Lessons from the root apex - Intercellular Communication in Plants - page 219

periclinal cell divisions associated with the formation of phloem poles (Plate 8.1)
(Bonke et al. , 2003). Recently, a couple of informative Arabidopsis pattern forma-
tion mutants have been able to shed some light on the underlying mechanisms of
vascular patterning in roots. In the wooden leg ( wol ) mutant cell divisions in the
vascular bundle are reduced, leading to a phenotype in which only approximately
9-11 cells occupy the vascular cylinder and after germination all vascular cells
differentiate into protoxylem. This phenotype is first seen during the torpedo stage
of embryogenesis (Scheres et al. , 1995; Mahonen et al. , 2000). When wol was intro-
duced into the fass background, phloem was restored. Thus, WOL does not appear
to directly influence cell fate within the vascular bundle but has a more indirect
influence by controlling the number of cells in the vascular cylinder.
WOL encodes a two-component hybrid-type receptor molecule (Mahonen et al. ,
2000) and is identical to CRE1/AHK4 (Inoue et al. , 2001; Suzuki et al. , 2001), a
cytokinin receptor. It is expressed in the vascular initials already during the glob-
ular stage of embryogenesis, linking cytokinin signalling to vascular embryonic
development.
Recently, Bonke et al. (2003) identified the gene ALTERED PHLOEM DEVEL-
OPMENT ( APL )asaMYB-CC transcription factor that is required for phloem
development throughout the plant. In the apl mutant, phloem patterning is affected;
the phloem-specific cell divisions occur less frequently and the cells in the phloem
pole area take on a xylem identity.
APL has a complex gene expression pattern mirroring the dynamic nature of
phloem development. It is expressed in developing protophloem and also further
up in the companion cells and metaphloem. APL expression can first be detected
during embryogenesis. Even though the phloem-specific asymmetric divisions are
delayed in the apl mutant, APL expression is initiated only after these have occurred.
This could indicate that APL acts as a cell non-autonomous factor to control these
divisions. However, a GFP-APL protein fusion appears to be expressed in a spatially
similar manner as APL RNA, indicating that APL itself probably does not act as the
cell non-autonomous factor. It is also possible that metabolic defects resulting from
loss of functional phloem may be the cause for the delay in the phloem-specific cell
divisions (Bonke et al. , 2003).
APL expression driven by the procambium-specific WOL promoter showed that
ectopic APL expression is able to fully suppress xylem differentiation in the pro-
toxylem pole position and to some extent also in the metaxylem position. Impor-
tantly, the affected protoxylem cells retained their nucleus, which indicates that
they did not change fate to phloem identity, meaning that APL is necessary, but not
sufficient, for phloem identity. It has recently been established that in the aerial part
of the plant, class III HD-ZIP and KANADI family transcription factors regulate
the distribution pattern of xylem and phloem in stems and leaves (Bowman et al. ,
2002; Emery et al. , 2003). These genes however do not lead to phloem defects. It
remains to be studied if the class III HD-ZIP and KANADI genes regulate APL.
The exact mechanisms of vascular patterning are still relatively poorly under-
stood, but the results so far seem to point to a temporal model where auxin is