Auxin as an intercellular signal - Intercellular Communication in Plants - page 13

for visualization and manipulation of PAT components as well as for visualization of
auxin and its activity enabled researchers to address more specifically the role of PAT-
dependent auxin distribution in each of these developmental processes. These studies
convincingly demonstrated that regulated, local auxin acummulation underlies most
of auxin-mediated development.

1.5.1 Monitoring of auxin distribution in planta
One of the major obstacles in studies addressing the role of auxin in plant develop-
ment was the inability to visualize auxin distribution in planta .Animportant break-
through was brought through the discovery of genes that are rapidly upregulated by
auxin (reviewed in Hagen & Guilfoyle, 2002). The consensus sequence TGTCTC
(auxin response element, AuxRE) was identified within promoters of these genes,
which confers the auxin responsiveness (Ulmasov et al ., 1995). Multiple repeats
of the AuxREs yielded synthetic promoters DR5 or DR5rev (another variant with
inverse repeats), which are highly responsive to auxin and, therefore, can be used
for indirect monitoring of auxin levels. Indeed, multiple strategies were utilized
to show the correlation between DR5 activity and auxin accumulation. Firstly, di-
rect measurements of auxin content within Arabidopsis roots demonstrated elevated
levels at the tip, where the highest DR5 activity was also detected (Casimiro et al .,
2001). Secondly, exogenously supplied auxin was able to induce DR5 activity in all
cells and inhibition of PAT changed the DR5 expression pattern, suggesting that the
spatially restricted signals in untreated plants visualize differences in auxin levels
between cells. Finally, the accumulation of auxin itself (monitored using an anti-
IAA antibody) mirrors the DR5 pattern (Plates 1.1J and 1.1K). In summary, despite
the theoretical limitations for the use of DR5 and related tools, so far it seems that
DR5 activity can be used as a reasonable approximation for auxin levels at least in
embryonic and meristematic tissues. Indeed, the DR5::GUS and/or DR5rev::GFP
constructs have been instrumental in detecting the spatial pattern of auxin accumu-
lation in many tissues including Arabidopsis embryos (Plate 1.1I), roots (Plate 1.1J)
and organ primordia (Plate 1.1H), and have demonstrated a universal role for auxin
gradients in plant development (Sabatini et al ., 1999; Friml et al. , 2002a,b, 2003;
Benkova et al. , 2003).

1.5.2 Embryonic axis formation

Embryogenesis is a process that transforms a single-celled zygote into the embryo
containing all basic pattern elements of the future plant. The mature embryo dis-
plays an axis of polarity, with the shoot meristem at the apical end and root meristem
at the basal end. This remarkably uniform apical-basal pattern has been traced in
Arabidopsis back to the earliest stages of embryogenesis. The first manifestation
of the apical-basal axis is the asymmetric division of the zygote, which produces a
small apical cell and a larger basal cell. The apical cell divides vertically and gen-
erates the 'proembryo', which later gives rise to the most regions of the seedling.
The basal cell continues to divide horizontally and produces the suspensor - a file