The Angiosperm Life Cycle 1. Male gametophyte = pollen grain, develops in the anther. Produces sperm 2. Female gametophyte = embryo sac, develops in the ovule of the ovary. Produces egg
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 sperm B. Tube cell – will make pollen tube 4. The 2 cells enclosed in thick wall => pollen grain
Development of the Female Gametophyte (Embryo Sac) 1. Ovule = female sporangium 2. 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.
1. Synergids – attract and guide pollen tube to the egg 2. Antipodal cells – unknown function 3. 2 polar nuclei – eventually fuse with a sperm to make the 3n endosperm
Angiosperm Reproduction 1. Pollen grain lands on stigma (= pollination) 2. Generative cell divides by mitosis to form 2 sperm cells 3. Tube cell forms pollen tube 4. Sperm travel down pollen tube and enter embryo sac 5. Double fertilization – A. Egg + sperm zygote B. 2 polar nuclei + sperm 3n nucleus that becomes the endosperm
Maturation 1. Endosperm begins to divide to form structure that provides nutrients to developing embryo 2. Embryo divides to form cotyledons (= seed leaves) and meristems 3. Ovule is now a seed – dehydrates & becomes dormant (low metabolism, no growth). 4. Ovary tissues divide & mature into fruit
Germination 1. Dormant seed becomes a seedling 2. Seed needs proper conditions to break dormancy 3. Steps: A. Water uptake by seed causes expansion B. Embryo begins to grow C. Enzymes digest endosperm & transfers nutrients to embryo D. Radicle (embryo root) emerges E. Hypocotyl (embryo shoot) raises cotyledons above ground F. True leaves form & PSN begins
Asexual Reproduction 1. How? 2. Detached fragments of plant can develop into new plants (fragmentation) 3. Stolons, rhizomes vegetative propagation 4. Clones of parent 5. Benefit? A. Rapid expansion in suitable environment B. Daughters not as fragile as seedlings 6. Trade off? A. Sexual reproduction produces genetic variation
Plant Responses to Internal and External Signals
Plants Respond to the Environment! 1. For example, plants can…. A. send signals between different parts of the plant B. track the time of day and the time of year C. sense and respond to gravity and the direction or wavelength of light 2. How do they respond? A. by adjusting their growth pattern and development Example = Etiolation
Hormones and Plants 1. Hormone = chemical signal produced by one part of a plant and translocated to other parts where it triggers a response in target cells and tissues 2. Environmental stimuli cause increases or decreases in levels/ratios of hormones in the plant
How do hormones elicit a plant response? 1. 3 steps of signal processing: A. Reception B. Transduction/amplification C. Response
A. Reception a. Receptor proteins (on cell membrane) receive the chemical signal (hormone) & undergo conformational change b. Ex. absorption of a specific wavelength of light by a pigment
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.
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.
c. Some responses occur rapidly, regulating physiology: i. Abscisic acid (ABA) stimulation of stomatal closing d. Other responses take longer, especially if they require changes in gene expression. ii. Control of development by affecting cell division, elongation, and differentiation.
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 changes 3. Morphogenic response– morphological response (change in shape, growth) Ex. Onset of flowering
1. Auxin – regulates: A. Cell elongation & differentiation B. Root growth C. Branching D. Apical dominance E. Fruit development F. Phototropism & gravitropism
Auxin can also: G. Stimulate roots to grow from cuttings H. Be used as an herbicide (very high levels of auxin inhibit growth I. Stimulate fruit development without pollination seedless fruits!
2. Cytokinin – regulates: A. Root growth & differentiation B. Cell division (cytokinesis) & differentiation C. Germination D. Prevents leaf senescence/aging (florists spray cytokinins to keep flowers fresh) E. Control of apical dominance
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.
3. Gibberellin – regulates: A. Fruit growth B. Release of some seeds and buds from dormancy C. Stem elongation (act with auxin to acidify cell wall) D. Bolting of inflorescence
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.
4. Abscisic Acid (ABA) – regulates: A. Initiation of dormancy/ inhibition of germination B. Stimulates production of proteins that allow seed to withstand dehydration C. Water washes ABA away, gibberellins stimulate germination D. Inhibits growth E. Counteracts first 3 growth hormones. Ratio of ABA to others determines outcome F. 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
5. Brassinosteroids A. Inhibit root growth & leaf abscission B. Promote xylem differentiation
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
6. Ethylene – regulates: A. Fruit ripening a. Conversion of starches to sugars b. Fruit picked green, then gassed with ethylene to induce ripening B. Leaf abscission a. Leaves drop off plant in response to water stress, season change b. Ethylene stimulates enzymes to digest cell walls of the abscission layer of petiole.
C. Apoptosis = programmed cell death a. Death of leaves in Fall, yearly death of annuals b. Ethylene stimulates enzymes that break down cells D. Triple response to mechanical stress a. Theres a rock in the way! Ethylene production stimulates: i. Stem growth slows ii. Stem thickens iii. Stem curves & grows horizontally b. once past the rock, ethylene production declines & plant can grow up again
Examples of responses: 1. Light - phototropism 2. Gravity - gravitropism 3. Touch/ mechanical stimuli - thigmotropism 4. Responses to stress 5. Responses to herbivores & pathogens
1. Plant responses to light - phototropism A. Plants detect lights direction, intensity, & wavelength B. 2 classes of light receptors: a. Blue light – light-induced stomatal opening b. Phytochromes i. Red light receptors ii. Most important iii. Exist in 2 reversible forms: P r & P fr. Relative amounts in plant stimulates various responses Video
Phytochrome - Mediated Responses Inhibition of internode elongation Development of proper leaf shape Increase in number of stomata per leaf Increase in amount of chlorophyll Decrease in apical dominance Increased accumulation of carotenoid pigments in tomatoes Membrane permeability Seed germination Spore germination Chloroplast movement Internode extension, Hypocotyl hood formation, Leaflet movement, Geotropic sensitivity, Anthocyanin synthesis, Shade avoidance Circadian rhythms
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 eukaryotes 3. Plant examples: stomatal opening/closing, production of PSN enzymes 4. Mechanism: ???? Phytochromes receptors may train the biological clock to 24 hours.
Photoperiodism 1. Photoperiodism is a physiological response to day length. 2. Synchronization of plant events according to seasons 3. Plants detect the time of year by the photoperiod (the relative lengths of night and day).
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.
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.
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 cell C. Thus different rates of cell elongation on opposite sides of the root /shoot. D. Root grows down & shoot grow up
3. Plant response to mechanical stimuli: thigmotropism A. Directional growth in response to touch Ex. Vines winding around fence, tree B. Stimulus activates genes that affect cell wall properties Ex. Mimosa pudica - video
4. Plant Responses to Stress A. Drought a. Increase in ABA keeps guard cells closed b. Thus plant growth slows because cells cant elongate or photosynthesize B. Flooding a. Ethylene stimulates some root cells to die (apoptosis) to create air tubes in the roots C. Salt stress a. Problem: roots can lose water because soil water has lower potential b. Response: root cells produce extra organic solutes within the cell to create lower potential inside
D. Heat stress a. Production of heat-shock proteins which prevent cell enzymes from denaturation E. Cold stress a. Problem: cell membranes become less fluid and transport becomes difficult b. Response: cell replaces membrane fats with fats that remain fluid at lower temperatures
5. Plant response to herbivores & pathogens A. Morphological adaptations like thorns B. Production of toxic compounds when bitten C. Production of chemicals that attract predator to the herbivore – ex. Parasitic wasps D. Production of anti-microbial compounds E. Seal off the pathogen and initiate cell death to remove it