Agriculture Reference
In-Depth Information
IAA
−
Cytosol
Plasma membrane
Cell wall
?
pH 5.5
- Influx carrier
AUX1?
IAA
−
+
IAAH
H
+
pH 7.0
- Efflux carrier
PIN?
IAA
−
+
H
+
IAAH
- Unstable
component
?
IAA
−
IAA
−
- NBP
IAA
−
?
IAA
−
IAA
−
Figure 1.3
Model for cell-to-cell polar auxin transport. According to the chemiosmotic model, a
membrane pH gradient (maintained by plasma membrane H
+
-ATPases) drives accumulation of IAA
within cell. At the higher pH of the cytoplasm, some of the protonated auxin molecules dissociate and
are 'trapped' inside the cell and can only exit by efflux carrier systems. Asymmetry in the distribution
of the efflux carrier results in a directional transport of auxin through the cell file. Efflux carrier is a
protein complex containing auxin efflux catalyst and NPA-binding protein (NBP) linked with an
unstable component.
et al.
, 1998; Muller
et al.
, 1998; Swarup
et al
., 2001; Friml
et al.
, 2002a,b). These
findings provide a molecular confirmation that at least in some tissues, influx and
efflux in concert facilitate the polar translocation of auxin (Friml, 2003).
1.2.3
Multicomponent auxin efflux carrier system
Subsequent physiological studies, especially those focused on the mechanism of
the effect of AEIs such as TIBA or NPA on PAT, provided important insights into
the composition of the auxin efflux carrier complex. Finding that plant protein ex-
tracts specifically bind NPA, taken together with the NPA effect on auxin efflux,
