EEB 162 Lecture 15 - Auxins
Exam Latest Update
*What are the 5 classical plant growth regulators (hormones)? - Answer 1) Auxin
2) Gibberellins
3) Cytokinins
4) Ethylene
5) Abscisic acid (ABA)
Why is auxin required in plants? - Answer - Auxin, with cytokinin, is required for viability
- No mutants lacking either auxin or cytokinin have been found
*What are the "stress" hormones"? - Answer Abscisic acid
Ethylene
Jasmonic acid
How was auxin response first discovered? - Answer - Darwin and his pointed to an auxin
response (phototropism of the grass coleoptile)
- Coleoptile will grow towards the light, but if it's covered, this response is blocked
- Concluded that there was a signal that was produced in the tip and moved to the
growth zone
How was auxin finally acquired? - Answer - Went showed that a chemical could be
collected from coleoptiles → causes bending in the absence of light
- Substance was named auxin ("to grow" in Greek)
What is auxin's chemical identity? - Answer *Indole-3-acetic acid (IAA)*
Most abundant and physiologically important
In which parts of the plant is IAA biosynthesis observed? - Answer - Associated with
rapidly dividing and growing tissues, especially shoots
- All plant tissues appear capable of producing low levels of IAA, but shoot apical
meristems and young leaves are the primary synthesis sites, as well as root apical
meristems
, What happens to IAA after biosynthesis? - Answer - In young leaves, IAA accumulates at
the margins of leaves and directs vascular differentiation
- IAA is transported into the cytosol and into the nucleus → binds to a receptor →
complex regulates gene expression
What is axial polarity, and where is it observed? - Answer - Depends on the polarity of
auxin transport
- Independent of orientation w.r.t. gravity
- Observed in excised tissues
What causes an auxin gradient to develop? - Answer Polar transport
*Which are the "growth" hormones? - Answer Auxin
Gibberellins
Cytokinins
*What are the 3 other recently discovered plant hormones? - Answer Brassinosteroids
Jasmonate
Salicylic acid
*Which hormones are volatile compounds? - Answer Ethylene
Jasmonate
Salicylic acid
*What is polarized auxin transport? - Answer - Proceeds in a cell-to-cell fashion rather
than via the symplast
- Auxin exits the cell through the plasma membrane → diffuses across the cell wall →
enters the next cell through its plasma membrane
- Velocity of polarized auxin transport is faster than diffusion but slower than phloem
translocation
- Major site is the vascular parenchyma tissues
*Why does polarized auxin transport require energy? - Answer Depends on proton
pumping out of the cell into the cell wall (chemiosmotic model)
Describe what occurs during polarized auxin transport. - Answer 1) IAAH diffuses into
the cell, OR IAA- is pumped into the cell by secondary active transport, via an H+-IAA-
symporter
Exam Latest Update
*What are the 5 classical plant growth regulators (hormones)? - Answer 1) Auxin
2) Gibberellins
3) Cytokinins
4) Ethylene
5) Abscisic acid (ABA)
Why is auxin required in plants? - Answer - Auxin, with cytokinin, is required for viability
- No mutants lacking either auxin or cytokinin have been found
*What are the "stress" hormones"? - Answer Abscisic acid
Ethylene
Jasmonic acid
How was auxin response first discovered? - Answer - Darwin and his pointed to an auxin
response (phototropism of the grass coleoptile)
- Coleoptile will grow towards the light, but if it's covered, this response is blocked
- Concluded that there was a signal that was produced in the tip and moved to the
growth zone
How was auxin finally acquired? - Answer - Went showed that a chemical could be
collected from coleoptiles → causes bending in the absence of light
- Substance was named auxin ("to grow" in Greek)
What is auxin's chemical identity? - Answer *Indole-3-acetic acid (IAA)*
Most abundant and physiologically important
In which parts of the plant is IAA biosynthesis observed? - Answer - Associated with
rapidly dividing and growing tissues, especially shoots
- All plant tissues appear capable of producing low levels of IAA, but shoot apical
meristems and young leaves are the primary synthesis sites, as well as root apical
meristems
, What happens to IAA after biosynthesis? - Answer - In young leaves, IAA accumulates at
the margins of leaves and directs vascular differentiation
- IAA is transported into the cytosol and into the nucleus → binds to a receptor →
complex regulates gene expression
What is axial polarity, and where is it observed? - Answer - Depends on the polarity of
auxin transport
- Independent of orientation w.r.t. gravity
- Observed in excised tissues
What causes an auxin gradient to develop? - Answer Polar transport
*Which are the "growth" hormones? - Answer Auxin
Gibberellins
Cytokinins
*What are the 3 other recently discovered plant hormones? - Answer Brassinosteroids
Jasmonate
Salicylic acid
*Which hormones are volatile compounds? - Answer Ethylene
Jasmonate
Salicylic acid
*What is polarized auxin transport? - Answer - Proceeds in a cell-to-cell fashion rather
than via the symplast
- Auxin exits the cell through the plasma membrane → diffuses across the cell wall →
enters the next cell through its plasma membrane
- Velocity of polarized auxin transport is faster than diffusion but slower than phloem
translocation
- Major site is the vascular parenchyma tissues
*Why does polarized auxin transport require energy? - Answer Depends on proton
pumping out of the cell into the cell wall (chemiosmotic model)
Describe what occurs during polarized auxin transport. - Answer 1) IAAH diffuses into
the cell, OR IAA- is pumped into the cell by secondary active transport, via an H+-IAA-
symporter
