Biol 126 - unit 5 - plant growth responses

Explain etiolation and de-etiolation

• Potato growing in darkness produces thin pale shoots with no leaves and few short roots

  • Roots and leaves are unnecessary at this point of a potato’s life

  • Morphological adaptations for growing in darkness, called etiolation

• If exposed to light, potato undergoes changes called de-etiolation, in which shoots and roots grow normally (informally called greening)

• De-etiolation activates enzymes that:

  • Function in photosynthesis

  • Supply chemical precursors for chlorophyll production

  • Affect levels of plant hormones that regulate growth

What are the 3 main stages of signal transduction pathway?

1. Reception - detected by receptors, proteins that change in response to stimuli

2. Transduction - second messengers transfer and amplify signals from receptors to proteins that cause responses

3. Response - regulation of cell activities, often by increased enzyme activity

What are plant hormones?

• Plant hormones (aka plant growth regulators) = chemical signals that modify or control one or more specific physiological processes

• Tiny amounts can have significant effects

• Coordinate and control growth, development, and responses to stimuli

  • Affect division, elongation, and differentiation of cells

• Effects depend on many factors, including stage of plant growth and hormone concentrations

Explain auxin

• Promotes cell elongation

• First discovered in phototropism experiments

• Produced mostly in  shoot tips (apical meristem)

  • and is transported down the stem (unidirectional transport, not gravity)

• According to the acid growth hypothesis, auxin stimulates proton pumps (H+) in plasma membrane

• Proton pumps lower pH in cell wall, activating expansins (enzymes that loosen cell wall’s fabric)

  • With cellulose loosened, cells can elongate

• Important in many aspects of plate spatial organization/architecture:

  • Role in phyllotaxy (arrangement of leaves on stems)

  • Role in leaf venation pattern

  • Role in the production of woody tissue

• Practical uses:

  • IBA stimulates adventitious roots and used in propagation of plants

  • An overdose of synthetic auxins can be used to kill weeds

    • 2, 4-D is used as an herbicide on dicots

Explain cytokinins

• Stimulate cytokinesis (cell division)

• Produced in actively growing tissues such as roots, embryos, and fruits

• Work together with auxin to control cell division and differentiation

  • Both hormones need to be at a specific concentration ratios for cell growth and differentiation

• Apical dominance

• Anti-aging effects: slow aging of some plant organs by:

  • Inhibiting protein breakdown

  • Stimulating RNA and protein synthesis

  • Mobilizing nutrients from surrounding tissues

Explain apical dominance

• Interaction between cytokinins, auxins, and strigolactone

• Auxin triggers the synthesis of strigolactone to repress bud growth

  • Cytokinins stimulates axillary bud growth

• If terminal bud removed (primary source of auxin), plant becomes bushier (inhibition of axillary buds is removed)

Explain gibberellins

• Stem elongation (along with auxin)

• Fruit growth (along with auxin)

• Produced in young roots and leaves

• Seed germination:

  • After water is absorbed, release of gibberellins from the embryo signals seeds to germinate

1. Water → gibberellin

2. Synthesis of digestive enzyme (eg. amylase)

3. Mobilized stored nutrients are used to support growth

4. Emerge from seed coat

Explain abscisic acid (ABA)

• Slows growth

• Two main effects:

1. Seed dormancy: Ensures that seed will germinate only in optimal conditions

2. Drought tolerance: Primary internal signal that enables plants to withstand drought

Explain ethylene

• Produced by all plant tissues in response to stresses, such as drought, flooding, mechanical stress, injury, and infection

• Four effects include:

1. Response to mechanical stress

• Induces triple response, which allows growing shoot to avoid obstacles

• Triple response consists of a slowing of stem elongation, a thickening of the stem and horizontal growth

2. Aging/senescence

• Senescence = programmed death of cells or organs

• Burst of ethylene associated with apoptosis, programmed destruction of cells, organs, or whole plant

3. Leaf abscission

• Cool temps in autumn change in balance of auxin and ethylene, controls leaf abscission

• Digestion of cell walls where leaf meets stem, healed over by corky leaf scar

4. Fruit ripening

• Burst of ethylene production in fruit triggers ripening process

  • Positive feedback mechanism

Response to light

• Light triggers many key events in plant growth and development

• Plant detect not only presence of light but also direction, intensity, and wavelength (colour)

• Action spectrum graph shows relative response of process to different wavelengths

  • Action spectra useful in studying any process that depends on light

  • Spectrum for blue-light-stimulated phototropism in maize cleoptiles

Two major classes of light receptors

• Two major classes of light receptors

  • Blue-light photoreceptors

  • Phytochromes

• Various blue-light photoreceptors control stem elongation, stomatal opening, and phototropism

• Phytochrome pigments regulate many plant responses to light, including seed germination and shade avoidance

Explain phytochromes

• Phytochromes exist in two photoreversible states (Pr (red) to Pfr (far-red light))

• Phytochrome conversion triggers many developmental responses

• Red light triggers conversion of Pr to Pfr

  • Happens throughout daytime

  • Stimulates cellular responses that lead to germination

• Far-red light triggers conversion of Pfr to Pr

  • Happens overnight

  • Inhibits germination response

Explain photoperiodism

• Photoperiod, relative lengths of night and day, is the environmental stimulus plants use the most to detect time of year

  • “Recorded” over time through conversion of phytochrome forms - marks sunrise and sunset to help set internal biological clock

  • Collectively “data” is used to detect season

• Photoperiodism = physiological response to photoperiod (seasonal changes in day length)

  • eg. flowering

explain photoperiodism - flowering

• Plants that flower when light period is shorter than a critical length called short-day plants

  • Flower in late summer, fall, and winter

• Plants that flower when light period is longer than a certain number of hours called long-day plants

  • Flower in late spring and early summer

• Flowering in day-neutral plants controlled by plant maturity, not photoperiod

• In 1940s, researchers discovered flowering and other responses to photoperiod controlled by night length, not day length

• Short-day plants (long night plants) governed by set minimum number of hours of darkness

• Long-day plants (short night plants) governed by set maximum number of hours of darkness

• A flash of red light shortens the dark period, but a subsequent flash of far-red cancels the red flash’s effect

• Photoperiod detected by leaves, which cue buds to develop as flowers

• Flowering sign called florigen (unidentified molecule)

Explain response to gravity

Response to gravity

• Aka gravitropism

• Roots show positive gravitropism (grow downwards)

• Shoots show negative gravitropism (grow upwards)

• Statolith hypothesis:

  • Plants may detect gravity by settling of statoliths, cytoplasmic components/plastids containing dense starch grains (other organelles likely involved too)

Explain response to touch

Response to touch

• Aka thigmotropism

• Occurs in vines and other climbing plants

• eg. rubbing stems of young plants twice daily results in plants shorter than controls

• eg. Mimosa pudica folds its leaflets and collapses in response to touch

• Due to loss of turgor in leaf motor cells on one side