The Plant Vascular System: Evolution, Development and FunctionsF

Figure 7. Model of OCTOPUS (OPS) and ALTERED PHLOEM DEVELOPMENT (APL) action.
Insights into Plant Vascular Biology 305
(A) OPS is located at the apical plasma membrane in the procambium and phloem lineage. OPS interprets vascular signals for phloem
differentiation, such as APL.
(B) In the ops mutant, phloem differentiation is delayed. Procambial cell number is increased and gaps of undifferentiated cells are visible in
the protophloem strand.
(C) In the apl mutant, the initiation of phloem differentiation is largely unperturbed, however proper protophloem fails to emerge. In their
place, protophloem/protoxylem hybrid cells appear.
plants. This dramatic effect is restricted to the phloem poles;
other cell types appear similar to wild-type (WT) plants.
As mentioned above, asymmetrical cell divisions establish
the phloem poles in wild-type plants; periclinal divisions establish
CCs and tangential divisions establish SEs. In the apl
mutant, these cell divisions are often delayed, but they still
take place, so APL does not appear to be required for these
asymmetric cell divisions. However, since the subsequent
differentiation of SEs cannot be observed, it can be concluded
that APL is responsible for phloem differentiation, rather than
the establishment of the phloem cell lineage.
It has also been proposed that APL acts as an inhibitor to
xylem differentiation. In the apl mutant, ectopic xylem strands
are seen in the place of the phloem poles. When APL is
expressed ectopically in vascular bundles, xylem formation is
inhibited. Recently, novel imaging techniques were employed
to analyze the apl mutant in more detail (Truernit et al. 2008).
With this increased resolution, it was discovered that protophloem
differentiation proceeds normally in this mutant until
2 dag. At this time, cells in the protophloem position display the
normal characteristic shape and cell wall thickening. However,
after this period, the previously described acquisition of xylem
characteristics was observed, although the cells in place of
the SEs still formed sieve plates. This finding suggests that
these cells can be classified as hybrids between phloem and
xylem (Truernit et al. 2008). Thus, APL is absolutely required
for the later stages of phloem development, although it is
not the earliest factor acting in this process. The identity of
such a factor, or factors, has yet to be determined. Additional
support for the presence of such factor(s) was provided by Lee
et al. (2006), who pointed out that there are numerous phloem
markers with earlier SE expression than APL.