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What allows for indeterminate growth in plants?
they have stem cells that continue to exist into adult life, which allow them to grow throughout their lifetime
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Why do humans have determinate growth?
only have stem cells in the embryo
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Best way to figure out function of a gene?
get rid of it and see the effect on the organism
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Arabidopsis plants grown in dark vs. light
- dark: long hypocotyl because it is trying to find the light
- light: short hypocotyl
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Application of siRNAs to medicine?
therapeutics for gene regulation -- if you make too much of a bad protein, you can suppress it by using small RNA
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What two conceptual features of stem cells are shared between plants & animals?
- Cell Division
- Self-Renewal
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What happens during self-renewal process in stem cells?
- asymmetric cell divisions create two daughter cells with different cell fates
- One daughter cell goes on to form a specialized cell
- Other daughter remains a stem cell (self-renewal)
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How do animals and plants differ in the persistence of totipotent vs. pluripotent stem cells?
- animal stem cells are totipotent only in early embryo
- all plant cells are totipotent
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Where are plant stem cells located?
In meristems found in the 16 cell embryo
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What is a stem cell niche?
where stem cells are maintained undifferentiated by short-range diffusible signals
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As the stem cells divide and move out of the niche...
the daughter cells that are placed outside the reach of the signal begin to differentiate
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What do apical meristems control?
primary growth of roots and shoots, enabling indeterminate growth (make it longer)
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Shoot apical meristem (SAM) contributes to...
primary shoot growth
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Root apical meristem (RAM) contributes to...
primary root growth
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The three zones of growth:
- zone of differentiation
- zone of elongation
- zone of cell division
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What is the Shoot Apical Meristem?
dome-shaped mass of actively dividing cells at the shoot tip
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What do axillary buds develop from?
meristematic cells left at the basses of leaf primordia
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What do lateral meristems control?
secondary growth (make it wider)
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Vascular Cambium
cylinder of meristematic cells one cell thick that gives rise to secondary vascular tissue (xylem and phloem)
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Two types of meristems and their location
- - Apical Meristems: (primary growth) -- Located at the tips of roots and in the buds of shoots
- Lateral Meristems: (secondary growth)
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Cell division vs. Cell expansion
- cell division: increases the potential for plant growth
- cell expansion: accounts for actual increase in plant size
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What is the force for cell expansion?
water uptake
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What is the direction of cell expansion controlled by?
orientation of cellulose microfibrils in the cell wall, which is dictated by the microtubules in the cytoplasm
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Advantages of using Arabidopsis mutants to identify genes controlling plant development?
- Smallest plant genome (28,000 genes)
- Small stature
- Brief life cycle (6-10 weeks)
- Self-pollinates (can isolate homozygous mutations)
- Can be out-crossed (can do genetics)
- Can isolate mutants in development
- Can map the mutations to the genome to identify genes controlling plant development
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What does the first asymmetric division of the embryo set up?
- Shoot/root polarity
- big basal cell (roots) vs. small apical cell (shoots)
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How does the stem cell know to stay undifferentiated?
by short-range diffusible signals from localized source cells
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When the shoot apical meristem undergoes a phase transition, what does it give rise to?
floral meristem, from which the floral organs are derived
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How does SAM transition from vegetative phase to floral meristem?
stem cells move away from the apical meristem and form a new meristem
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What does the order of emergence have to do with organ identity? What is that order?
- Order of emergence determines organ fate
- 1. sepal
- 2. petal
- 3. stamen
- 4. carpel
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What is the innermost organ in a plant?
Carpel
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epidermal cells become specialized after...
asymmetric division
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Wild-type epidermis vs. mutant?
- Wild-type: stomata are separated from each other by at least one pavement cell
- Scream mutant: all epidermal cells are stomata
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What kind of mutation allowed for scream mutants?
Gain-of-function
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Gnom mutant of Arabidopsis is defective in what? Why is that important?
- the first asymmetrical division of zygote
- division is symmetric so plant doesn't know which side is shoot vs. root
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What does the gnome gene control?
controls polar auxin transport (via PIN1 auxin transporter) that establishes the shoot-root axis in the early embryo
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What are homeotic mutants responsible for?
Involved in determining where, when, and how body segments develop
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Homeotic mutants in animals vs. plants?
- animals: In flies, homeotic mutants result in legs growing in place of antennae
- plants: flower homeotic mutants show abnormal numbers of floral parts
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Three rules of ABC model of floral development?
- 1. Each gene acts in two whorls
- 2. Combination of gene products determine organ identity
- 3. Gene A and C are mutually exclusive
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Gene A mutant
carpel, stamen, stamen, carpel
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Gene B mutant
sepal, sepal, carpel, carpel
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Gene C mutant
sepal, petal, petal, sepal
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A mutant in all three genes (A,B, and C) makes..
flowers composed of leaves
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What does the wild-type superman gene do?
- normally suppresses gene B activity in 4th whorl
- but in superman mutant, gene B is expressed in the 4th whorl, so it becomes a stamen instead of a carpel
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What is the suppressor of the superman mutant called?
Clark Kent
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Function of sepal?
enclose the flower
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Function of petals?
brightly colored, attract pollinators
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What are the two sterile floral organs?
petals and sepals
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Function of stamens?
- male reproductive organ
- produce pollen, anther + filament
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Function of carpels?
- female reproductive organs
- produce ovules, stigma + style + ovary
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What do the anthers give rise to?
pollen grain, which is what fertilizes the egg
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How does double fertilization occur?
- pollen lands on sigma
- pollen grain grows down pollen tube into the ovary and discharges two sperm cells near the embryo sac
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What do the two sperm fertilize during double fertilization?
- sperm 1: fertilizes the egg to make zygote (2n)
- sperm 2: combines with polar nuclei -> endosperm (3n)
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Function of endosperm?
becomes food source for the embryo
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Major benefit of double fertilization in angiosperms?
to coordinate developmental timing between the embryo and its food stores
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Development of embryo
- First mitotic division of embryo is asymmetric
- Large basal cell forms suspensor cell
- Terminal apical cell divides several times to form spherical pro-embryo
- Cotyledon appear at either side of apical meristem
- After germination, apical and root meristems sustain primary growth of seedling
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Advantages of seeds?
can be alive for 100s of years before germination
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Seed dormancy
when the embryo is alive but not growing, seed is desiccated yet protects embryo from drying out
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When can identical twins be made?
in the first four days after fertilization
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How does the orientation of microtubule band determine its plane of cell division?
- Transverse microtubule band = anti-clinal division (vertical)
- Radial microtubule band = peri-clinal division (horizontal)
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Apical meristems are made in the _____ and persist throughout ______
embryo, adult life
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Went Experiment (1926)
- auxin mediates phototropism
- extracted chemical messenger into agar block, when agar was put on top of tip, it still bent
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What does auxin do?
hormone that makes cells elongate
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What does the GUS protein report?
- auxin content of cell
- cell where this protein is expressed appear blue, promoted by DR5, which is stimulated by Auxin
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How does accumulation of auxin cause bending toward light?
accumulates on dark side, making them elongate, which makes the plant bend
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How are plant and animal hormones similar?
- Are small diffusible molecules
- Are produced in very low concentrations
- Minute amount of hormone can have a profound effect on the growth and development
- Hormones are transported- plant hormones are made in one location and transported to another
- Hormones are perceived by receptors and signals transduced to effect changes in gene expression
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Growth enhancing hormones?
- auxin (cell elongation)
- cytokinin (cell division)
- Brassinosteroid (cell division and elongation of stems)
- Gibberllin (promotes seed germination)
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Growth inhibiting hormones?
- Abscisic acid (promotes dormancy of embryo)
- Ethylene (promotes senescence and fruit ripening)
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How does ethylene work?
causes cell wall to break down and become softer and makes starch become glucose (sweeter)
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Acid growth hypothesis
- auxin transport across a cell wall causes cell expansion
- 1. transport of auxin stimulates plasma membrane proton pumps
- 2. lowering the pH in the cell wall
- 3. which activates expansin enzyme that breaks down carbs/proteins in cell wall
- 4. pressure potential exerted by cell wall decreases, causing osmotic uptake of water into cell
- 5. uptake of water elongates the cell
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Auxin is made in ____ and transported to ______
shoot apex, roots via phloem
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Cytokinin is made in ____ and transported to _____
roots, up to shoots via xylem
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Ratio of auxin to cytokinin determines...
shoot vs. root development
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High auxin, low cytokinin
roots
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Low auxin, high cytokinin
shoots
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equal amounts of auxin and cytokinin
callus (undifferentiated mass of cells)
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What is routine used for?
used in horticulture to stimulate roots from leaf cuttings
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Which plant hormone looks/functions like animal hormones?
Brassinosteroid
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How do you know whether a mutant is defective in synthesis/perception of a hormone?
- add that hormone, if it does not grow you know that the plant is defective in the receptor or signal transduction part
- if hormone starts to be created, then synthesis is not defective
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Precocious germination
embryo is germinating while it is still on the ear
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ABA vs. GA
- ABA: promotes seed dormancy
- GA: promotes germination
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How does embryo withstand extreme dehydration?
ABA levels are high, which induce production of special proteins that help it survive
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How does GA promote germination?
- promotes conversion of starch into glucose, which is what feeds the growing embryo
- so embryo starts to grow
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"triple response"
- slowing of stem elongation
- stem thickening
- curvature to enable horizontal growth
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What does triple response do?
- allows a growing shoot to avoid obstacles (rocks in soil) as it elongates
- releases ethylene to grow around it
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How do plants that are not exposed to ethylene?
they grow straight
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Hormone synthesis mutants
mutants that can be rescued by adding hormone or blocking hormone synthesis, are defective in hormone production
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Receptor/signal transduction mutants
cannot be rescued by adding back the hormone (or blocking its production) are defective in either hormone perception (ex: receptor) or signal transduction component
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Maize mutants defective in ABA signal transduction cause...
- precocious germination
- because ABA cannot transduce its signal to inhibit growth of embryo
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What does action spectrum depict?
- relative response of a process to different wavelengths
- tells which light quality affects a process
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3 major photoreceptors:
- phototropins
- crytochromes
- phytochromes
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What is a photoreceptor?
is a protein that is covalently bound to a chromophore (a non protein pigment) that can absorb light of a particular wavelength
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crytochromes vs. phytochromes?
- crytochromes: blue light, light inhibition of hypocotyl elongation
- phytochromes: red/fr light, flowering time regulation
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What are hypotocotyl mutants impaired in?
red/blue light perception or signal transduction
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Protein doman 1 vs domain 2 in phytochrome?
- 1: photoreceptor activity
- 2: kinase activity
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Red vs. far-red light and their effects on phytochrome
- red: activates phytochrome, induces germination
- far-red: inactivates phytochrome
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How do plants measure 24 hours?
- circadian clock
- level of constans fluctuates throughout the day, which helps them keep track of length of day and night
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When is the flowering time gene activated?
When the plant senses long days
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What does the flowering time gene do?
mobile signal that goes up to shoot apical meristem and activates ABC genes to make plant flower
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Mosses were the first land plants to...
evolve embryo protection
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What do mosses lack?
a vascular system, can't transport over long distances
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Ferns were the first land plants to... and the first to have...
- evolve leaves and a vascular system
- have a dominant sporophyte generation
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What does sporopollenin do?
prevents exposed zygotes from drying out
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Traits present in all land plants but absent in green algae?
- alternation of generations and multicellular embryos
- multicellular organs that produce gametes
- walled spores produced in sporangia to aid in dispersal of spores
- apical meristems
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Evolutionary advantage of seeds
- food supply- provided by endosperm inside seed
- desiccation- protects embryo from drying out
- seed dispersal- used to transport embryo to new environment
- viable but dormant embryo
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4 phyla of gymnosperms
- Cycadophyta
- Gingkophyta
- Gnetophyta
- Coniferophyta
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Cycadophyta
- thrived during mesozoic
- large cones, plam-like leaves
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Gingkophyta
high tolerance to air pollution
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Coniferophyta
600 species, forests
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All seed plants share 5 traits
- 1. reduced gametophytes
- 2. heterospory (male/female)
- 3. ovules
- 4. pollen
- 5. seeds
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Why are gymnosperms called naked?
- naked seeds not enclosed in ovaries
- they are exposed on modified leaves
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Advantages of flowers/fruits?
- fruits aid in seed dispersal
- help attract pollinators
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What does the integument become?
the seed coat
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