Agriculture Reference
In-Depth Information
The data accumulated so far provide an extensive body of evidence to argue that
PIN proteins are involved in some important aspects of auxin transport, probably in
auxin efflux. Nevertheless, the central question of whether PIN proteins represent
transport or regulatory component of auxin efflux machinery still remains a topic
for future investigations.
1.3.3 ABC transporters
Recently, another protein family has been implicated in auxin transport - MDR
proteins, a subfamily of the ABC transporters (Noh
et al
., 2001). Members of this
family are known to enhance the export of chemotherapeutic substances in mam-
malian systems. Two of them, AtMDR1 and AtPGP1, were isolated by NPA-affinity
chromatography and were also able to bind NPA
in vitro
or when expressed in yeast
cells. Moreover, the corresponding mutants and double mutants show lower rate of
PAT and other phenotypic aberrations somewhat resembling defects in PAT. Never-
theless,
Atmdr1
mutants still exhibited up to 60% of the NPA binding found in wild
type and NPA is effective in reducing PAT to background levels in the mutant (Noh
et al
., 2001). So the question whether MDR/PGPs represent the elusive NPB, which
regulates PAT, remains open. A mechanism how the MDR/PGP proteins modulate
PAT has not been identified so far, but a direct auxin transport function or a positive
regulation of a PIN-type auxin efflux carriers has been suggested. Another possi-
bility is that MDR proteins are involved in correct localization of auxin carriers.
Indeed, it has been reported that in all cell types of
Atmdr1
and
Atpgp1
hypocotyls
the usual basal localization of PIN1 is replaced with more punctuate pattern (Noh
et al
., 2003). However, previous reports identified PIN expression associated only
with vascular tissue (Friml
et al.
, 2002a), and so the biological meaning of this
observation remains unclear.
1.4
Subcellular dynamics of auxin carriers
The widely accepted chemiosmotic model on PAT conveys the idea that auxin influx
and efflux carriers reside in the plasma membrane where they exhibit transport func-
tions. However, physiological studies already suggested that a fraction of the auxin
efflux complexes have a short half-life in the plasma membrane and cycle rapidly
through an unidentified intracellular compartment (Morris, 2000). These findings
were unexpected and difficult to reconcile with classical models. After isolation of
PAT components and with the availability of tools to visualize them, the subcellular
dynamics of these proteins was addressed directly using cell biological approaches.
1.4.1 Constitutive recycling of PIN proteins
The fungal toxin BFA is in both animal and plant systems a well-characterized com-
pound that interferes with subcellular vesicle trafficking. The molecular targets of
