The Plant Vascular System: Evolution, Development and FunctionsF

Microarray analyses of gene expression have revealed a
simultaneous expression of many genes involved in both secondary
wall formation and PCD (Demura et al. 2002; Milioni
et al. 2002; Kubo et al. 2005; Pesquet et al. 2005; Ohashi
et al. 2010). As mentioned above, recent results have demonstrated
that a transcriptional regulatory system, composed of
transcription factors such as VND6 and a TERE-cis sequence,
regulates the simultaneous expression of genes related to
both secondary wall formation and PCD in tracheary elements
(Ohashi-Ito et al. 2010). These findings indicate that tracheary
element-differentiation-inducing master genes initiate at least
a part of PCD directly by activating PCD-related genes through
binding the TERE sequence in their promoters. This suggests
that, in contrast to apoptosis in animals, in which a common
intracellular signaling system induces PCD, in plants, various
developmental processes involving PCD may be regulated independently
involving their own specific developmental steps.
Nitric oxide (NO) and polyamine have been suggested as
signals involved in the cell death induction in xylem development.
NO production is largely confined to xylem cells; removal
of NO from the cultured Zinnia cells, with its scavenger PTIO,
results in dramatic reductions in both PCD and in the formation
of tracheary elements (Gabaldon et al. 2005). Thus, NO might
well be an important factor mediating PCD during tracheary
element differentiation.
Execution of PCD in developing tracheary elements involves
expression and vacuole-accumulation of several hydrolytic
enzymes, such as the cysteine proteases XCP1 and XCP2
(Zhao et al. 2000; Funk et al. 2002; Avci et al. 2008), the Zn 2+ -
dependent nuclease ZEN1 (Ito and Fukuda 2002) and RNases
(Lehmann et al. 2001). Ca 2+ -dependent DNases were also
detected in secondary xylem cells, and their activity dynamics
were closely correlated with secondary xylem development
(Chen et al. 2012a). ATMC9 encodes a caspase-like protein,
which does not function as a caspase but as an arginine/lysinespecific
cysteine protease (Vercammen et al. 2004). ATMC9
was specifically expressed in differentiating vessels but not
in fully-differentiated vessels (Ohashi-Ito et al. 2010). This
result suggested the involvement of ATMC9 in PCD. However,
application of caspase inhibitors significantly delays the time of
tracheary element formation and inhibits DNA breakdown and
appearance of TUNEL-positive nuclei in Zinnia xylogenic cell
culture (Twumasi et al. 2010).
Recently, the protease responsible for developing xylemrelated
caspase-3-like activity was purified and identified to
be 20S proteasome (Han et al. 2012). The fact that treatment
with a caspase-3 inhibitor Ac-DEVD-CHO causes a defect in
veins in Arabidopsis cotyledons, and the proteasome inhibitor
clasto-lactacystin β-lactone delays tracheary element PCD in
VND6-induced Arabidopsis xylogenic culture, strongly suggest
that the proteasome is involved in PCD during this process of
differentiation (Han et al. 2012). Consistent with this notion, the
Insights into Plant Vascular Biology 311
26S proteasome inhibitors lactacystin and MG132 also delay
or block the differentiation of suspension-cultured tracheary
elements (Woffenden et al. 1998; Endo et al. 2001). Autophagy
has also been suggested to be involved in tracheary element
PCD (Weir et al. 2005). A recent finding that a small GTP
binding protein RabG3b plays a positive role in PCD during tracheary
element differentiation by activating autophagy (Kwon
et al. 2010) provided support for this notion.
The PCD of xylem fibers is less well characterized compared
to that of xylem tracheary elements, likely due to the fact that
this process proceeds slowly in these cell types. Microarray
analyses revealed that a large number of genes encoding
previously-uncharacterized transcription factors, as well as
genes involved in ethylene, sphingolipids, light signaling and
autophagy-related factors, are expressed preferentially during
xylem fiber development (Courtois-Moreau et al. 2009). Further
comparison of genes related to PCD between xylem fibers
and tracheary elements, in a model species like poplar, may
help in advancing our understanding of PCD as it occurs in
plants.
Control over master transcription factors and crosstalk
between signaling pathways
It is now well established that xylem cell differentiation is
regulated by various factors, both at the cell-autonomous and
non-cell-autonomous level. Auxin, cytokinin, brassinosteroids
and CLE peptides act, cooperatively, at different stages of
xylem cell differentiation. Importantly, an as-yet-unidentified
intracellular signaling system initiates the expression of genes
for master transcription factors such as VND6, VND7 and
SND1/NST1, each of which in turn induces distinctive xylem
cell-specific gene expression. Further advances in our understanding
of the events underlying xylem differentiation will
be gained by studies on the related intracellular signaling
pathways and the nature of the crosstalk that occurs between
these specific signaling pathways.
Spatial & Temporal Regulation
of Vascular Patterning
Vascular organization in leaves
The leaf vascular system is a network of interconnecting
veins, or vascular strands, consisting of two main conducting
tissue types: the xylem and the phloem. While the specialized
conducting elements are composed of tracheary or vessel
elements in the xylem and SEs in the phloem, the vascular
system also contains non-conducting supporting cells, such as
parenchyma, sclerenchyma and fibers. Thus, the development
of cell types within the radial arrangement of the vascular
bundle must be precisely spatially coordinated along with the