4 A. Male Gametophyte (Pollen) 1. Anther is composed of pollen sacs (malesporangium).2. Inside pollen sac: 2n cells calledmicrosporocytes undergo meiosis to form 4 haploidmicrospores.Each microspore divides by mitosis to make 2 cells:a. Generative cell – will make spermb. Tube cell – will make pollen tubeThe 2 cells enclosed in thick wall pollen grain
5 B. Female Gametophyte (Embryo Sac) 1. Ovule = female sporangium2. 2n cell in ovule (megasporocyte) divides bymeiosis to form 4 haploid megaspores.3. Only one megaspore survives and divides bymitosis 3 times to make 8 haploid nuclei.
6 4. Supporting Cellsa. Synergids – attract and guide pollen tube to the eggb. Antipodal cells – unknown functionc. 2 polar nuclei – eventually fuse with a sperm tomake the 3n endosperm
9 II. Angiosperm Reproduction A. Pollination1. Pollen grain lands on stigma (= pollination)2. Generative cell divides by mitosis to form 2sperm cells3. Tube cell forms pollen tube4. Sperm travel down pollen tube and enterembryo sac5. Double fertilization
10 5. Double Fertilization a. Egg + sperm zygote b. 2 polar nuclei + sperm 3n nucleus that becomes theendospermFig 38.5
11 B. Maturation structure that provides nutrients to developing embryo 1. Endosperm begins to divide to formstructure that provides nutrients to developingembryo2. Embryo divides to form cotyledons (= seedleaves) and meristems3. Ovule is now a seed – dehydrates &becomes dormant (low metabolism, no growth).4. Ovary tissues divide & mature into fruit
16 Plant Responses to Internal and External Signals
17 I. Introduction A. General Ideas for example, plants can…. 1. send signals between different parts of the plant2. track the time of day and the time of year3. sense and respond to gravity and the direction or wavelength of light
18 B. How do they respond?1. by adjusting their growth pattern and developmentExample = Etiolation
19 2. Hormone = chemical signal produced by one part of a plant and translocated to other parts where it triggers a response in target cells and tissues3. Environmental stimuli cause increases or decreases in levels/ratios of hormones in the plant
20 II. Cell Signaling: B. Transduction/amplification C. Response A. ReceptionB. Transduction/amplificationC. Response
22 A. Reception1. Receptor proteins (on cell membrane) receive the signal (hormone ) & undergo conformational change2. Absorption of a specific wavelength of light by a intracellular pigment
23 B. Transduction/Amplification 1. Membrane Bounda. G-proteinb. Tyrosine Kinases2. Secondary Messengersa. cAMP or cGMP.b. Kinases/Phophotases.c. Calcium
24 Examples of Second Messengers: G proteins – active when GTP bound. Activate:Cyclic nucleotides – cAMP or cGMP. Activate:Protein kinases – enzymes that phosphorylate & thusactivate other proteins such as transcription factors.Cascade of protein kinases amplify the signal.Calcium – a mineral that can bind to activate proteinkinases.
28 C. Response 1. Amplified signal induces the regulation of a specific cellular activity.Fig. 11.9
29 2. Mechanisms: a. Transcriptional regulation – activated transcription factors bind to DNA & control transcription of specificgenesFig. 18.9Fig. 18.8
30 b. Post – translational modification of proteins – by phosphorylation by protein kinasesFig. 39.4
31 c. Rapid, regulating physiology: d. Slow, gene expression. i. stimulation of stomatal closingd. Slow, gene expression.i. Control of development by affecting cell division, elongation, and differentiation.
32 III. Types of Plant Responses A. Tropism – growth responses toward oraway from a stimulus (Photo. or Gravi.)B. Nastic response – non-growth responseEx. Venus flytrap mechanism; turgorchangesC. Morphogenic response – morphologicalresponse (change in shape, growth) Ex.Onset of flowering
33 IV. Six Major Plant Hormones A. Auxin (IAA)B. CytokininsC. Gibberellins (GA)D. BrassinosteroidsE. Abscisic acid (ABA)F. Ethylene
35 A. Auxin 1. Production Site 2. Effects a. Cell elongation & differentiationb. Root growthc. Branchingd. Apical dominancee. Fruit developmentf. Phototropism & gravitropism
36 Auxin can also: g. Stimulate roots to grow from cuttings h. Be used as an herbicide (very high levels of auxininhibits growth)i. Stimulate fruit development without pollination seedless fruits!
37 B. Cytokinin 1. Production Site 2. Effects a. Root growth & differentiationb. Cell division (cytokinesis) & differentiationc. Germinationd. Prevents leaf senescence/aging (florists spray cytokinins to keep flowers fresh)e. Control of apical dominance
38 (Aside) Apical Dominance 1. Auxin travels down stem & inhibits axillary bud growth causing the shoot to lengthen.2. Cytokinins travel up from roots to stimulate axillary bud growth.3. If SAM removed, auxin concentration drops & cytokinins stimulate axillary buds to grow.4. Lower bud thus grow before higher ones since they are closer to the cytokinin source than the auxin source.
40 C. Gibberellins 1. Production Site 2. Effects a. Fruit growth b. Release of some seeds and buds from dormancyc. Stem elongation (act with auxin to acidify cell wall)d. Bolting of inflorescence
41 (Aside) Dormancy and Germination 1. High concentration of gibberellins in seeds & embryo.2. The release of gibberellins signals seeds to break dormancy and germinate.3. Imbibed water (& other environmental cues) stimulates gibberellin release.
42 D. Abscisic Acid (ABA) 1. Production Site 2. Effects a. Initiation of dormancy/ inhibition of germinationb. Stimulates production of proteins that allow seedto withstand dehydrationc. Water washes ABA away, gibberellins stimulategerminationd. Inhibits growth
43 e. Counteracts first 3 growth hormones e. Counteracts first 3 growth hormones. Ratio of ABA to others determines outcomef. Stomatal closure during water stressg. Root water stress stimulates ABA production, travels up to leaves to “warn” them to close stomata before wilting occurs
44 E. Brassinosteroids 1. Production Site 2. Effects a. Inhibit root growth & leaf abscissionb. Promote xylem differentiation
45 F. Ethylene 1. Production Site a. The only gaseous hormone. b. Diffuses through air spaces between plant cells.c. Produced in response to stresses: drought, flood, injury, infection
46 2. Effects a. Fruit ripening b. Leaf abscission Conversion of starches to sugarsFruit picked green, then gassed with ethylene to induceripeningb. Leaf abscissionLeaves drop off plant in response to water stress, seasonalchangeEthylene stimulates enzymes to digest cell walls of theabscission layer of petiole.
48 c. Apoptosis = programmed cell death Death of leaves in Fall, yearly death of annualsEthylene stimulates enzymes that break down cellsd. Triple response to mechanical stressThere’s a rock in the way! Ethylene production stimulates:slowing of stem growth, stem thickens, stem curves, & thengrows horizontallyOnce past the rock, ethylene production declines & plantcan grow up again
50 G. Minor Plant Hormones 1. Strigolactones 2. Florigen a. Production siteb. EffectsSeed germination, control apical dominance, attract mycorrhizal fungi2. Florigena. Production siteb. EffectsFlowering
55 3. Responses a. Inhibition of internode elongation b. Development of proper leaf shapec. Increase in number of stomata per leafd. Increase in amount of chlorophylle. Decrease in apical dominancef. Increased accumulation of carotenoid pigments in tomatoesg. Membrane permeabilityh. Seed germinationi. Spore germinationj. Chloroplast movementk. Inter-node extension, Hypocotyl hood formation, Leaflet movement, Geotropic sensitivity, Anthocyanin synthesis,l. Shade avoidancem. Circadian rhythms
56 (Aside) Circadian Rhythms and Clocks 1. Circadian rhythm = a physiological cycle with a frequency of about 26 hours that persists even when an organism is sheltered from environmental cues.2. all eukaryotes3. Plant examples: stomatal opening/closing, production of PSN enzymes (Others?)4. Mechanism: ???? Phytochromes receptors may “train” the biological clock to 24 hours.
57 B. Photoperiodism 1. Definition a. Photoperiodism is a physiological response to day length.b. Synchronization of plant events according to seasonsc. Plants detect the time of year by the photoperiod (the relative lengths of night and day).
58 a. Night length is the important factor (continuous hours of darkness) 2. Mechanisms (Trends)a. Night length is the important factor (continuous hours of darkness)b. Short–day (Long–night) plants - flower in late summer, fall, and winter.c. Long–day (short–night) plants - flower in late spring and summer.d. Day–neutral plants are unaffected by photoperiod.
60 e. Some plants flower after a single exposure to the proper photoperiod. f. Some require several successive days of the proper photoperiod to bloom.g. Still others respond to photoperiod only if they have been previously exposed to another stimulus. (e.g. vernalization)h. Leaves detect the photoperiod – send signals to buds to produce flowers.
61 C. Gravitropism 1. Definition a. Gravity provides stimulus for plants to grow upout of ground, no matter the seed orientation in thesoil.b. Gravitational pull on plant cell causes starchgrains to settle to bottom - stimulates an asymmetricproduction of auxin in the cell
62 2. Mechanisms a. Thus different rates of cell elongation on opposite sides of the root /shoot.b. Root grows down & shoot grow upFig 39.25
63 D. Thigmotropism 1. Definition 2. Mechanisms a. Directional growth in response to “touch”Ex. Vines winding around fence, treeb. Stimulus activates genes that affect cell wallproperties Ex. Mimosa pudica
64 D. Stress (Stressotropism) 1. Droughta. Increase in ABA keeps guard cells closedb. Thus plant growth slows because cells can’telongate or photosynthesize2. Floodinga. Ethylene stimulates some root cells to die(apoptosis) to create air tubes in the roots
65 3. Salt stress a. Problem: roots can lose water because soil water has lower potentialb. Response: root cells produce extra organic soluteswithin the cell to create lower potential inside
66 4. Heat stress 5. Cold stress a. Production of heat-shock proteins which prevent cell enzymes from denaturation5. Cold stressa. Problem: cell membranes become less fluid and transport becomes difficultb. Response: cell replaces membrane fats with fats that remain fluid at lower temperatures
67 6. Pests a. Morphological adaptations like thorns b. Production of toxic compounds when bittenc. Production of chemicals that attract predator to the herbivore – ex. Parasitic waspsd. Production of anti-microbial compoundse. Seal off the pathogen and initiate cell death to remove it
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