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Development & Morphogenesis (Environmental Complexity (Water (not a…
Development & Morphogenesis
morphogenesis=generation of the shape of the plant and its various organs :silhouettes:
differentiation=increase in complexity as some cells become different from each other
polarity=formation of the root/shoot axis
pattern establishment mechanism=informs each cell of its location relative to other epidermis cells
Environmental Complexity
Light
provides energy for photosynthesis
provides the gradient of light=direction of light
provides duration of light=information about time of year
Gravity
can cause stress to a plant
upright stems support more weight than stems at random
provides directions for roots
Touch
contact with a stone or branch is detrimental
touch with a nearby support system
Venus flytrap hairs are example of touch
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Temperature :fire:
metabolism usually does not proceed above one to seven degrees C
cold temps are required for the normal flowering of biennial & many perennials
lower temps required to induce dormancy in temperate trees and shrubs
Water
not a signal like other factors
roots grow towards water simply due to the favorable environment
roots do not scout out for water
scarcity triggers adaptive responses
Pests & Damage
bacteria, fungi, and insects attack chemically with toxins and digestive enzymes
plants have recognition cells to recognize bits of bacterial flagella
Responding to Environmental Stimuli
Tropic Responses
tropic response=growth response oriented with regard to the stimulus
plagiotropism=growth at an angle(branches)
positively gravitropic=growing downward in response to gravity
negatively gravitropic=growing upward in response to gravity
thigmotropism=touch is the stimulus
chemotropism=growing along the style to the ovary by following a gradient of chemical released from the ovule
changing directions=differential growth
Nastic Responses :pencil2:
nastic response=stereotyped nongrowth response that is not oriented with regard to the stimulus
Venus flytrap stimulus
diurnal=active during the day
nocturnal=active at night
photonastic=cue that stimulates flower opening is the presence of or absence of light
nastic movements based on changes of turgor pressure
sleep movements=leaf blade elevated in day and lowered at night
Morphogenic Responses
causes a change in the quality of the plant
a fundamental change occurs in the metabolism of a tissue or even the entire plant
Taxis
taxis=a response in which a cell swims toward(positive) or away(negative) from a stimulus
chemotaxis=sperm cell swimming to egg cell
Communication Within the Plant
Perception and Transduction
statocytes=root cap cells that have large starch granules
statoliths=large starch granules that sink in response to gravity
presentation time=length of time the stimulus must be present for perceptive cells to complete transduction
threshold=level of stimulus that must be present during the presentation time to cause perception and transduction
all-or-none response=after threshold & ptime are met, stimulus is no longer important=response completely internal
dosage-dependent response=amount or duration of the stimulus affects the amount of duration of the response
Chemical Messengers
hormones=organic chemicals produced in one part of a plant and then transported to other parts where they initiate a response
hormones synthesized in regions of transduction
released for transport through the phloem or mesophyll
Auxins
first plant hormone discovered in 1926
indoleacetic acid=IAA
involved in positive phototropism in oat seedlings (1926)
conjugated form of IAA is safe from destruction
polar transport=from apex to the base of the plant
only carry information that the shoot apex is healthy & active
small amounts stimulate growth but large amounts inhibit growth
Cytokinins
artificial one=kinetin
two natural ones=zeatin & isopentenyl adenine
cytokinins are purines
Gibberellins
at least 125 are known
are numbered, not named-GA1, GA2..
all have gibberellane ring system
Abscisic Acid
ABA=abscisic acid
discovered in sycamores-abscission of fruits, leaves, and flowers
involved in many types of stress resistance
stimulus for ABA production is soil dryness
inhibitor
Ethylene
the only gaseous hormone
stimulates certain fruits to undergo developmental changes
moves rapidly through tissues by diffusion
Other Hormones
Brassinosteroids=leaf morphogenesis, root & stem growth & vascular differentiation
Jasmonic acid=defense against animals & fungi
Salicylic acid=involved in resistance to pathogens (viruses)
Signal Amplification Cascades
weak signals are amplified by the target cell
creates thousands of messenger molecules
increases chances of them entering the nucleus to encounter the right genes
cascades are like a domino effect
Activation and Inhibition of Shoots by Auxin
Cell Elongation
auxin triggers cell elongation just below the apical meristem
weaken the cell wall so the protoplast can press it and stretch it
Apical Dominance
apically produced auxin induces dormancy in axillary buds=apical dominance
result is each shoot tip has only one active apical meristem
is a threshold response
Differentiation of Vascular Tissues
third site of response to auxin=vascular cambium
auxin activates the dormant vascular cambium
auxin stimulates cambial cells to begin mitosis and cytokinesis
auxin causes new daughter cells to differentiate into xylem cells
Hormones as Signals of Environmental Factors
Leaf Abscission
young leaves produce lots of auxin
a mature leaf produces less auxin but is a steady flow
Tropisms
light is stimulus in phototropism
blue is most effective wavelength for phototropism
blue-light responses
coleoptile=outermost protective leaf on oat seedlings
auxin is transported from one side of coleoptile to the other
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creates a bend towards the light by growing faster on one side
Flowering
Ripeness to Flower
juvenile phase=when plants are incapable of being induced to flower
adult phase=plants are sensitive to floral stimulation
phase change=juvenile to adult
cold temps can stimulate phase change in
biennial plants
little is known about the phase change in perennial plants
Photoperiodic Induction to Flower
different mechanisms for different species
in certain annual species size is a big factor
peas & corn initiate flowers after a particular number of leaves have been produced
in many species the transition to flowering is triggered by the photoperiod
photoperiod=day length=season indicator
short-day plants=bloom in spring or fall
long-day plants=bloom in summer
day-neutral plants=don't respond to day length
phytochrome=pigment that detects the presence or absence of light
site of perception
(young leaf)
is not the site of response
(apical meristem)
Endogenous Rhythms and Flowering
endogenous rhythms=aspects of their metabolism cycle repeatedly between two states
sleep movements in prayer plants
in many flowers, nectar & fragrance are controlled by this
involves a negative feedback loop
in nature they are 24 hrs long but many aren't that long
if it is 24 hrs long=
Circadian rhythm
ABC Model of Flower Organization
appendages
outermost=sepals
petals
stamens
centermost=carpels
eudicot flowers
five sepals
five petals
five stamens
five carpels
monocots
three sepals
three petals
three stamens
three carpels
ABC model hypothesis
postulates that genes A, B, and C control basic aspects of flower organization
gene A=affects the outermost, lowest regions of flower shoot apical meristem
primordial develop into whorl of sepals
gene C=controls uppermost, central region
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stamens produced with B & C
center of flower=primordial develop as carpels
gene B=partially overlaps the other two
petals develop with A & B genes
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hemeotic mutation=mutation that causes an organ to develop in an unexpected site