○ 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

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

○ Rhizoids : Passageways for water movement, composed of dead cells, less evolved than water-conducting tissue

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

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

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

③ Reproduction methods

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

○ Seeds : Formed through meiotic 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 (sphagnum, peat moss), liverworts

⑹ Classification 2. Ferns and Fern Allies

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

② Types : Ferns, horsetails (sometimes considered separate from ferns)

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

Table. 2. Comparison of Gymnosperms and Angiosperms

① Gymnosperms (Naked Seed Plants)

② Angiosperms (Enclosed Seed Plants)

○ Monocots are a unique trait of angiosperms

○ Angiosperms further classified into monocots and dicots

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

○ Periderm cells and water-conducting cells in secondary phloem, no cortex cells

○ Lignin : Depos

ited in secondary cell wall, makes it rigid

③ Secondary growth in trees

○ Heartwood : Inner layer of the secondarily thickened water-conducting cells, no longer functional, can be absent without affecting tree’s life, prevents invasion by fungi or insects through resins or other compounds

○ Sapwood : Outermost layer of the secondarily thickened water-conducting cells, functional water-conducting cells

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 companion cells, epidermal cells have no chloroplasts.

③ Companion cells : Related to plasmodesmata

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

○ 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 Non-vascular plants are more advanced

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

① Vascular bundle sheath cells surround it

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

③ Monocots have a reticulate venation, while dicots 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 : Part of cells with only primary pits in cells that have secondary cell walls

② Phloem : Adjacent to the cortex

○ Phloem in angiosperms : Companion cells, sieve elements, sieve plates, and sieve plate pores

○ Companion cells : Living cells but lack organelles like nucleus, ribosomes, and vacuole

○ Facilitates the movement of phloem sap

○ Sieve elements are located at the ends of companion cells

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 Transverse Section of Stem

① Secondary xylem : Develops spring wood and summer wood, forming growth rings

② Vascular bundle formation layer

③ Xylem

④ Phloem : Cork cambium, cork cambium formation layer, cork tissue

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 (Refer above)

⑶ Ground Tissue : Cortex

① Root cap

○ Observed in leguminous plants

○ Rhizobium : Nitrogen-fixing bacteria, susceptible to oxygen due to nitrogenase enzyme

○ 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. Rhizobia move to the cortex as infection threads, developing bacteroids in the peribacteroid membrane

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

○ 6^th. Decreased oxygen concentration occurs

○ 6^th - 1^st. Formation of lignin-rich casparian strip restricts oxygen movement

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

○ 7^th. Bacteroids in the nodule fix nitrogen using nitrogenase enzyme

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

② Endodermis : Tissue where lateral roots (lateral meristems) originate

○ Generates lateral meristematic tissue to thicken the root

○ Transports nutrients and ions to the vascular bundle cells

③ Pericycle : Innermost 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 pericycle, endodermis, and vascular bundle

② Vascular bundle

○ Dicotyledonous Plants : Xylem vessels are cross-shaped, and phloem vessels are located between the xylem vessels.

○ Monocotyledonous Plants : Xylem vessels form a structure of multiple tubes surrounding the phloem vessels, and phloem vessels are located between the xylem vessels.

○ Observation of the Casparian strip

⑸ Transverse section of the root

① Xylem

② Xylem Vessels

③ Phloem Vessels

④ Endodermis

⑤ Casparian Strip Present in the Endodermis

⑥ Cortex

⑦ Epidermis

⑹ Longitudinal section of the root

① Epidermis

② Differentiation Zone (Maturation Zone) : Observation of root hairs

③ Elongation Zone

④ Division Zone : Apical meristem (Zone of cell division)

⑤ Root Cap

6. Plant Physiology

⑴ Overview

① Material Transport Pathways

○ Apoplast Pathway : Pathway along the apoplast

○ Apoplast : Extracellular matrix located outside the plasma membrane, including cell walls and intercellular spaces, as well as dead cells such as tracheids and vessel elements.

○ Active transport through the Casparian strip and use of energy due to the H+ concentration gradient

○ Symplast Pathway : Pathway through the symplast

○ Symplast : Continuous network of living protoplasts within the plant

○ Material transport without the use of energy

○ Example : Intercellular spaces in the phloem

○ Transmembrane Pathway

② Water Potential

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

○ Lower turgor pressure in source tissue compared to sink tissue

○ Sink : Sinks are tissues that consume or store sugars, such as roots (sugar storage), fruits (sugar storage)

○ Storage as starch, transport as sugars

○ Reason : Sugars are non-reducing sugars

○ Types of sugar transporters : Monosaccharide Transporters (MST), Sucrose Transporters (SUT), Sucrose and Hexose Transporter (SWEET)

○ Pressure Flow Model : Water is pushed into the sieve tube elements of source tissues and then 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

○ Bidirectional sugar transport across sink tissues through translocation

⑵ Root Material Transport

① Casparian Strip

○ Impermeable layer made of suberin

○ For water to reach the vascular cylinder of the root, it must pass through the endodermis and its symplast

○ Prevents unnecessary or toxic substances from entering

○ Prevents leakage of substances stored in the vascular cylinder 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 used during active transport of H⁺.

○ H⁺ - mineral cation co-transporter used during mineral cation absorption

○ 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 (Zeacretein)

○ 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 their concentration gradients

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

○ 1^st - 6^th. Increase in sucrose 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 sugar production and sucrose 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 loss 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 → 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, protein translation and modification

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, induction of unit fruit formation

④ Polar Transport

○ Auxin moves only from the stem apex to the base

○ Movement towards the root, not gravity-related

⑤ Canalization 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 (within the cell) : Passive diffusion or secondary active transport

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

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 synergistically

○ 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, promotes RNA and protein synthesis

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

○ Increases during fruit growth, 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 released into endosperm, hydrolyzing starch

④ Function 2. Promotion of stem elongation

⑤ Function 3. Seedless grapes → Enhanced grape growth

⑥ Function 4. Formation of second-year plant rosettes

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 vessel formation

○ Promotes growth of vascular cambium in woody plants

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. Drought, etc., leads to excessive abscisic acid secretion

○ 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, 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

○ Thiourea and sulfuric acid derivatives inhibit ethylene, used to extend flower shelf life

○ 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, 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

○ Induction of Cell Division

○ Induction of Leaf Bud Formation (Senescence)

○ Promotion of Root and Root Hair Formation

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 phosphatase

○ 3^rd. Phosphatase 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 circulating carbon dioxide

○ Phosphates : High levels of phosphates inhibit ethylene production

⑻ Photoreceptors

① Red Light Receptor : Phytochrome (Blue Light)

○ Structure : Consists of a 4-domain structure formed by two identical polypeptides, each containing chromophore-active regions (including pigments) and phosphorylation-active domains

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

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

○ Pr to Pfr Conversion

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

○ Red light exposure converts Phytochrome from Pr to Pfr

○ Far-red light exposure converts Phytochrome from Pfr to Pr

○ Due to instability, Pfr spontaneously converts to Pr

○ Pfr primarily has biological activity

○ 1^st. Pfr 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

○ Monocarpic Plants: Flower when nights are longer than critical night length (Pr dominant)

○ Polycarpic Plants: Flower when nights are shorter than critical night length (Pfr dominant), more common

○ Day: Pr to Pfr transition due to more red light than far-red light

○ Night: Pfr spontaneously converts to Pr

○ During memory acquisition, white flash converts Pr to Pfr, subsequent far-red light eliminates white flash effect

○ Sufficiently long memory may prevent photoreversibility

○ Life Cycles and Circadian Rhythms : Synchronize with environmental cues

○ Photobleaching Phenomenon

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

○ Decreased Pfr/Pr under dark conditions induces photobleaching

② Cryptochrome 1. Blue Light Receptor : 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 Cryptochrome 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 Cryptochrome 3. Zeaxanthin : Stomatal Aperture Regulation

⑼ Flowering Hormone

① Floral Inductor (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 phloem

○ 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 indeterminate growth to determinate 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 days

⑽ Defense Proteins

① Specific Immune Response to Pathogens (Avirulence genes, Avr, and Resistance genes, R)

○ Avirulence genes (Avr)

○ Resistance genes (R)

○ If the pathogenic Avr gene does not match the plant’s Resistance gene, the plant 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 water vessels 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. Stimulus triggers starch granule movement, releasing Ca2+ from statocyte vacuoles

○ 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

○ Contact Stimulation

○ 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, 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 : Only covers angiosperm sexual reproduction. Applicable to both monocots and dicots

⑴ Flowers

① Structure of Flowers

○ Sepals : Protects flower bud, photosynthesis

○ Petals

○ Carpels : Carpels + Ovary

○ Stamens : Composed of anther + filament + pollen

② ABC Hypothesis : Model of floral organ development, involvement of organ identity genes

Figure. 6. ABC Hypothesis

○ Homeotic genes A, B, C determine positions of floral organs through interactions

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

⑵ Formation of Gametophytes

① Formation of Pollen Grains

○ 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

○ In maturation, generative cell undergoes mitosis to become 2 cells

② Formation of Embryo Sac

○ 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 cell division to form an embryo sac with 8 nuclei

○ 4^th. Embryo sac = Egg cell + Synergids × 2 + Antipodal cells × 2 + Central cell × 3

⑶ 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

⑷ Self-Incompatibility : Rejection of pollen from one’s own flowers

① Sporophytic Self-Incompatibility

○ S-alleles of sporophyte (pollen) production is crucial

○ S1 pollen from (S1, S2) sporophyte cannot germinate on (S2, S3) stigma due to inhibition by S2 allele

② Gametophytic Self-Incompatibility

○ S-alleles of pollen tube growth is crucial

○ Pollen tubes with S1 allele of (S1, S2) sporophyte germinate on (S2, S3) stigma, while those with S2 allele do not

③ Prevention of Self-Pollination by Flower Morphology

Example : Different flower types: Pin and thrum flowers

④ Weak Adhesion of Same Species Pollen

⑸ 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 sperm cells

○ 5^th. Chemicals secreted by synergids attract pollen tube, it grows to ovule

○ 6^th. Once pollen tube reaches ovule, one synergid degenerates, creates pathway

○ 7^th. Two sperm cells move from pollen tube to embryo sac

○ 8^th. One sperm fertilizes egg cell, forms zygote

○ 9^th. Other sperm fuses with two polar nuclei, forms endosperm nucleus (3n)

② Double Fertilization : Angiosperms

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

○ Sperm cell (n) + Central cell (n) × 2 → Endosperm (3n)

③ Single Fertilization : Gymnosperms

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

○ Embryo sac nucleus (i.e., female gametophyte, n) develops into endosperm (n) without fusion

⑹ Seed Development

① Dicotyledonous Plants

○ Embryo consists of hypocotyl, radicle, epicotyl, plumule

○ Mainly from hypocotyl

② Monocotyledonous Plants

○ Embryo consists of plumule, coleoptile, radicle, coleorhiza

○ Mainly from endosperm

⑹ Seed : Fertilized ovule that develops from integument

① Seed Initiation

○ One of the two daughter cells formed through suspensor (embryonic stalk) becomes embryo, the other becomes suspensor

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

○ Embryo Development

○ 1^st. Adequate endosperm development

○ 2^nd. Fusion of polar nuclei to form triploid nucleus (3n)

○ 3^rd. Inner cells form rounded embryo

○ 4^th. Outer cells form suspensor

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

○ Leaf development (in dicots) : Heart-shaped embryo grows into cotyledons, develops plumule from cotyledon junction (monocots form single cotyledon) → Cotyledon absorbs endosperm, forms hypocotyl → Shoot apical meristem forms at point of cotyledon separation → Develops into epicotyl

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

② Initiation of Three Primary Meristems

○ Protoderm : Outer surface of embryo → Develops into epidermis

○ Ground meristem : Central region of embryo axis → Forms primary cortex and pith

○ Procambium : Between protoderm and ground meristem

⑺ Fruit : Develops from ovary after fertilization (e.g., corn, grains)

① Simple Fruit : Develops from one to 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

⑻ Germination and Plant Propagation

① Germination

② Asexual Reproduction

9. Asexual Reproduction

⑴ Vegetative Propagation

① Cuttings : Nutrient propagation from transplanted branches

② Bulbils

③ Leaf Propagation

④ Vine Stems : Potatoes reproduce through tuberous stems

⑤ Tubers

⑥ Adventitious Roots : Sweet potatoes develop from storage root becoming a functional organ

⑦ Tuber and Tuberous Stem

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

578

Chapter 25. Botany

1. Plant Kingdom