surface of the soil
5. Some seeds do not germinate immediately
a) A mature seed is often dormant and will not germinate even if
growing conditions are ideal
b) Dormancy may be caused by the presence of inhibitors, such as the
plant hormone abscisic acid
c) Dormancy may be caused by hard, thick seed coats that prevent
oxygen and water from entering
d) In nature, hard seed coats are worn away by passing through the
digestive tracts of animals or through the activity of soil bacteria
B. Dicots and monocots exhibit characteristic patterns of early growth
1. The embryonic root, or radicle, is the first part to emerge from the seed
2. The shoot emerges next, but lacks a protective cap as is present in the
growing root
a) The stem of dicot seedlings curves and forms a hook, and pulls the
cotyledons and stem tip through the soil
b) Monocots have a sheath called the coleoptile that protects the
emerging stem tip
3. Indeterminate growth is unlimited growth, exhibited by stems and roots
4. Leaves and flowers exhibit determinate growth, in that they stop growing
after they have reached a certain size
II.
Light cues affect flowering and other plant responses
A. The response of a plant to the lengths of day and night is photoperiodism
1. Flowering is a photoperiodic response
2. Plants may be classified as short-day, long-day, and day-neutral
a) Short-day plants flower when the length of uninterrupted darkness is
equal to or greater than a critical period
(1) Short-day plants flower in late summer or fall, when the nights
grow long
b) Long-day plants flower when the length of the night is less than or
equal to some critical period
(1) Long-day plants flower in spring and early summer, when the
days grow long
c) Intermediate-day plants do not flower when the night length is either
too long or too short
d) Day-neutral plants flower in response to some other stimulus
(1) Day-neutral plants are common in the tropics where day length
does not vary much during the year
B. Phytochrome detects varying periods of day length
1. The photoreceptor in plants is a family of about 5 pigments called
phytochrome
a) Each pigment is encoded by a different gene, and the different
pigments may have overlapping and even antagonistic functions
b) Phytochrome is in cells of all vascular plants
c) Phytochrome readily converts from Pr to Pfr, which absorb in the red
and far-red wavelengths (660 and 730 nm, respectively)
d) When Pfr absorbs far-red light, it converts to Pr
e) When Pr absorbs red light, it converts to Pfr
f) Sunlight is composed of much more red than far-red light, so during
the day, the Pr is converted to Pfr
C. Competition for sunlight among shade-avoiding plants involves phytochrome
1. Many plants grow taller when in the shade of other plants, a phenomenon
called shade-avoidance
2. Shade avoidance occurs because leaves absorb more red light than far-red
light, and the low r/fr ratio stimulates stem growth
D. Phytochrome is involved in many other responses to light
1. Phytochrome is involved in the light requirement for germination
2. Other responses include sleep movements, shoot dormancy, leaf abscission
and pigment formation
E. Phytochrome acts by signal transduction
1. Phytochrome responses may be either short-term or long-term
2. Red light causes potassium channels to open; far-red light causes them to
close
3. Slower responses involve the regulation of numerous genes
4.
When light is absorbed by phytochrome, it triggers a conformational
change in the protein to which it is attached, which in turn triggers signal
transduction pathways
a) The active form of phytochrome moves to the nucleus where it
activates a transcription factor, identified as PIF-3
b) The transcription factor then binds to the promoter of a gene and
either turns the gene on or off
c) This pathway is switched on by red light and switched off by far-red
light
F. Light influences circadian rhythms
1. Circadian rhythms are responses that approximate a 24-hour cycle
2. Examples of circadian rhythms are the opening and closing of stomata and
sleep movements of bean and other plants
3. Circadian rhythms persist even under constant environmental conditions
4. Circadian rhythms involve both phytochrome and cryptochrome, a bluelight absorbing pigment
5. Circadian rhythms are thought to help organisms synchronize their
activities to the rising and setting of the sun
III.
Plant responses to stimuli other than light
A. Changes in turgor induce nastic movements
1. Mimosa, the sensitive plant, responds to touch or other stimuli
2. Pulvini are located at the bases of the leaflets and petioles
a) Pulvini receive an electrical signal, and potassium ions exit the cells,
water follows by osmosis and the leaf droops
b) These turgor or nastic movements are temporary and reversible
B. A tropism is growth in response to an external stimulus from a specific
direction
1. Directional growth is a tropism, and may be positive or negative to the
stimulus
2. Phototropism is growth in response to light
a) Most shoots are positively phototrophic
b) Phototropism is triggered by blue light after being absorbed by a
receptor called phototropin
3. Gravitropism is growth in response to gravity
a) Shoots are typically negatively gravitropic
b) Roots are positively gravitropic due to the sedimentation of
amyloplasts (starch-containing plastids) in the root cap
c) Auxin is involved with gravitropism as well
4. Thigmotropism is a response to a mechanical stimulus
a) Curling of tendrils is an example
5. Heliotropism (solar tracking) is the phenomenon in which leaves or flower
of certain plants follow the sun
a)
b)
IV.
Heliotropism is a pulvinar movement
Heliotropism is triggered by blue light
Plant hormones and development
A. Hormones are chemical messengers that regulate growth and development
1. Plant hormones are effective in extremely small amounts
2. Plant hormones have many different responses
B. Plant hormones act by signal transduction
1. Mutants of Arabidopsis have been helpful in studying hormones
2. The mechanism of action of plant hormones is similar to animal hormones
a) The hormone binds a receptor
b) Binding activates a G protein
c) The G protein triggers the production of a second messenger, such as
calcium or cyclic AMP
d) The second messenger then activates enzymes such as protein kinases
e) Protein kinases produce a variety of effects including changes in
membrane permeability or gene activation/inactivation
C. Auxin promotes cell elongation
1. Darwin was the first to provide evidence for auxin
a) Darwin and his son demonstrated positive phototropism in grass
shoots
b) When the coleoptile tip was covered, the shoot did not bend toward
light
c) This hormone was later named auxin, meaning "increase"
2. The term auxin refers to a group of both natural and synthetic hormones
that regulate growth, typically by promoting cell elongation
a) The primary auxin in plants is IAA (indoleacetic acid)
b) Auxin changes cell walls so they can expand, and is involved in
phototropism, gravitropism, and thigmotropism
3. Auxin is responsible for apical dominance
a) Auxin produced by the apical meristem inhibits lateral growth
4. Auxin stimulates development of fruit and can cause the formation of fruit
in flowers that have not been fertilized
5. Synthetic auxins are used in horticulture and agriculture
a) Other synthetic auxins include 2,4-D and 2,4,5-T and areherbicides,
which kill broadleaf dicots, but do not harm monocots
D. Gibberellins promote stem elongation
1. A Japanese biologist first identified gibberellin, produced by a fungus that
caused the "foolish seedling" disease, affecting rice seedlings
2. Gibberellins cause bolting, stimulate flowering, affect fruit development,
and stimulate germination of seeds
E. Cytokinins promote cell division
1. Cytokinins were first identified in herring sperm and coconut water
2.
3.
4.
5.
Cytokinins are a group of molecules that promote mitosis, hence the name
Cytokinins are a requirement for tissue culture
Cytokinins and auxins act in antagonistic ways influencing apical
dominance
Cytokinins delay senescence
F.
Ethylene stimulates abscission and fruit ripening
1. Ethylene is the only gaseous hormone affecting plants
2. Ethylene promotes ripening
a) As a fruit ripens, it produces ethylene, which further accelerates
ripening and even more production of ethylene
b) Ethylene is used in the commercial ripening of bananas
3. Ethylene is involved in thigmomorphogenesis, the developmental
responses to mechanical stimuli
4. Ethylene is also involved in leaf abscission
G. Abscisic acid promotes seed dormancy
1. Abscisic acid is primarily involved in the onset of dormancy
2. Abscisic acid is called the plant "stress hormone," as its effects are
important in protecting the plant from unfavorable conditions
3. Abscisic acid levels increase in response to cold, drought, and high salt
levels
4. Abscisic acid is involved in bud and seed dormancy
H. Additional classes of signaling molecules are involved in plant growth and
development
1. Brassinolides are steroids whose role is unclear in plants
2. Salicylic acid is a phenolic compound that helps to protect plants against
insect pests and other pathogens
3. Systemin is a polypeptide that is transported systemically in response to
wounding by insects
4. Oligosaccharins are composed of sugar residues and limit the spread of
plant pathogens
5. Jasmonates are derivatives of fatty acids and are produced in response to
insect pests and other disease-causing organisms
I. Unidentified plant hormones remain to be discovered
1. Florigen is a hypothetical hormone that promotes flowering
2. To date, these flower promoters and flower inhibitors have not yet been
isolated or chemically characterized
Research and Discussion Topics

How are plant hormones, like gibberellin and auxins, used in agriculture or the
nursery business? There is a great need for trained horticulturalists, and there may
be a student here or there that chooses that as a career. Think about it. How many
nurseries are within a quick drive from your home or place of business, including
places that sell plants that come from big commercial nurseries?
What about the agricultural end? What do you see with respect to the intersection of
plant genetics and agriculture? Do you think there will be sufficient numbers of
trained people in both genetics and agriculture to fill the need we’ll see, for example,
in the next decade?

Compare and contrast the production and action of plant and animal hormones (see
chapter 47).

"Fast plants" are commonly used in biology teaching laboratories. What are "fast
plants"? Why are they special? What is already known about their genome, their
developmental processes (e.g. development of leaves, flowers, roots, etc.)?