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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
Why do humans have determinate growth?
only have stem cells in the embryo
Best way to figure out function of a gene?
get rid of it and see the effect on the organism
Arabidopsis plants grown in dark vs. light
- dark: long hypocotyl because it is trying to find the light
- light: short hypocotyl
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
What two conceptual features of stem cells are shared between plants & animals?
- Cell Division
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)
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
Where are plant stem cells located?
In meristems found in the 16 cell embryo
What is a stem cell niche?
where stem cells are maintained undifferentiated by short-range diffusible signals
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
What do apical meristems control?
primary growth of roots and shoots, enabling indeterminate growth (make it longer)
Shoot apical meristem (SAM) contributes to...
primary shoot growth
Root apical meristem (RAM) contributes to...
primary root growth
The three zones of growth:
- zone of differentiation
- zone of elongation
- zone of cell division
What is the Shoot Apical Meristem?
dome-shaped mass of actively dividing cells at the shoot tip
What do axillary buds develop from?
meristematic cells left at the basses of leaf primordia
What do lateral meristems control?
secondary growth (make it wider)
cylinder of meristematic cells one cell thick that gives rise to secondary vascular tissue (xylem and phloem)
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)
Cell division vs. Cell expansion
- cell division: increases the potential for plant growth
- cell expansion: accounts for actual increase in plant size
What is the force for cell expansion?
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
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
What does the first asymmetric division of the embryo set up?
- Shoot/root polarity
- big basal cell (roots) vs. small apical cell (shoots)
How does the stem cell know to stay undifferentiated?
by short-range diffusible signals from localized source cells
When the shoot apical meristem undergoes a phase transition, what does it give rise to?
floral meristem, from which the floral organs are derived
How does SAM transition from vegetative phase to floral meristem?
stem cells move away from the apical meristem and form a new meristem
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
What is the innermost organ in a plant?
epidermal cells become specialized after...
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
What kind of mutation allowed for scream mutants?
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
What does the gnome gene control?
controls polar auxin transport (via PIN1 auxin transporter) that establishes the shoot-root axis in the early embryo
What are homeotic mutants responsible for?
Involved in determining where, when, and how body segments develop
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
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
Gene A mutant
carpel, stamen, stamen, carpel
Gene B mutant
sepal, sepal, carpel, carpel
Gene C mutant
sepal, petal, petal, sepal
A mutant in all three genes (A,B, and C) makes..
flowers composed of leaves
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
What is the suppressor of the superman mutant called?
Function of sepal?
enclose the flower
Function of petals?
brightly colored, attract pollinators
What are the two sterile floral organs?
petals and sepals
Function of stamens?
- male reproductive organ
- produce pollen, anther + filament
Function of carpels?
- female reproductive organs
- produce ovules, stigma + style + ovary
What do the anthers give rise to?
pollen grain, which is what fertilizes the egg
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
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)
Function of endosperm?
becomes food source for the embryo
Major benefit of double fertilization in angiosperms?
to coordinate developmental timing between the embryo and its food stores
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
Advantages of seeds?
can be alive for 100s of years before germination
when the embryo is alive but not growing, seed is desiccated yet protects embryo from drying out
When can identical twins be made?
in the first four days after fertilization
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)
Apical meristems are made in the _____ and persist throughout ______
embryo, adult life
Went Experiment (1926)
- auxin mediates phototropism
- extracted chemical messenger into agar block, when agar was put on top of tip, it still bent
What does auxin do?
hormone that makes cells elongate
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
How does accumulation of auxin cause bending toward light?
accumulates on dark side, making them elongate, which makes the plant bend
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
Growth enhancing hormones?
- auxin (cell elongation)
- cytokinin (cell division)
- Brassinosteroid (cell division and elongation of stems)
- Gibberllin (promotes seed germination)
Growth inhibiting hormones?
- Abscisic acid (promotes dormancy of embryo)
- Ethylene (promotes senescence and fruit ripening)
How does ethylene work?
causes cell wall to break down and become softer and makes starch become glucose (sweeter)
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
Auxin is made in ____ and transported to ______
shoot apex, roots via phloem
Cytokinin is made in ____ and transported to _____
roots, up to shoots via xylem
Ratio of auxin to cytokinin determines...
shoot vs. root development
High auxin, low cytokinin
Low auxin, high cytokinin
equal amounts of auxin and cytokinin
callus (undifferentiated mass of cells)
What is routine used for?
used in horticulture to stimulate roots from leaf cuttings
Which plant hormone looks/functions like animal hormones?
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
embryo is germinating while it is still on the ear
ABA vs. GA
- ABA: promotes seed dormancy
- GA: promotes germination
How does embryo withstand extreme dehydration?
ABA levels are high, which induce production of special proteins that help it survive
How does GA promote germination?
- promotes conversion of starch into glucose, which is what feeds the growing embryo
- so embryo starts to grow
- slowing of stem elongation
- stem thickening
- curvature to enable horizontal growth
What does triple response do?
- allows a growing shoot to avoid obstacles (rocks in soil) as it elongates
- releases ethylene to grow around it
How do plants that are not exposed to ethylene?
they grow straight
Hormone synthesis mutants
mutants that can be rescued by adding hormone or blocking hormone synthesis, are defective in hormone production
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
Maize mutants defective in ABA signal transduction cause...
- precocious germination
- because ABA cannot transduce its signal to inhibit growth of embryo
What does action spectrum depict?
- relative response of a process to different wavelengths
- tells which light quality affects a process
3 major photoreceptors:
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
crytochromes vs. phytochromes?
- crytochromes: blue light, light inhibition of hypocotyl elongation
- phytochromes: red/fr light, flowering time regulation
What are hypotocotyl mutants impaired in?
red/blue light perception or signal transduction
Protein doman 1 vs domain 2 in phytochrome?
- 1: photoreceptor activity
- 2: kinase activity
Red vs. far-red light and their effects on phytochrome
- red: activates phytochrome, induces germination
- far-red: inactivates phytochrome
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
When is the flowering time gene activated?
When the plant senses long days
What does the flowering time gene do?
mobile signal that goes up to shoot apical meristem and activates ABC genes to make plant flower
Mosses were the first land plants to...
evolve embryo protection
What do mosses lack?
a vascular system, can't transport over long distances
Ferns were the first land plants to... and the first to have...
- evolve leaves and a vascular system
- have a dominant sporophyte generation
What does sporopollenin do?
prevents exposed zygotes from drying out
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
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
4 phyla of gymnosperms
- thrived during mesozoic
- large cones, plam-like leaves
high tolerance to air pollution
600 species, forests
All seed plants share 5 traits
- 1. reduced gametophytes
- 2. heterospory (male/female)
- 3. ovules
- 4. pollen
- 5. seeds
Why are gymnosperms called naked?
- naked seeds not enclosed in ovaries
- they are exposed on modified leaves
Advantages of flowers/fruits?
- fruits aid in seed dispersal
- help attract pollinators
What does the integument become?
the seed coat