Lessons from the root apex - Intercellular Communication in Plants - page 209

2003). The hobbit ( hbt ) mutation, which has a slightly later but similar phenotype
to the mp , bdl and axr6 mutants, is located in a subunit of the anaphase promot-
ing complex (APC), another ubiquitin protein ligase class. hbt mutants accumulate
IAA17, which suggests that the APC regulates stability of the AUX/IAA proteins
(Blilou et al. , 2002).
On the basis of the studies above, auxin may be considered as a positional cue
to specify the position of the QC and columella initials. In embryos the cells of the
QC and columella are descendants of the hypophyseal cell. In the mp , bdl and axr6
mutants (in which auxin signalling is disturbed) this cell fails to take the hypophyseal
identity (Friml et al. , 2003). A similar defect can be seen in the hbt mutant, which
shows loss of QC and root meristem (Blilou et al. , 2002).
Currently it is not known whether this failure to differentiate could be the result
of not receiving the auxin signal, not having normal auxin signal transduction in
the uppermost suspensor cell or due to a combination of these reasons. Also, it is
currently unknown whether the same pathways are active both during embryogenesis
and after germination.
In addition to auxin, other types of signals have been implicated in regulating the
integrity of the root meristem. As described above, in post-embryonic roots it has
been established that the QC plays an important role in the maintenance of the root
meristem. Ablation of a QC cell leads to differentiation of the initial cells previously
attached to it (van den Berg et al. , 1997). Observations on mutants missing post-
embryonic root cell divisions further supported the view that the QC functions by
inhibiting differentiation of the surrounding initial cells (van den Berg et al. , 1997).
Umeda et al. (2000) came to a similar conclusion after examination of plants with
altered CAK (cyclin-dependent kinase-activating kinase) expression, which showed
differentiation of the columella initials. The role of QC as a regulatory unit to keep
the surrounding cells undifferentiated can be compared to the function of certain
cells in the shoot apical meristem that express the WUSCHEL ( WUS ) gene (Laux
et al. , 1996). Together with the CLAVATA receptor complex (Clark et al. , 1993)
WUS is responsible for the maintenance of meristem function. The current model
of the CLAVATA signalling pathway is as follows. CLV3 protein is secreted by the
shoot meristem stem cells and some of it moves to the surrounding cells. In both
the stem cells and the cells that receive CLV3, it is bound by the CLV1 receptor and
as a result CLV3 represses the WUS promoter. In the daughter cells this means that
they cannot become stem cells themselves and they initiate differentiation. In the
organizing centre (OC) downward movement of CLV3 is restricted by strong CLV1
expression, which sequesters the ligand and prevents it from moving further. This
allows for WUS expression in the OC, ensuring that the stem cell population and
activity is maintained (Lenhard & Laux, 2003; see also Chapter 6, this volume).
In roots, however, there have only recently been some hints of a CLAVATA-
like pathway, which influences meristem maintenance (Casamitjana-Martinez et al. ,
2003). Over-expression of the CLV3 homologue CLE19 in the root meristematic
zone leads to a loss of meristematic cells and a reduced cell elongation zone. While
QC and initial cell identity is maintained, their activity is also lessened. A similar