○ Double fertilization: Pollen tube delivers two sperm cells, one fertilizes the egg cell to become the zygote (2n), the other fuses with two polar nuclei to form the endosperm (3n)

③ No nutrient provisioning until the egg cell is fertilized

④ Assisted by animals, wind, water, etc., for fertilization

⑶ Comparison between Algae and Plants

① Habitat: Algae inhabit water, plants inhabit land

② Roots: Algae have rhizoids, plants have roots

③ Stems: Algae supported by water buoyancy, plants supported by stems

④ Leaves: Algae photosynthesize throughout the body, plants mainly in leaves

⑷ Classification of Plant Kingdom

Classification	Differentiation of Root, Stem, Leaf	Presence of Vascular Tissue	Reproduction Method	Presence of Flowers	Double Fertilization
Angiosperms	Differentiated (root)	Xylem	Seed	Flowering plants	Present
Gymnosperms	Differentiated (root)	Tracheid	Seed	Flowering plants	Absent
Ferns	Differentiated (root)	Tracheid	Spore	Non-flowering plants	Absent
Mosses (Bryophyta)	Not differentiated (rhizoids)	None	Spore	Non-flowering plants	Absent

Table 1. Classification of Plant Kingdom

① Organs responsible for root function

○ Rhizoids: Simply for attachment to the substrate, minimal water-absorbing capacity

○ Roots: Penetrate into the soil for plant anchorage, absorb water and nutrients

② Organs responsible for conducting water

○ Tracheids: Passageways for water movement, composed of dead cells, less evolved than xylem

○ Xylem: Passageways for water movement, composed of dead cells, more evolved than tracheids

○ Phloem: Passageways for transporting organic substances synthesized in leaves, composed of living cells

○ Vascular plant: Possesses xylem, phloem, and embryo. All vascular plants have vascular bundles

③ Reproduction methods

○ Spores: Formed through mitotic cell division, haploid (n), no nutrient content

○ Seeds: Formed through mitotic cell division like spores, diploid (2n), contain nutrients

④ Presence of seed coat and double fertilization: Seed coat encloses the embryo (zygote) to protect it

○ Gymnosperms: No seed coat, no double fertilization

○ Angiosperms: Seed coat present, double fertilization present

○ Gymnosperms: Embryo is naked

○ Angiosperms: Embryo is enclosed by the seed coat

⑸ Classification 1. Bryophytes

① Intermediate stage between aquatic and terrestrial life

② Primarily found in moist habitats

③ Lacks vascular bundles but possesses rosette-shaped cellulose synthesizing enzymes

④ Types: Mosses (hair cap mosses, liverworts)

⑹ Classification 2. Ferns and Fern Allies

① Some have roots, some have rhizoids in their life cycle

② Types: Bracken ferns, lycophythes (sometimes considered separate from ferns)

⑺ Classification 3. Seed Plants: Collective term for gymnosperms and angiosperms

	Gymnosperms	Angiosperms
Presence of Seed Coat	No seed coat (seeds are exposed)	Seed coat present
Vascular Tissue	Tracheid + Phloem	Xylem + Phloem
Presence of Cambium	Present (volumetric growth possible)	See below
Examples	Pine tree, Ginkgo tree	See below

Table 2. Comparison of Gymnosperms and Angiosperms

① Gymnosperms (Naked Seed Plants)

② Angiosperms (Enclosed Seed Plants)

○ Cotyledons are a unique trait of angiosperms

○ Angiosperms further classified into monocots and dicots

	Monocotyledons	Dicotyledons
Cambium	Absent (longitudinal growth only)	Present (volumetric growth possible)
Number of Cotyledons	1 leaf	2 leaves
Leaf Venation	Parallel venation	Reticulate venation
Vascular Bundle	Scattered vascular bundles	Ring-shaped vascular bundles
Root	Fibrous root	Taproot system (primary + lateral roots)
Number of Holes in Pollen	1 hole	3 holes
Flower Part Multiples	Multiples of 3	Multiples of 4 or 5
Examples	Barley, Rice, Wheat, Corn	Hibiscus, Rose, Chrysanthemum, Sunflower

Table 3. Comparison of Monocots and Dicots

2. Common Structure of Plants

⑴ Levels of plant organization

Cells → Tissues → Tissue systems → Organs → Organisms

⑵ Cellulose Synthesis

① H+-ATPase and cellulose synthase are present in plant cell membranes

② Orientation of microtubules attached to the inner cell membrane determines the direction of cellulose fiber formation outside the cell membrane

③ Cellulose microfibrils align perpendicular to the direction of cell expansion (longitudinal axis)

⑶ Growth stages

① Primary growth: Young plant parts develop leaves, stem and root lengths increase

○ Primary cell wall: Young plant cells have thin and easily deformable primary cell walls

○ Middle lamella: Composed of pectin, connecting adjacent primary cell walls

② Secondary growth: Stems and roots thicken in older regions

○ Secondary cell wall: Composed of primary cell wall, lignin, suberin

○ Not applicable to sclerenchyma fiber cells, xylem water-conducting cells, and phloem sieve tubes.

○ Lignin: Deposited in secondary cell wall, making it rigid.

③ Secondary growth in trees

○ Heartwood: The inner layers of the secondary xylem. They no longer function in water conduction, and the tree can survive without this part. It typically appears dark-colored due to the presence of resins or other compounds that help prevent the invasion of fungi and insects.

○ Sapwood: The outermost layers of the secondary xylem, which are the most recently formed. These layers are actively involved in water conduction.

3. Plant Structure: Leaves

⑴ Three primary tissues: Epidermis, ground tissue, vascular tissue

⑵ Epidermal tissue: Equivalent to animal epithelial tissue

① Cuticle layer: Wax layer (cutin), prevents water loss, prevents microbial invasion

② Epidermal cells

○ Guard cells are a type of epidermal cells

○ Except for guard cells, epidermal cells have no chloroplasts.

③ Companion cells: Related to stomata

○ The higher the K+ and Cl- content inside guard cells, and the higher the turgor pressure of guard cells, the larger the size of stomata.

○ Among the epidermal cells of chloroplasts, only guard cells have chloroplasts.

⑶ Ground Tissue: Refers to mesophyll tissue responsible for photosynthesis

① Mesophyll tissue: Tissue composed of specialized parenchyma cells for photosynthesis, divided into palisade mesophyll and spongy mesophyll

② Palisade mesophyll tissue: Tissue with densely packed mesophyll cells

③ Spongy mesophyll tissue: Tissue with loosely arranged mesophyll cells

○ Example: Vascular bundle sheath cells: Surround the vascular bundle

④ More developed in shade plants.

⑷ Vascular Bundle Tissue: Refers to the vascular bundle of leaves

① Surrounded by vascular bundle sheath cells.

② Upper side is the xylem and lower side is the phloem

③ Dicots have a reticulate venation, while monocots have parallel venation

⑸ Leaf Structure of C3 and C4 Plants

Figure. 1. Leaf structure of C3 and C4 plants

① Leaf structure of C3 plants shows distinct separation between palisade mesophyll and spongy mesophyll.

② Leaf structure of C4 plants does not show a distinct separation between palisade mesophyll and spongy mesophyll.

4. Plant Structure: Stem

⑴ Three Tissues: Epidermis, Ground Tissue, Vascular Bundle Tissue

⑵ Epidermis Tissue: Epidermis

⑶ Ground Tissue

① Cortex

② Pith: Mainly composed of parenchyma cells

⑷ Vascular Bundle Tissue

① Xylem

○ Composed of dead cells

○ Adjacent to the pith

○ Pit: The part of a cell that possesses only a primary cell wall, despite the presence of a secondary cell wall in other regions of the same cell.

② Phloem: Adjacent to the cortex

○ Phloem in angiosperms: Composed of sieve elements, companion cells, sieve plates, and sieve pores.

○ Sieve elements: Living cells, but they lack organelles such as a nucleus, ribosomes, and vacuoles.

○ Facilitates the smooth flow of phloem sap.

○ Companion cells: Connected to sieve elements via plasmodesmata; responsible for the functions of the nucleus and most organelles on behalf of the sieve elements.

○ Sieve plates are located at the ends of the sieve elements.

③ Monocots have a bundle sheath surrounding the veins, while dicots have scattered bundle sheaths

④ In each vascular bundle, the phloem is located more externally than the xylem

⑸ Meristematic Tissue

① Apical meristem

② Axillary meristem

③ Lateral meristem: Vascular cambium + Cork cambium

⑹ Cross-section of a Stem

① Bark: All tissues outside the vascular cambium, including secondary phloem, cortex, and periderm

② Periderm: Composed of cork and cork cambium

③ Dicot stem structure (from center outward): Pith – Xylem – Phloem – Sclerenchyma (fibers) – Cortex – Epidermis

⑺ Cross-section of a Tree Trunk

① Secondary xylem: Includes heartwood and sapwood; forms growth rings through early wood (spring wood) and late wood (autumn wood)

② Vascular cambium

③ Bark: Composed of secondary phloem, cortex, and periderm

④ Periderm: Composed of phelloderm, cork cambium, and cork

5. Plant Structure: Root

⑴ Three Tissues: Epidermis, Ground Tissue, Vascular Bundle Tissue

⑵ Epidermis Tissue

① Root hairs: Absorb water and minerals, exchange cations

⑶ Ground Tissue: Cortex

① Root cap

○ Observed in leguminous plants

○ Rhizobium: An aerobic bacterium; its nitrogen-fixing enzyme is sensitive to oxygen.

○ Symbiosis: Plants provide nutrients to Rhizobium, which supplies nitrogen compounds to plants

○ 1^st. Substances secreted by Rhizobium induce root hair curling

○ 2^nd. Rhizobium secretes enzymes to degrade cell walls

○ 3^rd. Rhizobium penetrates into the root hair

○ 4^th. A vesicle containing Rhizobium migrates through the cortical and endodermal cells and then differentiates into a bacteroid.

○ 5^th. Infected cortex and endodermal cells continue to grow, forming root nodule

○ 6^th. Decreased oxygen concentration occurs

○ 6^th - 1^st. The formation of a lignin-rich sclerenchyma cell layer limits the diffusion of oxygen.

○ 6^th - 2^nd. Leghemoglobin (iron-containing) binds oxygen, leading to oxygen depletion

○ 7^th. Bacteroids in the root nodule fix nitrogen using its nitrogen-fixing enzyme.

○ 8^th. Nitrogen fixation: N₂ → NH₃ → NH₄⁺, requires 16 ATP

② Pericycle: The tissue from which lateral roots originate.

○ Generates lateral meristematic tissue to thicken the root

○ Transports nutrients and ions to the vascular bundle cells

③ Endodermis: The innermost cell layer of the cortex.

○ Unlike other cortex cells, it has suberin in the cell walls, restricting water movement

⑷ Vascular Bundle Tissue

① Stele: Composed of the pericycle, xylem, and phloem

② Xylem vascular bundle

○ Dicotyledons: The xylem is arranged in a cross shape, and the phloem is located between the arms of the xylem.

○ Monocotyledons: The xylem forms multiple ring-shaped cylinders surrounding the pith, with the phloem located between the xylem bundles.

○ Pits are observed.

⑸ Transverse section of the root

① Pith

② Xylem Vessels

③ Phloem Vessels

④ Pericycle

⑤ Casparian Strip Present in the Endodermis

⑥ Cortex

⑦ Epidermis

⑹ Longitudinal section of the root

① Epidermis

② Differentiation Zone (Maturation Zone): Root hairs are observed.

③ Elongation Zone

④ Division Zone: Apical meristem

⑤ Root Cap

6. Plant Physiology

⑴ Overview

① Material Transport Pathways

○ Apoplast Pathway: Pathway along the apoplast

○ Apoplast: The cell wall located outside the plasma membrane, the extracellular space, and the interior of dead cells such as tracheids and xylems.

○ Due to the need to cross the endodermal cell membrane through secondary active transport that relies on the H⁺ gradient, energy is required.

○ Symplast Pathway: Pathway through the symplast

○ Symplast: A continuous cytoplasmic network connecting all living cells in a plant through plasmodesmata.

○ Material transport without the use of energy

○ Example: The internal space of phloem sieve elements, and plasmodesmata.

○ Transmembrane Pathway

② Water Potential

○ It is important when explaining the mechanism of water transport from roots to leaves.

○ Water moves from areas of high water potential to areas of low water potential.

○ Important for explaining the mechanism of water movement from roots to leaves

○ Water potential = Solute potential + Pressure potential

○ Solute potential: π = -CRT (C: Molar concentration), related to osmotic pressure

○ Pressure potential: Turgor pressure, pressure exerted by cell walls on cells

○ Gradient of water potential: Soil > Root epidermis > Root cortex > Root stele > Stem > Leaf > Atmosphere

○ Primary driving force: Cohesion-Tension Theory

○ Cohesion-Tension Theory: Transpiration at leaves leads to water molecules with strong cohesion being pulled upward due to evaporation

○ Secondary driving force: Root Pressure

③ Phloem Loading and Transport

○ Source of sugar: Tissues where photosynthesis and starch breakdown lead to net production of sugars (e.g., leaves)

○ At the source region, the phloem has a lower osmotic pressure compared to the xylem.

○ Sink of sugar: Tissues that consume or store sugars, such as division tissues (sugar consumption), roots (sugar storage and consumption), and fruits (sugar storage).

○ Sugars are stored in the form of starch, but transported in the form of sucrose.

○ Reason: Sucroses are non-reducing sugars

○ Types of sugar transporters: Monosaccharide (MST), sucrose (SUT), hexose (SWEET)

○ Pressure Flow Model: Water is pushed into the phloem sieve tube elements of source tissues and then sap moves to sink tissues due to pressure differences

○ Bulk flow: Movement of solution driven by pressure potential

○ Sink tissues have higher osmotic pressure, so sap is drawn in

○ Source tissues have lower osmotic pressure to allow sap to be expelled

○ Sugar concentration is higher in source tissues

○ Sucrose can also be delivered to leaves on the opposite side of the source through phloem cross-transport.

⑵ Root Material Transport

① Casparian Strip

○ Impermeable layer made of wax called suberin.

○ To enter the stele of the root, water must pass through the symplast of the endodermis.

○ Prevents unnecessary or toxic substances from entering

○ Prevents leakage of substances stored in the xylem into the soil

② Cation Exchange: Root hairs actively transport H⁺ into the soil, allowing absorption of mineral cations

○ Electric gradient leads to separation of mineral cations from soil particles, facilitating absorption

○ H⁺-ATPase is used during active transport of H⁺.

○ H⁺ co-transporter is used during mineral cation absorption into root.

○ H⁺ re-enters the cell along its concentration gradient, leading to increased cell membrane potential, compensated by anion uptake

③ Nitrate Transporter

○ High Affinity Transporter (HAT): Functions regardless of nitrate concentration

○ Low Affinity Transporter (LAT): Leads to rapid membrane depolarization (nitrate uptake) if nitrate concentration is high

○ Cycloheximide: Inhibitor of low affinity transporters

⑶ Stem Material Transport

① Movement of sap occurs due to bulk flow

② Main component of sap: Sugars

⑷ Leaf Material Transport

① Stoma (stomate, pore)

○ Gap between two specialized epidermal cells (guard cells)

○ Pathway for water and carbon dioxide

② Stomatal Opening and Closure

○ Cellulose microfibers radiate from guard cell walls

○ Increase in guard cell turgor pressure leads to guard cells expanding radially → stomatal opening

③ Mechanism of Stomatal Opening

○ 1^st. Blue Light Signal

○ 1^st - 1^st. Blue light → Blue light receptor (Zeaxanthin)

○ 1^st - 2^nd. Signal transmission by the blue light receptor

○ 1^st - 3^rd. Proton pumps in guard cells export H+ outside the cell through the plasma membrane

○ 1^st - 4^th. H⁺ and K⁺ enter the cell through cotransport along the concentration gradient of H⁺

○ 1^st - 5^th. Cl^- enters the cell to balance charge

○ 1^st - 6^th. Increase in osmotic concentration in guard cells

○ 1^st - 7^th. Increase in water entry into guard cells

○ 1^st - 8^th. Increase in guard cell turgor pressure

○ 1^st - 9^th. Guard cells swell, leading to stomatal opening

○ 2^nd. Red Light Signal

○ 2^nd - 1^st. Increased photosynthesis rate leads to increased sucrose production and osmotic concentration → 1^st - 6^th

○ 2^nd - 2^nd. Decreased CO₂ pressure leads to K⁺ and Cl^- uptake → 1^st - 4^th

○ Every CO₂ molecule results in the evaporation of 40 water molecules

○ Blue light response is faster than red light response

④ Mechanism of Stomatal Closure

○ Evening: Decrease in blue light, decrease in sugar → reverse mechanism of stomatal opening → water loss from guard cells → stomatal closure

○ Water stress

○ 1^st. Increased ABA: Cells with chloroplasts produce ABA, mainly transported by phloem. Increases with pH.

○ 2^nd. Without water stress: ABA-H+ form → uptake by mesophyll cells → reduced accessibility to guard cells → maintained stomatal opening

○ 3^rd. ABA^- reaches guard cells → signal transduction pathway

○ 4^th. K⁺, Cl^-, and water are expelled from guard cells

○ 5^th. Stomatal closure

7. Plant Hormones

⑴ Overview

① Function: Governs plant development and responses

② Signal transduction processes

○ Perception

○ Transmission

○ Response: Transcription regulation, post-translational modification of proteins

③ Characteristics

○ Plant hormones are concentration- and tissue-specific, resulting in diverse and unclassifiable functions

○ Cell growth by auxins and cytokinins only applies to dicotyledonous plants

⑵ Auxin (Indole-3-Acetic Acid, IAA)

① Formation of auxin by transformation of tryptophan

② Mechanism of action: Auxin → Binding to ABP 1 → Gene transcription promotion → Increased physiological activity

③ Effects

○ Stem elongation: Promotion of stem elongation at low concentrations, inhibition at high concentrations

○ Promotion of lateral root formation

○ Delayed leaf abscission

○ Apical dominance: Inhibition of lateral bud growth

○ Synthetic auxin: Selective removal of dicot weeds embedded among monocot crops or grasses

○ Fruit development regulation: Fruit growth after pollination is controlled by auxin, inducing unit fruit formation.

④ Polar Transport

○ Auxin moves only from the stem apex to the base

○ Movement towards the root, not gravity-related

⑤ Redistribution Hypothesis

○ Auxin moves from the side illuminated by light to the shaded side

○ Asymmetric elongation induced on the shaded side, causing bending towards the light

⑥ Acid Growth Hypothesis

○ 1^st. Auxin increases H+ pump activity

○ 2^nd. Decreases cell wall pH

○ 3^rd. Activation of expansin proteins due to low pH

○ 4^th. Expansins cleave cross-linking glycans, loosening the cell wall

○ 5^th. Cell expansion due to turgor pressure

⑦ Characteristics

○ Growth-promoting concentration varies with organ: Roots < Buds < Stems

Figure. 2. Increase in root and stem length according to auxin concentration

○ Polar transport (inside the cell): Passive diffusion or secondary active transport

○ Polar transport (outside the cell): Passive proteins located only below. Requires ionization within cells for transport protein.

⑶ Cytokinin

① Composed of zeatin, isopentenyl adenine, kinetin, etc.

② Synthesized in root apex meristem, transported upward through the xylem

③ Pyrimidine derivatives

④ Effects

○ Regulation of cell division and differentiation

○ Auxin and cytokinin act antagonistically

○ High cytokinin levels in callus → Shoots

○ High auxin levels in callus → Roots

○ Axillary bud growth promotion: Apical dominance

○ Delay of leaf senescence (prevents abscission): Inhibits protein degradation, and promotes RNA and protein synthesis

○ Prolongs shelf life of vegetables (mainly leaves) and flowers

○ Increases during fruit growth, and decreases as fruit matures

⑷ Gibberellin (GA)

① Produced by Gibberella fungi, isoprenoid compounds

② Signal transduction process

○ GID1: Gibberellin receptor

○ TF: Transcription factor that promotes transcription of gibberellin-responsive genes

○ DELLA: Inhibits TF

③ Function 1. Promotion of germination

○ 1^st. Adequate external conditions generate germination signals in seeds

○ 2^nd. Metabolism, water absorption → Decreased ABA → Gibberellin synthesis in embryo

○ 3^rd. Gibberellin reaches aleurone layer through endosperm

○ 4^th. Gibberellin stimulates synthesis of α-amylase in aleurone

○ 5^th. α-amylase is released into endosperm, hydrolyzing starch

④ Function 2. Promotion of stem elongation

⑤ Function 3. Seedless grapes → Enhanced grape growth

⑥ Function 4. Induces bolting in biennial plants.

⑸ Brassinosteroids

① Compounds derived from triterpenoids extracted from mustard family plants

② Exhibit function even in small amounts

③ Effects

○ Inhibits root growth

○ Delays leaf abscission

○ Promotes xylem vessel formation

○ Hypocotyl elongation in dicot seedlings

⑹ Abscisic Acid (ABA)

① Acts as a stress hormone, aiding in adverse environmental conditions

② Function 1. Induces dormancy in apical meristems for winter buds

③ Function 2. Induces seed dormancy

④ Function 3. Stomatal closure

○ 1^st. When transpiration is excessive due to drought or other factors, abscisic acid secretion increases.

○ 2^nd. Decreased turgor pressure in guard cells results in stomatal closure

Figure. 3. Mechanism of stomatal opening and closing by abscisic acid

⑤ Other functions

○ Inhibits seed germination

○ Suppresses plant defense response by inhibiting SA synthesis

⑺ Ethylene

① Volatile gas synthesized in nodes, fruits, etc.

② Function 1. Triple response to mechanical stress (e.g., obstacles)

○ 1-1. Inhibition of stem elongation

○ 1-2. Thicker stem, increased stem girth

○ 1-3. Increased curvature of the stem

Figure. 4. Comparison of cultivated plants grown in darkness (X) and those treated with ethylene in darkness (Y)

③ Function 2. Ripening of fruits: Ethylene destroys chlorophyll, and promotes cell wall degradation

○ Positive feedback: Activates other ethylene

○ As a gas, ethylene spreads to neighboring fruits through diffusion

○ Bananas: Ethylene turns them from green to yellow to brown

○ Silver thiosulfate is used to extend the shelf life of flowers because it inhibits ethylene.

○ Respiratory climacteric phenomenon: Rapid CO2 emission (ethylene inhibition) during fruit ripening

○ Reduction in starch content as fruit matures

④ Function 3. Apical hook

○ Present in dicot plants, and protects apical meristem tissue until emerging from soil

○ 1^st. Phytochrome acts as a light receptor

○ 2^nd. Pfr, activated by far-red light, inhibits ethylene production

○ 3^rd. Induces apical hook opening

○ 3^rd - 1^st. Outer side receives more red light, leading to stem elongation

○ 3^rd - 2^nd. Inner side becomes shaded, causing ethylene accumulation and stem inhibition

○ 3^rd - 3^rd. Growth discrepancy between inner and outer sides results in hook formation

⑤ Other functions

○ Induces programmed cell death.

○ Promotes leaf abscission (leaf drop).

○ Stimulates the formation of roots and root hairs.

⑥ Biosynthetic Pathway

○ 1^st. Methionine → AdoMet (Catalyst: ACC Synthase Enzyme)

○ 2^nd. AdoMet → ACC (Catalyst: ACC Oxidase Enzyme)

○ 3^rd. ACC → Ethylene: Processing ACC, the substrate of the final step, leads to ethylene production without stimulation

⑦ Signal Transduction Pathway

○ 1^st. Ethylene inhibits ethylene receptors

○ 2^nd. Ethylene receptors promote kinase activity

○ 3^rd. Kinase inhibits EIN2

○ 4^th. EIN2 activates transcription factors

○ 5^th. Transcription factors induce gene expression (triple response)

⑧ Regulation of Activity

○ Thiourea: Ethylene action inhibition by binding with ethylene receptors

○ CO2: Interferes with ethylene accumulation by carbon dioxide circulation.

○ Phosphates: High levels of phosphates inhibit ethylene production

⑻ Photoreceptors

① Red Light Receptor: Phytochrome (Blue)

○ Structure: A quaternary structure composed of two identical polypeptides, each containing two domains: a photoreceptor active site (including a chromophore) and a kinase active site.

○ P_r: Pigment that maximally absorbs red light (660 nm)

○ P_fr: Pigment that maximally absorbs far-red light (730 nm)

○ P_r to P_fr Conversion

○ Phytochrome regulates early plant growth: Phytochrome is in the P_r form when not exposed to light

○ Red light exposure converts Phytochrome from P_r to P_fr

○ Far-red light exposure converts Phytochrome from P_fr to P_r

○ Due to instability, P_fr spontaneously converts to P_r

○ P_fr primarily has biological activity

○ 1^st. P_fr activates G-protein

○ 2^nd. GTP converts to cGMP

○ 3^rd. Ca²⁺ channels open

○ 4^th. Calmodulin binds to Ca²⁺

○ 5^th. Specific gene activation

○ Example: Gibberellin synthesis induction (stimulation of germination), flowering, phototropic responses

○ Circadian Clock and Flowering

○ Critical night length

○ Short-day Plants: Flower when nights are longer than critical night length (P_r dominant)

○ Long-day Plants: Flower when nights are shorter than critical night length (P_fr dominant). More common.

○ Day: P_r to P_fr transition due to more red light than far-red light

○ Night: P_fr spontaneously converts to P_r

○ During night, white flash converts P_r to P_fr, subsequent far-red light eliminates white flash effect

○ Sufficiently long night may prevent photoreversibility

○ Life Cycles and Circadian Rhythms: Synchronize with environmental cues

○ Etiolation

○ After germination, rapid stem elongation and chlorophyll deficiency may happen under dark conditions (e.g., bean sprouts)

○ Decreased Pfr/Pr under dark conditions induces etiolation

② Blue Light Receptor 1. Cryptochrome: Inhibition of Hypocotyl Elongation

○ Signal transduction pathway not yet known

○ Regulates circadian rhythms and flowering in response to daytime length

○ Short stem growth under sufficient light, providing plants with working time

③ Blue Light Receptor 2. Phototropin: Phototropic Response, Stomatal Opening, Chloroplast Movement

○ Signal transduction: PHOT1 undergoes autophosphorylation, signal transduction pathway not yet known

○ Growth towards light of wavelengths necessary for stem and photosynthesis

④ Blue Light Receptor 3. Zeaxanthin: Stomatal Opening Regulation

⑼ Flowering Hormone

① Florigen

_○ Example 1: _Flowering Locus T (FT) protein encoded by FT gene: Transcription factor

○ Example 2: SOC1: Flowering-inducing transcription factor

○ 1^st. Accumulation of CO protein in companion cells of leaves under appropriate light stimulation

○ 2^nd. Accumulated CO protein promotes expression of FT gene

○ 3^rd. FT protein is produced in leaves and transported through the vascular tissue

○ 4^th. Acts in apical meristems or lateral meristems

○ 5^th. Expression of Bud genes

○ 6^th. Expression of ABC genes

○ 7^th. Transition to flower meristem: Induces transition from infinite growth to finite growth

② Flowering Inhibitory Protein: FLC inhibits FT and SOC1

③ Vernalization: Induces flowering by inhibiting flowering inhibitory protein, typically exposed to temperatures of 0-10°C for several weeks

⑽ Defense Proteins

① Specific immune response related to pathogens’ avirulence genes, Avr, and resistance genes, R

○ Avirulence genes (Avr)

○ Resistance genes (R)

○ If a plant lacks the resistance gene corresponding to a specific pathogen’s avirulence gene, it becomes susceptible to disease.

② Process

Figure. 5. SAR Mechanism

○ 1^st. Hypersensitive Response (HR): Defense substances are produced in primary infection sites, causing localized cell death

○ Defense substances: Nitric oxide, hydrogen peroxide, phytoalexins, salicylic acid, etc.

○ Phytoalexin: Has antifungal and antimicrobial functions

○ 2^nd. Accumulation of salicylic acid (SA) leads to conversion to methylsalicylic acid (MeSA)

○ Enzyme: AtBSMT1

○ 3^rd. Methylsalicylic acid diffuses as a gas or is transported via transporter proteins in xylem throughout the plant.

○ 4^th. Methylsalicylic acid converts to salicylic acid in secondary sites slightly distant from the infection site

○ Enzyme: MSE

○ WRKY11: Transcription factor required for salicylic acid synthesis during hypersensitive response

○ WRKY11 binds to Swe1 protein within the nucleus

○ Upon HR signal, Swe1 is phosphorylated and translocates to the cytoplasm for degradation

○ 5^th. Salicylic acid induces production of pathogen defense proteins (e.g., PR proteins)

○ PR proteins (pathogenesis-related proteins) (e.g., PR1)

○ 6^th. Systemic Acquired Resistance (SAR): Plant develops resistance throughout its system

⑾ Response to Plant Stimuli

① Gravity (Gravitropism)

○ Positive (Roots), Negative (Stems)

○ 1^st. Starch masses (statoliths) distribute in the gravity direction

○ 2^nd. The movement of amyloplasts acts as a stimulus, triggering the release of Ca²⁺ from the smooth endoplasmic reticulum.

○ 3^rd. Calcium ions bind to calmodulin, initiating signal transduction

○ 4^th. Auxin concentration increases in gravity direction

○ 5^th. Growth regulation (roots positive, stems negative)

② Mechanical Stimuli

○ Thigmotropism

○ Trees on windy mountaintops: Short and stout growth due to strong wind

○ Tendril formation

○ Mimosa Pudica folding upon touch: Stimulus → Potassium release → Water expulsion → Reduced turgor pressure

③ Response to Various Environmental Stimuli

○ Drought: Increased Abscisic Acid (ABA)

○ Inhibits young leaf growth: Reduces leaf surface area due to desiccation.

○ Inhibits surface root growth, and promotes deeper root growth

○ Flooding: Decreased oxygen in soil → Increased ethylene → Apoptosis of root epidermis → Movement to dead space

○ Salinity: Synthesis of organic solutes, some plants develop salt glands

○ High Temperature: Closure of stomata to conserve moisture. Increased heat shock proteins function as chaperones.

○ Low Temperature: Increased unsaturated fatty acids, accumulation of substances like sugars in cytoplasm, synthesis of antifreeze proteins

8. Sexual Reproduction

⑴ Overview

① Only the sexual reproduction of angiosperms is covered here.

② This applies to both dicotyledons and monocotyledons.

⑵ Flowers

① Structure of Flowers

○ Sepals: Protect flower buds; photosynthesis

○ Petals

○ Stamen: Consists of filament and anther

○ Carpel (Pistil): Made up of stigma, style, and ovary

② ABC Hypothesis: Model of floral organ development. Organ identity genes are involved.

Figure. 6. ABC Hypothesis

○ Homeotic genes A, B, and C act individually or in combination to determine the positions of the corresponding floral organs.

○ Interaction of A and B genes results in petal formation at position II

○ Interaction of C and B genes results in stamen formation at position III

○ A gene inhibits C gene action at positions I and II, and C gene inhibits A gene action at positions III and IV

⑶ Formation of Gametophytes

① Formation of male gametophyte

○ 1^st. Microsporocytes (2n) undergo meiosis to produce 4 microspores (n)

○ 2^nd. Each microspore undergoes mitosis to produce a generative cell (n) and a tube cell (n)

○ 3^rd. Pollen grain = Generative cell (n) × 1 + Tube cell (n) × 1

○ Tube cell: Involved in pollen tube growth

○ During fertilization, generative cell undergoes mitosis to become 2 cells

② Formation of female gametophyte

○ 1^st. Megaspore mother cell (2n) undergoes meiosis to produce 4 megaspores (n)

○ 2^nd. Only 1 of the 4 megaspores survives

○ 3^rd. Surviving megaspore undergoes 3 rounds of nuclear division without cytoplasm division to form an embryo sac with 8 nuclei

○ 4^th. Female gametophyte = Egg cell + Synergids × 2 + Polar Nuclei × 2 + Antipodal cells × 3

⑷ Pollination

① Anemophily: Petals, nectar absent; abundant pollen, sticky or feathery stigmas

② Entomophily: Pollination by bees, moths & butterflies, birds, flies, bats; various floral adaptations

③ Hydrophily

⑸ Self-Incompatibility: Rejection of pollen from one’s own flowers

① Sporophytic Self-Incompatibility

○ The S allele of the sporophyte that produced the pollen is important.

○ When S1 pollen from a sporophyte with (S1, S2) lands on a pistil with (S2, S3), the pollen tube fails to germinate because the S1 pollen was produced by a sporophyte carrying the S2 allele.

② Gametophytic Self-Incompatibility

○ S-alleles of pollen tube is crucial

○ When pollen grains with S1 and S2 pollen tubes, produced by a sporophyte with (S1, S2), land on a pistil with (S2, S3), the pollen with the S1 pollen tube is able to germinate, while the pollen with the S2 pollen tube fails to germ.

③ Prevention of Self-Pollination by Flower Morphology

○ Example: Different flower types: Pin and thrum flowers

④ Pollen from the same species binds weakly to the stigma through cell-to-cell signaling mediated by the cell wall.

⑹ Fertilization

① Process

○ 1^st. Pollen consists of generative cell (n) and tube cell (n)

○ 2^nd. Pollen lands on stigma, absorbs moisture, germinates

○ 3^rd. Pollen tube grows through style, elongates into ovary, reaches ovule

○ 4^th. Generative cell in pollen tube undergoes mitosis to become two generative cells

○ 5^th. Chemicals secreted by the synergids attract pollen tube, guiding it to grow toward the embryo sac.

○ 6^th. When the pollen tube reaches the embryo sac, one of the synergid cells degenerates, providing a pathway into the ovule.

○ 7^th. Two generative cells move from pollen tube to ovule

○ 8^th. One generative cell fertilizes egg cell, forming zygote

○ 9^th. The oher generative cell fuses with two polar nuclei, forming triploid (3n).

② Double Fertilization: Angiosperms

○ Generative cell (n) + Egg cell (n) → Embryo (2n)

○ Generative cell (n) + Polar nuclei (n) × 2 → Endosperm (3n)

③ Single Fertilization: Gymnosperms

○ Generative cell (n) + Egg cell (n) → Embryo (2n)

○ The primary endosperm cell (i.e., the female haploid, n) develops without fertilization, forming a haploid endosperm (n).

⑺ Seed Development

① Dicotyledonous Plants

○ Composed of the embryo, endosperm, cotyledon, and seed coat

○ Embryo = hypocotyl + radicle + epicotyl + plumule

○ Seeds primarily develop from the ovule

② Monocotyledonous Plants

○ Composed of the embryo, endosperm, scutellum (cotyledon), and seed coat/pericarp

○ Embryo = plumule + coleoptile + radicle + coleorhiza

○ Seeds primarily develop from the endosperm

⑻ Seed: A zygote that has developed from the fertilized ovule

① Embryogenesis

○ Of the two daughter cells formed through mitosis, one develops into the embryo, while the other becomes the suspensor, a supporting structure.

○ Development of endosperm: Initial endosperm nucleus undergoes successive nuclear divisions to enclose the zygote, then forms cell walls

○ Embryo Development

○ 1^st. Adequate endosperm development

○ 2^nd. Development of zygote to embryo

○ 3^rd. Inner cells form rounded embryo

○ 4^th. Outer cells form suspensor

○ 5^th. Lower cells of suspensor absorb water, expanding to become basal cells

○ Growth of cotyledons (in dicotyledons): The globular embryo develops into a heart-shaped form → the two lobes of the heart shape develop into cotyledons (in monocotyledons, a single cotyledon forms through unilateral growth) → the cotyledons absorb nutrients from the endosperm, leading to the torpedo stage → at the point where the cotyledons separate, the shoot apical meristem is formed → it develops into the epicotyl.

○ Hypocotyl (e.g., root apical meristem)

② Initiation of Three Primary Meristems

○ Protoderm: Outer surface of embryo → Develops into epidermis

○ Procambium: Central region of the embryonic axis → Primary phloem and xylem

○ Ground meristem: Located between the protoderm and the procambium

⑼ Fruit: A fertilized ovule developed from the ovary (e.g., corn, cereals)

① Simple Fruit: Develops from one or many fused carpels.

② Aggregate Fruit: Develops from separate carpels in a single flower.

③ Multiple Fruit: Develops from a cluster of flowers.

④ Accessory Fruit: Develops from structures other than the ovary.

⑽ Germination and Plant Propagation

9. Asexual Reproduction

⑴ Vegetative Propagation

① Cuttings: From transplanted branches

② Adventitious buds

③ Leaf propagation

④ Runner/stolon: In potatoes, tubers develop as reproductive organs.

⑤ Tuber

⑥ Rhizome: In sweet potatoes, the primary root develops into a storage root.

⑦ Bulbs and corms

⑵ Apomixis: 100% inheritance of genes to offspring

① Megaspore mother cell directly forms diploid embryo without undergoing meiosis

② Apomixis-capable species select apomixis pathway when favorable, but main evolutionary path remains sexual

⑶ Applications of Apomixis

① Grafting

② Tissue Culture

③ DNA Recombination

④ Cell Fusion

Input: 2015.07.XX XX:XX

Modification: 2020.02.22 23:25

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Chapter 25. Botany

1. Plant Kingdom

2. Common Structure of Plants

3. Plant Structure: Leaves

4. Plant Structure: Stem

5. Plant Structure: Root

6. Plant Physiology

7. Plant Hormones

8. Sexual Reproduction

9. Asexual Reproduction

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