4 The Angiosperm Life Cycle 1. Male gametophyte = pollen grain, develops in the anther. Produces sperm2. Female gametophyte = embryo sac, develops in the ovule of the ovary. Produces egg
5 Development of Male Gametophyte (Pollen) 1. Anther is composed of pollen sacs (sporangium).2. Inside pollen sac: 2n cells called microsporocytes undergo meiosis to form 4 haploid microspores.3. Each microspore divides by mitosis to make 2 cells:A. Generative cell – will make spermB. Tube cell – will make pollen tube4. The 2 cells enclosed in thick wall => pollen grain
6 Development of the Female Gametophyte (Embryo Sac) 1. Ovule = female sporangium2. 2n cell in ovule (megasporocyte) divides by meiosis to form 4 haploid megaspores.3. Only one megaspore survives and divides by mitosis 3 times to make 8 haploid nuclei.
7 1. Synergids – attract and guide pollen tube to the egg 2. Antipodal cells – unknown function3. 2 polar nuclei – eventually fuse with a sperm to make the 3n endosperm
12 Maturation1. Endosperm begins to divide to form structure that provides nutrients to developing embryo2. Embryo divides to form cotyledons (= seed leaves) and meristems3. Ovule is now a seed – dehydrates & becomes dormant (low metabolism, no growth).4. Ovary tissues divide & mature into fruit
16 Asexual Reproduction 1. How? 2. Detached fragments of plant can develop into new plants (fragmentation)3. Stolons, rhizomes vegetative propagation4. Clones of parent5. Benefit?A. Rapid expansion in suitable environmentB. Daughters not as fragile as seedlings6. Trade off?A. Sexual reproduction produces genetic variation
17 Plant Responses to Internal and External Signals
18 Plants Respond to the Environment! 1. For example, plants can….A. send signals between different parts of the plantB. track the time of day and the time of yearC. sense and respond to gravity and the direction or wavelength of light2. How do they respond?A. by adjusting their growth pattern and developmentExample = Etiolation
19 Hormones and Plants1. Hormone = chemical signal produced by one part of a plant and translocated to other parts where it triggers a response in target cells and tissues2. Environmental stimuli cause increases or decreases in levels/ratios of hormones in the plant
20 How do hormones elicit a plant response? 1. 3 steps of signal processing:A. ReceptionB. Transduction/amplificationC. Response
22 A. Receptiona. Receptor proteins (on cell membrane) receive the chemical signal (hormone) & undergo conformational changeb. Ex. absorption of a specific wavelength of light by a pigment
23 B. Transduction/Amplification a. Reception (step one) causes the formation of a secondary messengers within the cell.b. Second messengers are chemicals that amplify the signal by triggering a cascade of protein activations.
24 Examples of Second Messengers: G proteins – active when GTP bound. Activate:Cyclic nucleotides – cAMP or cGMP; Activate:Protein kinases – enzymes that phosphorylate & thus activate other proteins such as transcription factors. Cascade of protein kinases amplify the signal.Calcium – a mineral that can bind to activate protein kinases.
35 c. Some responses occur rapidly, regulating physiology: i. Abscisic acid (ABA) stimulation of stomatal closingd. Other responses take longer, especially if they require changes in gene expression.ii. Control of development by affecting cell division, elongation, and differentiation.
36 Types of Plant Responses 1. Tropism – growth response toward or away from a stimulus (Photo. or Gravi.)2. Nastic response – non-growth response Ex. Venus flytrap mechanism; turgor changes3. Morphogenic response– morphological response (change in shape, growth) Ex. Onset of flowering
37 Six Major Plant Hormones 1. Auxin (IAA)2. Cytokinins3. Gibberellins (GA)4. Brassinosteroids5. Abscisic acid (ABA)6. Ethylene
39 1. Auxin – regulates: A. Cell elongation & differentiation B. Root growthC. BranchingD. Apical dominanceE. Fruit developmentF. Phototropism & gravitropism
40 Auxin can also: G. Stimulate roots to grow from cuttings H. Be used as an herbicide (very high levels of auxin inhibit growthI. Stimulate fruit development without pollination seedless fruits!
41 2. Cytokinin – regulates: 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
42 Apical Dominance1. 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.
44 3. Gibberellin – regulates: A. Fruit growthB. Release of some seeds and buds from dormancyC. Stem elongation (act with auxin to acidify cell wall)D. Bolting of inflorescence
45 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.
46 4. Abscisic Acid (ABA) – regulates: A. Initiation of dormancy/ inhibition of germinationB. Stimulates production of proteins that allow seed to withstand dehydrationC. Water washes ABA away, gibberellins stimulate germinationD. Inhibits growthE. Counteracts first 3 growth hormones. Ratio of ABA to others determines outcomeF. Stomatal closure during water stress G. Root water stress stimulates ABA production, travels up to leaves to “warn” them to close stomata before wilting occurs
47 5. Brassinosteroids A. Inhibit root growth & leaf abscission B. Promote xylem differentiation
48 6. Ethylene A. The only gaseous hormone. B. Diffuses through air spaces between plant cells.C. Produced in response to stresses: drought, flood, injury, infection
49 6. Ethylene – regulates: A. Fruit ripening a. Conversion of starches to sugarsb. Fruit picked green, then gassed with ethylene to induce ripeningB. Leaf abscissiona. Leaves drop off plant in response to water stress, season changeb. Ethylene stimulates enzymes to digest cell walls of the abscission layer of petiole.
51 C. Apoptosis = programmed cell death a. Death of leaves in Fall, yearly death of annualsb. Ethylene stimulates enzymes that break down cellsD. Triple response to mechanical stressa. There’s a rock in the way! Ethylene production stimulates:i. Stem growth slowsii. Stem thickensiii. Stem curves & grows horizontallyb. once past the rock, ethylene production declines & plant can grow up again
57 Phytochrome - Mediated Responses Inhibition of internode elongationDevelopment of proper leaf shapeIncrease in number of stomata per leafIncrease in amount of chlorophyllDecrease in apical dominanceIncreased accumulation of carotenoid pigments in tomatoesMembrane permeabilitySeed germinationSpore germinationChloroplast movementInternode extension, Hypocotyl hood formation, Leaflet movement, Geotropic sensitivity, Anthocyanin synthesis,Shade avoidanceCircadian rhythms
58 Circadian Rhythms and Biological 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 enzymes4. Mechanism: ???? Phytochromes receptors may “train” the biological clock to 24 hours.
59 Photoperiodism1. Photoperiodism is a physiological response to day length.2. Synchronization of plant events according to seasons3. Plants detect the time of year by the photoperiod (the relative lengths of night and day).
60 Photoperiods Control Flowering 1. Night length is the important factor (continuous hours of darkness)2. Short–day (Long–night) plants - flower in late summer, fall, and winter.3. Long–day (short–night) plants - flower in late spring and summer.4. Day–neutral plants are unaffected by photoperiod.
62 A. Some plants flower after a single exposure to the proper photoperiod. B. Some require several successive days of the proper photoperiod to bloom.C. Still others respond to photoperiod only if they have been previously exposed to another stimulus. (e.g. vernalization)D. Leaves detect the photoperiod – send signals to buds to produce flowers.
63 2. Plant response to gravity: gravitropism A. Gravity provides stimulus for plants to grow up out of ground, no matter the seed orientation in the soil.B. Gravitational pull on plant cell causes starch grains to settle to bottom - stimulates an asymmetric production of auxin in the cellC. Thus different rates of cell elongation on opposite sides of the root /shoot.D. Root grows down & shoot grow up
65 3. Plant response to mechanical stimuli: thigmotropism A. Directional growth in response to “touch” Ex. Vines winding around fence, treeB. Stimulus activates genes that affect cell wall properties Ex. Mimosa pudica - video
66 4. Plant Responses to Stress A. Droughta. Increase in ABA keeps guard cells closedb. Thus plant growth slows because cells can’t elongate or photosynthesizeB. Floodinga. Ethylene stimulates some root cells to die (apoptosis) to create air tubes in the rootsC. Salt stressa. Problem: roots can lose water because soil water has lower potentialb. Response: root cells produce extra organic solutes within the cell to create lower potential inside
67 D. Heat stressa. Production of heat-shock proteins which prevent cell enzymes from denaturationE. 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
68 5. Plant response to herbivores & pathogens A. Morphological adaptations like thornsB. 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