Auxin as an intercellular signal - Intercellular Communication in Plants - page 18

In roots, the situation is more complex, since gravity is perceived in the root cap
but the growth response occurs in the elongation zone where elevated auxin levels on
the lower side inhibit growth, resulting in downward bending (Chen et al. , 2002). It
seems that following gravity stimulation, auxin is redistributed laterally toward the
lower side of the root cap, from where it is transported to the elongation zone (Saba-
tini et al. , 1999; Rashotte et al. , 2000). Localization and mutant analyses suggest that
the auxin influx component AUX1 facilitates auxin uptake in the lateral root cap and
epidermis region and that PIN2 efflux regulator mediates directional auxin translo-
cation toward the elongation zone (M uller et al. , 1998; Swarup et al. , 2001). The
notion that both auxin influx and efflux are required for root gravitropism is further
supported by the experiments with inhibitors of both processes (Parry et al. , 2001).
But how is gravity perception linked to the initial lateral auxin redistribution in
the root cap? Gravity is perceived by sedimentation of starch-containing organelles
(statoliths) in the columella root cap cells (Chen et al. , 2002). PIN3 is localized in
these cells under normal growth conditions without any apparent asymmetry at the
cell boundaries. Intriguingly, when roots are gravistimulated, already within 2 min
PIN3 changes its position and relocates, presumably to the new lower cell boundary
(Plate 1.1M) (Friml et al. , 2002a). It is conceivable that the rapid recycling of PIN3
between endosomes and the plasma membrane along the actin cytoskeleton provides
a mean for its rapid retargeting in response to the environmental stimulus. However,
how the statolith sedimentation and PIN3 relocation are linked remains unclear. It
is possible that actin reorganization following the statoliths sedimentation would
redirect intracellular traffic of PIN3 along the sedimentation routes and PIN3 would
preferentially accumulate at the lower side of the cell. It remains to be demonstrated
whether PIN3 relocation also mediates the auxin redistribution in the shoot. How-
ever, it is likely, since both statoliths and PIN3 are present in the shoot endodermis
(Friml et al. , 2002a), which is essential for shoot tropism responses (Fukaki et al .,
1998). Also the link between light perception and lateral auxin redistribution during
phototropism remains a topic for future investigations.

1.5.6 Downstream of auxin gradients

Recent studies on a role of PAT in plant development revealed that local gradients
in auxin accumulation underlie the developmental responses to auxin. So far, this
mechanism could be demonstrated for embryonic axis and postembryonic organ for-
mation, meristem pattern maintenance and tropisms. However, how can the accumu-
lation of a structurally rather simple molecule as IAA lead to such a wide variety of
different responses? Since it is clear that different cells respond to auxin by activation
of different developmental programs, part of the answer lies downstream of auxin
gradients. The ability of auxin to bring about diverse responses appears to result from
the existence of several independent mechanisms for auxin perception and from a
complex transcriptional network at the lower end of the auxin signaling pathway.
So far, we know little about auxin perception, since, despite the fact that several
auxin-binding proteins (most prominent among them ABP1) have been isolated