Lessons from the root apex - Intercellular Communication in Plants - page 207

directing auxin out of the embryo (Friml et al. , 2003). The auxin flow in the suspensor
is reversed, leading to a new auxin maximum in the topmost suspensor cell, which
becomes the hypophysis. Since pin7 mutants start to recover the apical-basal axis at
this stage, it could be concluded that PIN7 function becomes redundant at this stage.
At the late globular stage, PIN4 expression is first detected along the surface of the
hypophysis and at the basal end of the adjacent suspensor cell. It is thought to support
PIN1 and PIN7 functions. During the later stages of embryogenesis, its expression
pattern shifts slightly, disappearing from the suspensor cell, and appearing in the
provascular cells. Following the expression pattern of PIN4 , the DR5 marker was
visualized from the early heart stage onwards in the basal part of the embryo. In
pin4 mutants this expression pattern was altered significantly. The DR5 expression
domain was enlarged towards all sub-epidermal cells of globular stage embryos, and
in later stages, the expression domain was mostly located in the presumptive vascular
tissue, suggesting that PIN4 is necessary for the regulation of both positioning and
levels of the auxin response maximum during embryogenesis (Friml et al. , 2002).
Since there seems to be some functional redundancy between the different em-
bryonically expressed PIN efflux carriers, Friml et al. (2003) constructed a set of
double, triple and quadruple loss of function mutants of combinations of pin7 with
pin1 , pin3 and pin4. These mutants showed a series of defects that became increas-
ingly severe according to the number of knocked out PIN genes. Quadruple mutants
completely failed to recover the embryonic axis and showed strong gnom -like de-
fects. In the most severe cases the mutants were ball shaped, completely lacking
apical-basal polarity. Similar defects could be achieved in wild-type embryos by
interfering with the auxin homeostasis by chemical inhibition of auxin transport.
These results suggest that the recovery of the apical-basal axis at the globular stage,
which can be seen in the pin7 mutant, is also caused by a PIN-dependent auxin
distribution. This mechanism seems to be non-redundant.
In mature roots, PIN4 is localized in a polar manner at the basal ends of the
proximal meristem, suggesting an auxin flow through the vascular cylinder which
is directed towards the QC and columella initials. Both inhibition of auxin transport
with NPA and via pin4 mutants disrupted the DR5 expression pattern and shifted its
maximum to the cells in the vascular tissue that normally display polar localization
of PIN4 (Sabatini et al. , 1999; Friml et al. , 2002). This suggests that PIN4 drives
the formation of the auxin maximum in the root tip and that in the root tip auxin
is conducted almost exclusively through polar transport. Besides the change of the
location of the auxin maximum, cell fate within the root apical meristem (RAM) was
also changed. Several markers for QC were misexpressed or not expressed at all in
pin4 mutants. The cells at the location of the QC, columella initials and endodermis
also showed irregular cell divisions in the mutants. In addition, cells at the location
of and slightly above the new maximum acquired features of columella initials and
QC respectively, linking the auxin maximum to cell fate (Friml et al. , 2002).
Sterols have also been implicated in playing a role in embryo development. The
first evidence for their involvement was a mutation in the FA CKEL ( FK ) gene, which
encodes a sterol C-14 reductase that acts in the brassinosteroid (BS) biosynthesis