Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 158

CLV1/2 receptor complex is not only activated by CLV3 but also sequesters it in the
region above the organising centre, thus preventing CLV signalling in underlying
cells (Lenhard & Laux, 2003). This work also shows that fluorescently labelled
CLV3 protein accumulates in regions of the meristem that do not express CLV3 .
The non-cell autonomous accumulation of CLV3 apparently serves two purposes:
it prevents lateral expansion of the organising centre and promotes the transition
of central zone cells into the peripheral zone where they eventually differentiate
(Lenhard & Laux, 2003).
Despite the progress in understanding how stem cells are maintained in the meris-
tem, fundamental questions remain. For instance, how does WUS signal stem cell
identity in overlying cell layers? How many and what function do other RLKs play
in meristem homeostatis? In addition, what is the role of the recently identified
CLV3 -like ( CLE ; Cock & McCormick, 2001) and WUS -like ( WOX ; Haecker et al. ,
2004) genes in meristem function? To date, only one CLE gene has been examined
in detail ( CLE40 ; Hobe et al. , 2003) and this showed that targeted expression of
CLE40 in the shoot apex activates the CLV pathway. However, as CLE40 expres-
sion is broader than CLV3 expression, and does not accumulate to high levels in the
shoot apex, it is likely that CLE40 and CLV3 have different functions. Interestingly,
ectopic expression of CLE40 and CLV3 results in a loss of the root apical meris-
tem, suggesting that CLV signalling may occur in the root (see also Chapter 7, this
volume). Of the 14 WOX genes that have recently been reported in the Arabidopsis
genome, a subset is expressed in developing embryos and may therefore be involved
in early patterning events (Haecker et al. , 2004). This idea is supported by the find-
ing that wox2 mutants have aberrant divisions in the apical region of the embryo.
Mutations affecting other WOX genes do not affect embryo development, perhaps
suggesting that this family is highly redundant.

6.5 Maintaining indeterminate cells in the meristem requires

homeobox genes

Superimposed on the CLV signalling pathway are other pathways that keep meristem
cells in an indeterminate state until they reach the peripheral zone, where they are re-
cruited into organ formation. A highly conserved family of homeodomain transcrip-
tion factors encoded by the KNOX ( KNOTTED1 -like homeobox) genes perform this
function in the meristems of diverse species (see Reiser et al. , 2000). The founding
member of this family is the maize KNOTTED1 gene, which is expressed throughout
the meristem but excluded from founder cells and organ primordia (Vollbrecht et al. ,
1991; Jackson et al. , 1994). The close correlation between undifferentiated cells of
the meristem and KN1 expression suggests that KN1 may function by repressing
organ identity in the meristem and/or keeping meristem cells in an undifferentiated
state. Consistent with this is the finding that ectopic expression of KN1 or other
members of the KNOX family in leaves results in a less determinate pattern of leaf
development and may, if transcript levels are sufficiently high, induce the formation
of ectopic shoots (reviewed in Reiser et al ., 2000).