Chapter 19: Auxin: The Growth Hormone Flashcards
phototropism
The bending of plants toward light. This phenomenon is caused by differential growth
The youngest leaves are sheathed in a protective
organ called the coleoptile.
Coleoptiles are very sensitive to light, especially to blue light
The Principal Auxin in Higher Plants Is
Indole-3-Acetic Acid
IAA biosynthesis is associated with rapidly dividing and rapidly growing tissues, especially in shoots. Although virtually all plant tissues appear to be capable of producing low levels of IAA, shoot apical meristems, young leaves, and developing fruits and seeds are the primary sites of IAA synthesis
IAA moves mainly from the apical to the basal end (basipetally) in excised oat coleoptile sections.
This type of unidirectional transport is termed polar transport. Auxin is the only plant growth hormone known to be transported polarly
The standard method for measuring polar auxin transport. The polarity of transport is independent of orientation with respect to gravity.
The longitudinal gradient of auxin from the shoot to the root affects various developmental processes, including stem elongation, apical dominance, wound healing, and leaf senescence
Polar Transport Requires Energy and Is Gravity
Independent
Roots grow from the basal ends of these bamboo
sections, even when they are inverted. The roots form at the basal end because polar auxin transport in the shoot is independent of gravity
Polar transport proceeds in a cell-to-cell fashion, rather than via the symplast.
That is, auxin exits the cell through the plasma membrane, diffuses across the compound middle lamella, and enters the cell below through its plasma membrane
The loss of auxin from cells is termed auxin
efflux; the entry of auxin into cells is called auxin uptake or influx.
The overall process requires metabolic energy, as evidenced by the sensitivity of polar transport to O2 deprivation and metabolic inhibitors.
Auxin influx. The first step in polar transport is auxin
influx. According to the model, auxin can enter plant cells from any direction by either of two mechanisms:
- Passive diffusion of the protonated (IAAH) form
across the phospholipid bilayer - Secondary active transport of the dissociated (IAA–) form via a 2H+–IAA– symporter
The dual pathway of auxin uptake arises because the passive permeability of the membrane to auxin depends strongly on the apoplastic pH.
The undissociated form of indole-3-acetic acid, in which the carboxyl group is protonated, is lipophilic and readily diffuses across lipid bilayer membranes
Plant Cell Wall
Determine plant structure
Cell Wall glues cells together
Exoskeleton
Bulk flow requires cell walls
Beta-D-glucan chain
generates Cellulose microfibril
CesA —–> Rosette subunit ——> Rosette
Cellulose (makes up cell wall)
UDP-G
Donates glucose to the growing glucan chain in cellulose synthesis
Tip growth
confined to apical domain
Diffused growth
spread growth across
turgor pressure
extends cell wall
Galactomannan
From Guar gum —- Thickening agent
