Auxin as an intercellular signal - Intercellular Communication in Plants - page 7

successfully applied to identify mutants affected in PAT. Some mutants have been
selected on the basis of abnormal responses to auxin transport inhibitors or were
identified fortuitously in screens for developmental alterations and only later was
the connection to PAT discovered (Friml & Palme, 2002).

1.3.1 Auxin influx - AUX1 proteins
A mutant called auxin1 ( aux1 ), which confers a root agravitropic and auxin-resistant
phenotype, was instructive for identification of a gene possibly encoding an auxin
influx carrier. The AUX1 gene encodes a 485 amino acid long protein sharing signif-
icant similarity with plant amino acid permeases consistent with the role for AUX1
in the uptake of the tryptophan-like IAA (Bennett et al ., 1996). So far the defini-
tive biochemical proof of AUX1 function as an auxin uptake carrier is lacking, but
several lines of evidence (mainly based on detailed analysis of the aux1 phenotype)
strongly support that AUX1 is required for auxin influx. Strikingly, the aux1 root
agravitropic phenotype can be restored by treatment with a membrane permeable
auxin NAA in contrast to less permeable 2,4-D. Moreover, this rescue coincides
with restoration of basipetal auxin transport, which is defective in aux1 (Yamamoto
&Yamamoto, 1998; Marchant et al ., 1999). In addition, the main features of the
aux1 phenotype can be mimicked by growing seedlings on inhibitors of auxin influx
(Parry et al ., 2001). Other evidence that AUX1 participates in auxin influx came
from auxin uptake assays in aux1 and wild-type roots. They revealed that aux1 roots
accumulated significantly less radioactively labeled 2,4-D than did wild-type, and
that this difference was not found when the membrane-permeable 1-NAA or the
IAA-like amino acid tryptophan were assayed (Marchant et al ., 1999). Recently, the
AUX1 protein was localized within Arabidopsis root tissue (Swarup et al ., 2001).
The AUX1 protein was detected in a subset of stele, columella, lateral root cap
and epidermal cells exclusively in root tips. Considering the localized expression
of AUX1 only in root tips, it is surprising that aux1 mutant root tips contain lower
auxin levels, which rather suggests defects in long-distance supply to the root tip
(Swarup et al ., 2001). This paradox, taken together with localization of AUX1 at
the upper side of protophloem cells (see Plate 1.1A, following page 146), suggests a
role of the AUX1 protein in unloading of the phloem flow via the protophloem to the
root apical meristem (Swarup et al ., 2001). Thus, AUX1 would appear to provide a
molecular connection between nonpolar and polar auxin transport routes. AUX1 is
a member of the small gene family in Arabidopsis .However, the characterization
of the three other LIKE AUX1 ( LAX ) genes has not yet been reported.

1.3.2 Auxin efflux - PIN proteins

Another Arabidopsis mutant, pin-formed ( pin1 ), with its characteristic needle-like
stem had already been functionally associated with auxin efflux on the basis of its dra-
matic morphological aberrations, which can be phenocopied by inhibition of auxin
efflux. In addition, pin1 inflorescences show a drastic reduction in basipetal auxin
transport (Okada et al ., 1991). The PIN1 gene was cloned by transposon tagging