The Plant Vascular System: Evolution, Development and FunctionsF

316 Journal of Integrative Plant Biology Vol. 55 No. 4 2013
Protoxylem vessels in the root mature before the surrounding
tissues elongate; during cell expansion of these surrounding
cells, these protoxylem vessels are often destroyed. Thus, the
metaxylem vessels act as the primary water conducting tissue
throughout the main body of the plant (Esau 1965b). Metaxylem
cell differentiation is temporally separated from protoxylem
differentiation in that the outer metaxylem cells differentiate
only after protoxylem cells differentiate and the surrounding
tissues have completed their expansion. The inner metaxylem
vessel differentiates later than the outer two metaxylem cells.
Phloem tissue is composed of three cell types: protophloem
SEs to the outside, and metaphloem SEs to the interior of
the vascular cylinder, with CCs flanking the SEs. Protophloem
SEs differentiate earlier than the metaphloem SEs and their
associated CCs.
Detailed anatomical studies of the Arabidopsis root tip have
elucidated the earliest events in the timing and patterning of
vascular initial cell divisions that give rise to all vascular cell
types in the primary root (Mähönen et al. 2000). Just above
the QC, asymmetric cell divisions of vascular initial cells give
rise to the presumptive pericycle layer and protoxylem cells.
At a position close to the QC (∼9 µm), five xylem cells are
visible, and these will eventually differentiate into protoxylem
and metaxylem vessels (Figure 11A). Two domains of vascular
initials give rise to the phloem and procambial cell lineages,
and they are located between 3 µm and 6 µm above the
QC (Mähönen et al. 2000; Bonke et al. 2003). The number
and exact pattern of future procambial cell divisions is variable
between individual plants of the same species.
The full set of phloem cells (protophloem, metaphloem
and CC) can be observed at a distance above the QC
(∼27 µm) (Mähönen et al. 2000) (Figure 11A). Protophloem
and metaphloem SEs result from one tangential division of
precursor cells, whereas CCs arise from one periclinal division
of precursor cells (Bonke et al. 2003). At a further distance
above the QC (∼70 µm), the first histological evidence of
differentiation can be observed in protophloem SEs, as determined
by staining with toluidine blue (Mähönen et al. 2000).
Thus, protophloem SE differentiation occurs much earlier in
developmental time compared to protoxylem vessel formation
(Figure 11A). Metaphloem SEs and CCs differentiate at an
approximately similar time to the outer metaxylem SEs. However,
morphological analyses have determined that the spatial
patterning of xylem cells occurs temporally prior to the spatial
patterning of the phloem cells within the root.
Vascular proliferation—cytokinin signaling
Vascular initial cells or stem cells are the progenitor cell type for
all vascular cells within the primary root. Regulation of vascular
initial cell division is the first step in vascular development
and is accomplished, in part, by the two-component cytokinin
receptor WOL (Mähönen et al. 2000). WOL is expressed early
in the Arabidopsis embryo during the globular stage and is
present throughout the vascular cylinder during all subsequent
stages of embryo and primary root development (Figure 11B).
Interestingly, vascular defects within the embryonic root have
not yet been reported. In the primary root of a wol mutant,
there are fewer vascular initial cells, and the entire vascular
bundle differentiates as protoxylem. Although this suggests
that wol is deficient in procambial, metaxylem vessel and
phloem cell specification, a double mutant between wol and
fass (which results in supernumerary cell layers) produces
phenotypically normal procambial and phloem cells, as well as
both protoxylem and metaxylem vessels. This demonstrates
that the role of WOL is in vascular initial cell proliferation, and
that any influence on cell specification is secondary to this
defect.
Transcriptional master regulators
and xylem development
Xylem cell differentiation, as marked by secondary cell
wall synthesis and deposition, occurs much later in root
developmental time relative to protophloem cell differentiation
(Figure 11A). However, cells destined to become xylem cells
are morphologically identifiable immediately after division of
vascular initial cells. Based on gene expression data, a downstream
regulator of cytokinin signaling, the AHP6, an inhibitory
pseudophosphotransfer protein, is likely one of the earliest
regulators of protoxylem cell specification (Mähönen et al.
2006), but is unlikely to be the sole regulator (Figure 11B). AHP6
functions to negatively regulate cytokinin signaling through
spatial restriction of signaling within protoxylem cells. In a
wol mutant, therefore, there is a lack of cytokinin signaling,
a decrease in the asymmetric division of vascular initial cells
and ectopic protoxylem cell differentiation in the few remaining
vascular cells. AHP6 acts in a negative feedback loop with
cytokinin signaling – cytokinin represses AHP6 expression,
while AHP6 represses and spatially restricts cytokinin signaling
(Mähönen et al. 2006). Cytokinin regulates the spatial domain
of AHP6 expression in embryogenesis prior to when primary
root protoxylem differentiation occurs. Thus, it appears that
this negative regulatory feedback between cytokinin and AHP6
occurs upstream of protoxylem specification in the primary root
(Mähönen et al. 2006).
The earliest marker of protoxylem cell specification in the primary
root is achieved through a TARGET OF MONOPTEROS
5 (TMO5) promoter:GFP fusion, named S4 (Lee et al. 2006;
Schlereth et al. 2010). TMO5 is required for embryonic root
initiation, and expression of this bHLH transcription factor is
turned on shortly after division of vascular initial cells in the
primary root and is turned off prior to secondary cell wall
differentiation in protoxylem cells. This marker then turns on