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Lecture 19

BIOL120 Lecture Notes - Lecture 19: Auxin, Cell Membrane, Systemic Acquired Resistance

Course Code
Simon Chuong

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Gravitropism – growth toward (+) or away from (-) gravity
Shoots grow away from gravity and show negative gravitropism
Roots grow toward gravity and therefore show positive gravitropism
How do plants sense gravity? – Three current hypotheses attempt to explain how plants sense and respond
to gravity
1. Starch-statolith hypothesis
2. Protoplast pressure hypothesis (gravitational pressure hypothesis)
3. Tensegrity model (“tensional integrity”)
1. Starch-statolith hypothesis – this hypothesis proposes that sedimentable amyloplasts play the role of
statoliths (gravity sensors)
Gravitropic Response
Following gravity perception, auxin is redistributed such that concentrations are
higher on the lower side of the root
Root cells are sensitive to higher auxin concentrations, slowing growth
Recent findings implicate the PIN3 protein (an auxin efflux carrier) in this
Thimotropism – growth in response to touch (from the Greek, thigma, “touch”)
Plants produce ethylene in response to touch
2. Protoplast pressure hypothesis
– this hypothesis proposes that
the weight of the entire mass of
the protoplast is involved in
gravity perception
3. Tensegrity Model – this hypothesis
proposes that amyloplast
sedimentation disrupts the cytoskeletal
mesh (actin filaments) causing a
transient influx of calcium
Ethylene inhibits elongation growth and stimulates growth in width
Tendrils produce ethylene on the side that contacts the object which slows growth on that side
The untouched side is unaffected and continues to grow
**Plant responses to touch can be rapid as seen in “sensitive plants”, Mimosa pudica
An electrical impulse propagates the signal along the length of the Mimosa leaf causing the
remaining leaflets to fold.
Leaf movement result from changes in water pressures of pulvini (special motor cells at the base
of the leaf) and often initiated by contact with objects outside of plant
Thigmotropic Responses – a similar mechanism regulates closing of a Venus fly trap in response to insect
activity (stimulation depolarizes the membrane causing hinge mesophyll cells to take up water)
Defense Responses
Plants suffer diseases caused by microbial pathogens
Secondary metabolites/natural products (alkaloids, phenolic and terpenes) defend plants against
both herbivores and microbial pathogens such as bacteria and fungi
It has been suggested that these secondary compounds evolved primarily to protect against
photodamage and climate changes
Disease-causing organisms elicit complex responses
Infection of fungi, bacteria or viruses induces local as well as systemic responses
Locally, molecules (elicitors) from the pathogen bind to specific receptors in the plasma
membrane leading to a localized reaction called a hypersensitive response (HR)
HR limits the spread of infection by causing localized cell death in combination with the
production of antimicrobial compounds
The localized cell death is “programmed” and results in a noticeable lesion on the leaf at the site of