Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 27
Plant Reproduction and
Development
(Sections 27.6 - 27.10)
Albia Dugger • Miami Dade College
27.6 Patterns of
Development in Plants
• An embryonic plant with
shoot and root apical
meristems formed as
part of the seed
• As the seed matures
and dries, the embryo
enters dormancy
Patterns of
Development
in Plants
seed coat fused
with ovary wall
endosperm cells
cotyledon
coleoptile
plumule
(embryonic
shoot)
embryo
hypocotyl
radicle
(embryonic
root)
Fig. 27.9, p. 436
Germination
• The embryo resumes development after germination
• Water seeps into a seed and activates enzymes that break
down stored starches into sugars
• The seed coat ruptures and oxygen diffuses into the seed
• Meristem cells divide, the embryo grows, and the
embryonic root breaks out of the seed coat
• germination
• Resumption of growth after dormancy
Triggers for Germination
• Germination requirements are evolutionary adaptations to life
in a particular environment, and maximize a seedling’s
chance of survival
• Triggers differ by species, and have a genetic basis:
• Some seed coats must be physically broken
• Some seeds require freezing, or exposure to light
• Some seeds require exposure to burning
Plant Development
• Sporophyte tissues and organs develop in characteristic
patterns with genetic and environmental components
• Patterns of early growth (increase in cell number and size)
vary by species
• Cell division occurs primarily at meristems – behind
meristems, cells differentiate and form specialized tissues
Early Growth of a Monocot (Corn)
Early Growth of a Monocot (Corn)
Fig. 27.10, p. 436
Early Growth of a
Monocot (Corn)
coleoptile
branch
root
coleoptile
hypocotyl
primary
root
radicle
A After a corn grain (seed) germinates, its
radicle and coleoptile emerge. The radicle
develops into the primary root. The coleoptile
grows upward and opens a channel through
the soil to the surface, where it stops growing.
Fig. 27.10a, p. 436
Early Growth of a Monocot (Corn)
Fig. 27.10b, p. 436
Early Growth of a Monocot (Corn)
primary leaf
coleoptile
B The plumule develops into the
seedling’s primary shoot, which
pushes through the coleoptile and
begins photosynthesis. In corn plants,
adventitious roots that develop from
the stem afford additional support for
the rapidly growing plant.
adventitious
(prop) root
branch root
primary root
Fig. 27.10b, p. 436
Early Growth of a Eudicot (Bean)
Early Growth of a
Eudicot (Bean)
Fig. 27.11a, p. 437
Early Growth of a
Eudicot (Bean)
seed
coat radicle
cotyledons (two)
hypocotyl
primary root
A After a bean seed germinates, its radicle emerges and
bends in the shape of a hook. Sunlight causes the
hypocotyl to straighten, which pulls the cotyledons up
through the soil.
Fig. 27.11a, p. 437
Early Growth of a
Eudicot (Bean)
Fig. 27.11b, p. 437
Early Growth of a
Eudicot (Bean)
primary
leaf
primary
leaf
withered
cotyledon
primary root
branch
root
root
nodule
B Photosynthetic cells in the cotyledons make food
for several days. Then, the seedling’s leaves take
over the task and the cotyledons wither and fall off.
Fig. 27.11b, p. 437
Key Concepts
• Growth and Development
• Plant development includes seed germination and other
events of the life cycle, such as root and shoot
development, flowering, fruit formation, and dormancy
• These events have a genetic basis, and are influenced by
the environment
ANIMATION: Plant Growth
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27.7 Plant Hormones and Other Signaling
Molecules
• Plant development depends on cell-to-cell communication,
which is mediated by plant hormones
• Environmental cues such as availability of water, length of
night, temperature, and gravity influence plants by triggering
the production and dispersal of hormones
• hormone
• Signaling molecule that is released into the body by one
type of cell and alters the activity of other cells
Plant Hormones
• Plant hormones stimulate or inhibit development
• When a plant hormone binds to a target cell, it may modify
gene expression, change solute concentrations, affect
enzyme activity, or activate another molecule in cytoplasm
• Five types of plant hormones interact in plant development:
gibberellins, auxins, abscisic acid, cytokinins, and ethylene
Gibberellins
• gibberellin
• Plant hormone that
induces cell division and
stem elongation
• Helps seeds break
dormancy
• Role in flowering in
some species
Auxins
• Auxins produced in apical meristems result in elongation of
shoots, cell differentiation in vascular cambium, fruit
development, and lateral root formation in roots
• Apical dominance: Auxin produced in a shoot tip prevents
growth of lateral buds along a lengthening stem
• auxin
• Plant hormone that stimulates cell division and elongation
Effects of Auxin
Effects of Auxin
Fig. 27.13a, p. 439
Effects of Auxin
time
A A coleoptile stops growing after its auxin-producing tip has
been removed. A block of agar that absorbs auxin from a cut tip
can stimulate a de-tipped coleoptile to resume growth.
Fig. 27.13a, p. 439
Effects of Auxin
Fig. 27.13b, p. 439
Effects of Auxin
time
B If an auxin-containing agar
block is placed to one side of
a cut tip, the coleoptile will
continue to grow, but bend as
it does.
Fig. 27.13b, p. 439
Abscisic Acid
• Abscisic acid inhibits growth, diverts photosynthetic products
from leaves to seeds, inhibits seed germination in some
species, and can cause stomata to close
• abscisic acid (ABA)
• Plant hormone that stimulates stomata to close in
response to water stress
• Induces dormancy in buds and seeds
Cytokinins
• Cytokinins form in roots and travel via xylem to shoots,
where they induce cell divisions in apical meristems
• cytokinin
• Plant hormone that promotes cell division
• Releases lateral buds from apical dominance
• Inhibits aging in leaves
Ethylene
• Ethylene gas is produced by damaged cells, in autumn in
deciduous plants, and near the end of the life cycle as part of
a plant’s normal process of aging
• Ethylene is widely used to artificially ripen fruit that has been
harvested while still green
• ethylene
• Gaseous plant hormone that inhibits cell division in stems
and roots
• Promotes fruit ripening
Major Plant Hormones and Their Effects
Commercial Uses of Plant Hormones
Other Signaling Molecules
• Brassinosteroids stimulate cell division and elongation
• Jasmonates helps inhibit germination and root growth
• FT protein is part of a signaling pathway in flower formation
• Salicylic acid helps plants resist attacks by pathogens
• Systemin helps transcription of genes for insect toxins
ANIMATION: Cell Shapes
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27.8 Adjusting the
Direction and Rate of Growth
• Plants respond to environmental stimuli by adjusting the
growth of roots and shoots
• These responses (tropisms) are typically mediated by
hormones
Responses to Gravity
• When a seed germinates, its primary root always grows
downward, and its primary shoot always grows upward
• In plants, a shift in dense starch grains (statoliths) to the
lowest part of the cell causes auxin to be redistributed to the
downward-facing side of roots and shoots
• gravitropism
• Plant growth in a direction influenced by gravity
Gravitropism
• Seedlings rotated
90° adjust by
redistributing
auxin
• Auxin transport
inhibitors prevent
seedlings from
adjusting direction
of growth
Gravity, Statoliths, and Auxin
• Ten minutes after root A was rotated 90°, statoliths are
already settling to the new “bottom” of the cells
Gravity,
Statoliths,
and Auxin
statoliths
Fig. 27.15a, p. 440
Gravity, Statoliths, and Auxin
Fig. 27.15b, p. 440
Responses to Light
• Phototropism orients plant parts to maximize light reception
for photosynthesis – phototropins absorb blue light and
control auxin production
• phototropism
• Change in the direction of cell movement or growth in
response to a light source
• solar tracking
• Plant parts change position in response to the sun’s
changing angle through the day
Phototropism
Responses to Contact
• We see thigmotropism when a vine’s tendril touches an
object and curls around it
• Mechanical stress, such as by wind exposure, inhibits stem
lengthening in a response related to thigmotropism
• thigmotropism
• Directional growth of a plant in response to contact with a
solid object
• Involves calcium ions and products of at least five genes
Effect of Mechanical Stress
Animation: Gravitropism
27.9 Sensing Recurring
Environmental Changes
• Seasonal shifts in night length, temperature, and light trigger
seasonal shifts in plant development
• Most organisms have a biological clock that governs the
timing of rhythmic cycles of activity
• biological clock
• Internal time-measuring mechanism by which individuals
adjust their activities seasonally, daily, or both in response
to environmental cues
Biological Clocks
• A bean plant holds its leaves horizontally during the day but
folds them close to its stem at night – these rhythmic leaf
movements are an example of a circadian rhythm
• Similar mechanisms cause flowers of some plants to open
only at certain times of day
• circadian rhythm
• A biological activity that is repeated about every 24 hours
Rhythmic Leaf Movements
• Despite being kept in the dark for 24 hours, the leaves of this
bean plant kept on folding and unfolding at sunrise (6 a.m.)
and sunset (6 p.m.)
Setting the Clock
• Sunlight resets biological clocks in plants by activating and
inactivating photoreceptors called phytochromes
• Active phytochromes activate genes that control important
processes such as germination and flowering
• phytochrome
• A light-sensitive pigment that helps set plant circadian
rhythms based on length of night
Phytochromes
• Red light activates phytochromes; far-red light inactivates
them
Phytochromes
red
far-red
660 nm 730 nm
red light
Pr
inactive
Pfr
far-red light
gene expression
activated
Pfr reverts to Pr
in darkness
Fig. 27.19, p. 442
When to Flower?
• Different species of plants flower at different times of the year
– in these plants, flowering is photoperiodic
• Long-day plants flower when the hours of darkness fall below
a critical value; short-day plants flower only when the hours of
darkness are greater than a critical value
• photoperiodism
• Biological response to seasonal changes in the relative
lengths of day and night
Control of Flowering
• In response to night length, companion cells in leaf phloem
transcribe more or less of the Flowering locus T (FT) gene
• Cells export FT protein into sieve tubes, where it migrates
from leaves to shoot tips
• FT protein interacts with a transcription factor to transcribe
floral identity genes in cells differentiating behind the
meristem
Photoperiodism
Photoperiodism
JANUARY
dormancy
FEBRUARY
MARCH
APRIL
seed germination or renewed
growth; short-day plant flowering
MAY
JUNE
long-day plant flowering
JULY
short-day plant flowering
AUGUST
SEPTEMBER
onset of dormancy
OCTOBER
dormancy
A
NOVEMBER
DECEMBER
14
12
10
8
Length of night (hours of darkness)
Fig. 27.20a, p. 443
Experiment: Photoperiodism
Experiment: Photoperiodism
Long-Day Plant:
critical night length
Short-Day Plant:
...does not flower
...flowers
B
C
...does not flower
...flowers
Time being measured (hours)
B A flash of red light interrupting a long night activates phytochrome.
It causes plants to respond as if the night were short, and long-day
plants flower.
C A pulse of far-red light, which inactivates phytochrome, cancels
the effect of the red flash, and short-day plants flower. Blue bars
indicate night length; yellow bars, day length.
Fig. 27.20b,c, p. 443
Vernalization
• Some plants flower only after exposure to cold winter
temperatures
• In these plants, the FT gene is silenced by a repressor which
stops being produced after a period of cold weather
• vernalization
• Stimulation of flowering in spring by low temperature in
winter
Vernalization
• Local effect of cold on
dormant buds of lilac
• Only buds exposed to
the low outside
temperatures resumed
growth and flowered in
springtime
Key Concepts
• Responses to Environmental Cues
• Plants respond to environmental cues, including gravity,
sunlight, and seasonal shifts in night length and
temperatures, by altering patterns of growth
• Cyclic patterns of growth are responses to seasons and
other recurring environmental patterns
27.10 Plant Defenses
• Plants protect themselves from predators in several ways:
• Thorns or nasty-tasting chemicals directly deter herbivores
• Damage to a leaf stimulate synthesis of jasmonates, which
stimulates production of certain genes products:
• Some products slow growth temporarily
• Some products release chemicals that attract wasps that
parasitize insect herbivores
Jasmonates in Plant Defense
Jasmonates in Plant Defense
Fig. 27.22a, p. 444
Jasmonates in Plant Defense
Fig. 27.22b, p. 444
Jasmonates in Plant Defense
Fig. 27.22c, p. 444
Jasmonates in Plant Defense
Fig. 27.22d, p. 444
Systemic Acquired Resistance
• The presence of a virus, bacteria, or fungus in one plant part
increases pathogen resistance in the entire plant
• Affected tissue releases molecular signals that cause cells in
other plant parts to produce compounds (hydrogen peroxide,
salicylic acid, jasmonates) that strengthen resistance
• systemic acquired resistance
• In plants, a long-term, systemic resistance to pathogens
Senescence
• Dropping of leaves (abscission) may be induced by any
stress; it also occurs in the normal life cycle of flowering
plants, as part of senescence
• abscission
• Process by which plant parts are shed in response to
seasonal change, drought, injury, or nutrient deficiency
• senescence
• Phase in a life cycle from maturity until death
Hormones and Abscission
• Hormones mediate abscission in the normal life cycle
• Example: A deciduous fruit tree
• In early summer, leaves and fruits produce auxin that
maintains growth
• As growing season ends, auxin production declines,
nutrients are routed to stems and roots
• Ethylene signals cells in abscission zones to drop leaves
and fruit
Abscission in a Horse Chestnut Tree
Abscission in a Horse Chestnut Tree
Fig. 27.23a, p. 445
Abscission in a Horse Chestnut Tree
Fig. 27.23b, p. 445
Plight of the Honeybee (revisited)
• Bees in hives affected by colony collapse disorder have large
amounts of ribosomal RNA fragments in their guts
• The problem may be picorna-like viruses, which hijack their
hosts’ protein synthesis machinery
• Bees that can’t make proteins can’t defend themselves
against infections, and are vulnerable to starvation