Lessons from the root apex - Intercellular Communication in Plants - page 205

Taken together, in each of these sets of experiments, cells had effectively under-
gone a post-embryonic change in their position and in response exhibited a change
in their developmental fate (van den Berg et al., 1995, 1997; Berger et al. , 1998;
Kidner et al. , 2000). This suggests that positional information is provided both em-
bryonically and post-embryonically to ensure appropriate cell specification in the
Arabidopsis root (Berger et al. , 1998; Kidner et al. , 2000). Below we will review
the molecular nature of such intercellular signalling.

8.4 Molecular genetics of root development

8.4.1 Distal patterning

Classical experiments already implicated auxin as a growth substance promoting de-
velopment of the whole root system. More recently, Sabatini et al. (1999) showed that
a promoter consisting of 7 tandem repeats of an auxin response element fused to the
GUS gene led to a maximum of GUS activity at the meristematic region where auxin
also accumulates, that is in the columella initials under the QC (Fig. 8.3). They also
showed that treatment with the chemical auxin transport inhibitor naphtylphtalamic
acid resulted in patterning defects, while introduction of a new auxin maximum else-
where in the root resulted in the formation of a new meristem. These results strongly
suggested that auxin is an important determinant of root meristem formation.
In mature plants, auxin is mainly produced in the shoot tissues and is most
likely transported to the site of action in two different manners: polar and non-polar
transport. Polar auxin transport is an active process involving specific efflux carriers.
Long-distance transport through the phloem, however, is thought to be non-polar.
This assumption is based on three arguments. Polar auxin transport is known to
have a velocity of approximately 10 mm/h, which would make it too slow to be an
efficient signalling method in large plant species. Also high concentrations of free
auxin have been measured in phloem exudates (Baker, 2000). Finally, aux1 mutants
are both unable to load auxin into the phloem in the shoot and unload auxin from
the phloem at the root. This means they cannot transport auxin between shoot and
root (Swarup et al. , 2001; Marchant et al. , 2002), indicating that AUX1, a putative
auxin permease, acts at both ends of the non-polar auxin transport route.
In embryos on the other hand, polar auxin transport seems to be the main mecha-
nism of achieving an auxin gradient. Throughout the formation of the embryo, auxin
seems to have an important role in defining the pattern formation. Already during
the earliest stages of embryogenesis, auxin is actively transported into the embryo
by the PIN7 efflux carrier (Friml et al. , 2003). PIN7 is localized at the apical mem-
brane of the basal cell just after the division of the zygote. Since in pin7 mutants the
apical cell fate is disrupted, it can be concluded that the actively maintained auxin
accumulation in the apical cell leads directly or indirectly to correct specification
of this apical cell (see also Chapter 1 for a more detailed exploration of the role of
auxin in signalling).