Plants Flashcards
In plants, what does meiosis result in?
A reduced second generation (not gametes) - alternation of generations.
Meiosis of sporangia forms spores (n) -> gametophytes -> gametangia -> gametes -> zygote (2n) -> sporophyte -> sporangia.
Simplified in gymnosperms.
What are the sporangia in conifers?
Cones.
Woody pine cone = female.
Cone cluster = male (release pollen).
What are spore mother cells?
Megasporocyte - female.
Microsporocyte - male.
What are sporophylls?
Carpels
(megasporophyll) - have ovules (contains megasporangia -> seeds) and form ovary (-> fruit)
Stamens (microsporophyll) - release pollen (microspores).
How are the embryo and endosperm (seed) formed?
Double fertilisation.
What are pollination syndromes?
Adaptive significance of flower:
Biotic - birds and insects.
Abiotic - wind and water.
What are the differences between monocots and eudicots?
Monocots: Flower parts in multiples of 3. Parallel leaf venation. Scattered vascular bundles (stem). 1 pore/furrow (pollen). 1 cotyledon (seeds/seedling). Eudicots: Flower parts in multiples of 4/5. Netlike leaf venation. Vascular bundles in ring. 3 pores/furrows. 2 cotyledons.
Relationship of gymnosperms and angiosperms?
Gymnosperms oldest, diverge first, then basal angiosperms (amborella, nymphaeales, magnoliids), monocots then other dicots and eudicots.
What are the basal angiosperms?
Same leaf venation and 2 cotyledons like eudicots, but pollen has only 1 pore, like monocots.
3% angiosperms.
What are the eudicots?
75% of angiosperms.
Fagaceae (oaks) and asteraceae (daisies).
What are the magnoliaceae?
Many tepals. Many stamens and carpels. All parts arranged in a spiral. Beetle pollinated. Magnolia have many fruits with one seed each - angiosperm flower but gymnosperm-like seeds.
An example of nymphaeales?
Nymphea - water lily.
What are the fagaceae?
Oak and relatives (beech, chestnut). Dominant, temperate, broadleaf trees. Reverted to wind pollination. Catkin inflorescence. Acorn fruit.
What are KCAG?
K = calix = sepals. C = corolla = petals. A = androecium = stamens. G = gynoecium = carpels, () = fused, _ = superior ovary.
What are the asteraceae?
Daisies (dandelions, sunflowers etc.) Inflorescence imitates a single flower (pseudanth), actually only 5 petals per flower. Ancestors similar to gentians. Calix -> pappus. Types of flower: Disc floret - C(5) A(5) Ginferior(2). Ray floret (female). Ligulate floret (hermaphrodite).
What are the monocots?
22% of angiosperms.
Poaceae (grasses) and orchidaceae (orchids).
Evolved from primitive aquatic magnoliids?
What are the poaceae?
Grasses.
Reversion to wind pollination.
KC2 A3 Gsup(2-3).
Spikelet encloses florets.
Florets: palea and lemma (tepals), ovary + stigmas, stamens.
Originated tropical forests, bamboos and rice. Spread (prairies, savannahs) correlated with high crown mammal dentition (herbivore resistance).
Human selection -> cereals.
What are the most diversified groups of angiosperms?
- Orchids - 24000.
- Daisies - 22000.
- Legumes - 17000.
What are the orchidaceae?
Orchids.
Most specialised monocot family.
Primarily tropical epiphytes.
Asymmetrical, extreme fusion of parts (column).
K3 C2+1 [A1-2 Ginf(3)].
Tiny seeds (like dust).
Insect pollination - highly specialised, eg bee mimicry.
How does fertilisation take place?
Pollen microspores -> microgametophytes and sperm.
Ovule w. megaspores -> megagametophyte (embryo sac) and egg.
Pollen tube delivers sperm to egg inside ovule.
What are the stages of embryo development?
Takes place inside ovule.
Stages = shapes:
1. Globular stage.
2. Early heart stage (can see cotyledons).
3. Late heart stage.
4. Mature embryo stage.
After fertilisation -> apical and basal cells.
Apical -> embryo.
Basal -> suspensor.
Embryo has 2 axes: apical/basal and radial ().
Apical/basal pattern of cell division with 2 groups of cells are left undifferentiated - the meristems.
Radial pattern: inside vascular, outside epidermis, cortex between.
Which factors are necessary to form the specific regions of the embryo?
Apical - gurke (gk).
Central - fäckel (fk).
^^both housekeeping proteins.
Basal - monopterous (mp), auxin responsive transcription factor.
What are roots made of?
Concentric layers of cells.
Central vascular strand - xylem and phloem.
Followed by gate layers - pericycle (with impermeable Casparian strip) and endodermis.
Then cortex and epidermis.
Root cap at end.
Cell-cell communication establishes layers.
What is the root meristem?
Where root development happens.
A stem cell niche (stem cells with defined roles) in contact with the quiescent centre - the organiser.
Plethora is expressed in QC and specifies the QC, division zone and elongation zone (gradient).
Plethora prevents the differentiation of the stem cell niche, which would stop root growth.
What happens if a stem cell around the QC is deleted?
Neighbour divides to replace it - cell communication.
What are scarecrow (scr) and short-root (shr)?
Transcription factors that cause the cortex/endodermis to differentiate.
SHR is produced in the stele but moves through plasmodesmata to cause the differentiation of the endodermis.
SCR is produced in the endodermis.
How do root hair cells (trichoblasts) differentiate?
Result from intercellular signals.
Cells in contact with 2 cortex cells -> RHCs.
Non-hair cells only in contact with 1.
Hair is default - GL2 gene determines non-hair, induced by Werewolf (product doesn’t move).
Werewolf also induces a gene whose product CPC is a repressor of GL2 (CPC can move to other cells to inhibit GL2, resulting in RHCs).
How do lateral roots form?
By division of pericycle cells, controlled by auxin (initiates growth).
Daughters organised into new root meristem.
Division controlled by mineral nutrient status - lateral roots detect nitrogen.
What is the shoot apical meristem?
Remains the same size throughout life of plant.
A pool of undifferentiated stem cells which separates during embryogenesis.
Produce differentiated organs and self-perpetuate.
Three zones:
Central zone - initial cells, slow proliferation, true stem cells, perpetuation.
Peripheral zone - rapid proliferation, leaf primordia differentiate.
Rib zone - vasculature/stem formation.
Three layers:
Tunica:
L1 - epidermis.
L2 - ground tissue, leaf, outer photosynthetic cells.
Corpus:
L3 - vascular tissue, leaf/stem.
What determines how cells in the SAM differentiate?
Their position.
However, most of shoot derived from few cells, each with multiple possible fates.
What is WUSCHEL?
Maintains initials, prevents differentiation.
Location of WUS expression (outside of central zone) acts as organiser, activates stem cells in central zone (initials).
A transcription factor - moves to the stem cells and binds to the CLV3 promotors to activate transcription. This limits WUS expression - autoregulatory loop.
What is Clavata1?
Antagonises WUS.
Promotes differentiation of initials.
Encodes leucine-rich repeat (LRR) transmembrane protein, acts as receptor in interaction with CLV2.
Also a kinase signalling protein.
What is Clavata3?
Expressed in initials.
Signals to limit WUS expression and therefore size of central zone.
Ligand for the CLV1/2 receptor.
What is WOX5?
Wuschel-like homeobox 5. A gene similar to Wuschel but signals cells around the root QC not to differentiate.
Signals to prevent stem cell differentiation conserved between root and shoot meristems.
What is stm?
Shoot meristem-less.
Required to prevent differentiation.
Lethal mutant - stem cells in embryonic shoot meristem differentiate, meristem lost.
What are the KNOX genes?
Knotted homeobox class of genes - STM and related genes, which prevent differentiation into leaves. Expressed in meristem central zone.
What are ARP genes?
Stop (repress) KNOX expression in the leaf primordia, allowing cells to differentiate to form leaves and initiate the SAM (Phan/Phab) to make the shoot.
Ectopic PHABULOSA converts roots to shoots and extra in embryo causes more WUS.
What is the difference between shoot growth and leaf growth?
Shoot growth is indeterminate - no limit (KNOX)
Leaf growth is determinate - predefined (ARP).
How do compound leaves form?
Extra KNOX.
Leaf primordia cells remain partially indeterminate.
What is TOPLESS?
Makes the SAM shoot. Represses PLETHORA (root).
Why is the shoot modular?
Organ production is sequential.
Leaf primordia are regularly spaced (phyllotaxy), with new primordia arising at the maximum possible distance from previous ones - lateral inhibition.
How does lateral inhibition take place?
Withdrawal of an activator - auxin.
Auxin causes differentiation of vascular tissue, which drains it - auxin maxima -> primordia, and withdrawal of auxin -> differentiation.
Local application of auxin generates new phyllotaxy.
What are the cell layers in a leaf?
Upper epidermis, palisade mesophyll, spongy mesophyll and lower epidermis (guard cells and stomata).
Mesophyll has vascular bundles - xylem, phloem and bundle sheath.
Each cell type differentiates individually - determined by genetic switches.
What are trichomes?
Giant single cells. Protrude from epidermis.
Trichome development activated by transcription factors (without them leaves are glabrous (bald)).
How are guard cells formed?
Epidermal cells specialise to become guard mother cells (GMCs), which then divide asymmetrically.
Guard cells inhibit the formation of others in the area (lateral inhibition), creating spacing.
What type of polarity do leaves have?
Dorsoventral (upper/lower, adaxial/abaxial).
Phan/Phab (orthologues) required for dorsal/adaxial fate - expressed in adaxial side of primordium (closer to SAM).
Without adaxial side, get needle-like leaves, abaxial all around.
What determines root vs shoot?
Root = Plethora. Shoot = Phan/Phab.
How are flowers produced?
Switch from vegetative to inflorescence meristem to change the type of lateral organ produced.
Involves gene Leafy - extra = advanced flowering.
How are the different floral organs made?
One or two classes of homeotic genes required to specify different whorls.
ABC model:
A - sterile sporophyll (Apetala1/2).
B - male/micro sporophyll (Apetala3/Pistillata).
C - fertile sporophyll (Agamous).
Sepals = A.
Petals = A+B.
Stamens = B+C.
Carpels = C.
Agents of Leafy - necessary and sufficient to determine each whorl.
Agamous also turns of floral meristem activity by terminating WUS (central zone).
What would happen in an ABC triple mutant?
Floral organs -> leaves.