Development and morphogenesis
- 2. Can flowers tell you the time of day?
- 3. Stimuli and a Stationary Life • Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock • Plants, being rooted to the ground, must respond to environmental changes that come their way – For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light
- 4. Signal transduction pathways link signal reception to response • A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots • These are morphological adaptations for growing in darkness, collectively called etiolation • After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
- 5. (a) Before exposure to light (b) After a week’s exposure to natural daylight
- 6. • A potato’s response to light is an example of cell-signal processing • The stages are reception, transduction, and response
- 7. Reception CELL WALL 2 31 Transduction CYTOPLASM Response Relay proteins and second messengers Activation of cellular responses Receptor Hormone or environmental stimulus Plasma membrane
- 8. Reception • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli • In de-etiolation, the receptor is a phytochrome capable of detecting light
- 9. Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses • Two types of second messengers play an important role in de- etiolation: Ca2+ ions and cyclic GMP (cGMP) • The phytochrome receptor responds to light by – Opening Ca2+ channels, which increases Ca2+ levels in the cytosol – Activating an enzyme that produces cGMP
- 10. Response • A signal transduction pathway leads to regulation of one or more cellular activities • In most cases, these responses to stimulation involve increased activity of enzymes • This can occur by transcriptional regulation or post- translational modification
- 11. De-Etiolation (“Greening”) Proteins • De-etiolation activates enzymes that – Function in photosynthesis directly – Supply the chemical precursors for chlorophyll production – Affect the levels of plant hormones that regulate growth
- 12. Plant hormones help coordinate growth, development, and responses to stimuli • Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant
- 13. The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism • In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present
- 14. Control Light Shaded side Illuminated side Boysen-Jensen Light Light Darwin and Darwin Gelatin (permeable) Mica (impermeable) Tip removed Opaque cap Trans- parent cap Opaque shield over curvature RESULTS They postulated that a signal was transmitted from the tip to the elongating region In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance
- 15. A Survey of Plant Hormones • Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ • In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells
- 16. Overview of Plant Hormones
- 17. Auxin • The term auxin refers to any chemical that promotes elongation of coleoptiles • Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA • Auxin is produced in shoot tips and is transported down the stem • Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell • Auxin also alters gene expression and stimulates a sustained growth response
- 18. The Role of Auxin in Cell Elongation • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric • With the cellulose loosened, the cell can elongate
- 19. Auxin’s Role in Plant Development • Polar transport of auxin plays a role in pattern formation of the developing plant • Reduced auxin flow from the shoot of a branch stimulates growth in lower branches • Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem • Polar transport of auxin from leaf margins directs leaf venation pattern • The activity of the vascular cambium is under control of auxin transport
- 20. Practical Uses for Auxins • The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings • An overdose of synthetic auxins can kill plants – For example 2,4-D is used as an herbicide on eudicots
- 21. Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division) Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin to control cell division and differentiation
- 22. Control of Apical Dominance • Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds • If the terminal bud is removed, plants become bushier
- 23. (a) Apical bud intact (not shown in photo) (b) Apical bud removed (c) Auxin added to decapitated stem Axillary buds Lateral branches “Stump” after removal of apical bud
- 24. Anti-Aging Effects • Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
- 25. Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination Stem Elongation • Gibberellins are produced in young roots and leaves • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division
- 26. Fruit Growth • In many plants, both auxin and gibberellins must be present for fruit to develop • Gibberellins are used in spraying of Thompson seedless grapes
- 27. (a) Rosette form (left) and gibberellin-induced bolting (right) (b) Grapes from control vine (left) and gibberellin-treated vine (right)
- 28. Germination • After water is imbibed, release of gibberellins from the embryo signals seeds to germinate
- 29. Brassinosteroids • Brassinosteroids are chemically similar to the sex hormones of animals • They induce cell elongation and division in stem segments • They slow leaf abscission and promote xylem differentiation
- 30. Abscisic Acid • Abscisic acid (ABA) slows growth • Two of the many effects of ABA – Seed dormancy – Drought tolerance
- 31. Seed Dormancy • Seed dormancy ensures that the seed will germinate only in optimal conditions • In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold • Precocious (early) germination can be caused by inactive or low levels of ABA
- 32. Red mangrove (Rhizophora mangle) seeds Maize mutant Coleoptile Precocious germination of wild-type mangrove and mutant maize seeds.
- 33. Drought Tolerance • ABA is the primary internal signal that enables plants to withstand drought • ABA accumulation causes stomata to close rapidly
- 34. Strigolactones • The hormones called strigolactones – Stimulate seed germination – Help establish mycorrhizal associations – Help control apical dominance • Strigolactones are named for parasitic Striga plants • Striga seeds germinate when host plants exude strigolactones through their roots
- 35. Ethylene • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection • The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening
- 36. The Triple Response to Mechanical Stress • Ethylene induces the triple response, which allows a growing shoot to avoid obstacles • The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth
- 37. Ethylene concentration (parts per million) 0.00 0.10 0.20 0.40 0.80 triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth
- 38. Senescence • Senescence is the programmed death of cells or organs • A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
- 39. Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
- 40. 0.5 mm Stem Petiole Protective layer Abscission layer
- 41. Fruit Ripening • A burst of ethylene production in a fruit triggers the ripening process • Ethylene triggers ripening, and ripening triggers release of more ethylene • Fruit producers can control ripening by picking green fruit and controlling ethylene levels
- 42. Responses to light are critical for plant success • Light cues many key events in plant growth and development • Effects of light on plant morphology are called photomorphogenesis
- 43. • Plants detect not only presence of light but also its direction, intensity, and wavelength (color) • A graph called an action spectrum depicts relative response of a process to different wavelengths • Action spectra are useful in studying any process that depends on light
- 44. • Different plant responses can be mediated by the same or different photoreceptors • There are two major classes of light receptors: blue-light photoreceptors and phytochromes
- 45. Blue-Light Photoreceptors • Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism
- 46. Phytochromes as Photoreceptors • Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life • These responses include seed germination and shade avoidance Phytochromes and Seed Germination • Many seeds remain dormant until light conditions change
- 47. RESULTS Red Red Red Red Red Red Far-red Far-redFar-redFar-red Dark (control) Dark Dark Dark How does the order of red and far-red illumination affect seed germination? Red light increased germination, while far-red light inhibited germination The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome.
- 48. Phytochromes and Shade Avoidance • The phytochrome system also provides the plant with information about the quality of light • Leaves in the canopy absorb red light • Shaded plants receive more far-red than red light The red mulch reflects far-red light from the ground to the tomato plants. The plants, absorbing the extra far- red light, react as if nearby plants are present.
- 49. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions
- 50. Sleep movements of a bean plant (Phaseolus vulgaris). Noon Midnight
- 51. • Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” • Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle • The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes
- 52. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues Photoperiodism and Responses to Seasons • Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year • Photoperiodism is a physiological response to photoperiod
- 53. Critical Night Length • In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
- 54. Photoperiodism and Control of Flowering What is the pattern? • many plants need a particular daylength for flowering • we call necessary light duration the critical period • the critical period is the number of hours in the light in a 24 hour day • some plants flower if light period is more than critical period: long-day plants (LDP) • some plants flower if light period is less than critical period: short-day plants (SDP) • remember that some plants are day neutral (day- neutral plants) controlled by plant maturity, not by photoperiod
- 55. 24 hours Light Flash of light Darkness Critical dark period Flash of light (b) Long-day (short-night) plant (a) Short day (long-night) plant Photoperiodic control of flowering.
- 56. • Some plants flower after only a single exposure to the required photoperiod • Other plants need several successive days of the required photoperiod • Still others need an environmental stimulus in addition to the required photoperiod – For example, vernalization is a pretreatment with cold to induce flowering
- 57. A Flowering Hormone? • Photoperiod is detected by leaves, which cue buds to develop as flowers • The flowering signal is called florigen • Florigen may be a macromolecule governed by the FLOWERING LOCUS T (FT) gene