The Plant Vascular System: Evolution, Development and FunctionsF

available regarding the regulation of vascular development.
The importance of signaling in the control of vascular morphogenesis
has become increasingly apparent. The primary agents
involved include well-known phytohormones such as auxin,
cytokinin and brassinosteroids, as well as other small regulatory
molecules. This level of understanding also involves the
transporters and receptors of these factors. There is increasing
evidence for the notion that xylem and phloem development
are highly coordinated. While many of the factors which will be
described in this section of the review act cell-autonomously,
the emerging importance of mobile regulatory factors will also
be highlighted.
Embryonic provascular and shoot vascular
development
During embryogenesis, the progenitor cells that will eventually
become the vascular tissues are first established as undifferentiated
procambial tissues. Surrounded by the epidermal and
ground tissue layers, this procambial tissue forms the innermost
domain of the plant embryo. By the late globular embryonic
stage, the four procambium cells have undergone periclinal
divisions to generate the future pericycle and the vascular
primordium. The complete root promeristem with all initials and
derived cell types is contained already in the early torpedo
stage embryo (Scheres et al. 1995). Asymmetric cell divisions
within the vascular primordium go on to establish the number
of vascular initials present in the seedling root meristem.
In the aerial parts of a seedling, the protovascular elements
are first specified and start to differentiate in the cotyledons and,
only subsequently, in the axis (Bauby et al. 2007). Protophloem
differentiation can first be observed in the midvein by the
typical cell elongation, followed by extension to the distal
loops and cotyledonary node. Before vasculature differentiation,
continuous procambial strands can already be observed
(Figure 5). The main agent in the establishment of this pattern
is the phytohormone auxin. The accumulation of auxin, through
polar transport mechanisms, as shown by the synthetic auxin
reporter DR5 and the auxin-induced pre-procambial marker
AtHB8, precedes the formation of vascular strands in leaves
(Scarpella et al. 2004). Facilitating this highly localized auxin
accumulation is the auxin transporter PIN1, which channels
auxin to the provascular regions. PIN1 is already expressed
before procambium formation.
After the initial differentiation, the vasculature develops in
bundles (Esau 1969). These bundles have a very distinct
radial pattern of abaxial phloem and adaxial xylem divided by
an active procambium (Figure 6). The radial patterning of the
vascular bundles is already established during embryogenesis
by the main factors of radial patterning, KANADI (KAN) and the
Class III HD-ZIPs, PHB, PHV, REV and CORONA/ATHB15
(McConnell et al. 2001; Emery et al. 2003). Auxin also plays
Insights into Plant Vascular Biology 301
a role in this determination of the radial vascular patterning
(Izakhi and Bowman 2007). PIN1 localization is affected in kan
mutants, showing the integration of the auxin transport pathway
and KAN signaling. The triple mutant phb phv rev has radialized
as well as abaxialized leaves and vascular bundles. In contrast,
gain-of-function Class III HD-ZIP mutants with faulty microRNA
(miRNA) regulation and kan1 kan2 kan3 have radialized and
adaxialized leaves and bundles. In addition, the Class III HD-
ZIP genes also appear to regulate vascular tissue proliferation.
As will be described in more detail later, the phloem translocation
stream, or phloem sap, contains not only photosynthate
but also a wide array of macromolecules, such as mRNA,
small RNAs and proteins. Both in animals and plants, small
RNAs have already been identified as important regulatory
factors controlling cell fate. A bidirectional cell-to-cell communication
network involving the mobile transcription factor
SHORTROOT (SHR) and microRNA165/166 species specifies
the radial position of two types of xylem vessels in Arabidopsis
roots (Carlsbecker et al. 2010; Miyashima et al. 2011). Since
microRNA165/166 is a factor restricting PHB activity, it also
regulates phloem development.
Recent studies have shown that CALLOSE SYNTHASE
3 (CALS3), a membrane-bound enzyme which synthesizes
callose, a β-1,3-glucan (Verma and Hong 2001; Colombani
et al. 2004), appears to be involved in regulating the cell-to-cell
movement of microRNA165/166 (Vatén et al. 2011). CALS3 is
expressed both in phloem and meristematic tissues. Gain-offunction
mutations in CALS3 result in increased accumulation
of PD callose, a decrease in the PD aperture, multiple defects
in root development, and reduced intercellular trafficking of
various molecules. Using an inducible expression system for a
modified version of CALS3 (CALS3m), Vatén et al. (2011) were
able to show that increased callose deposition inhibited SHR
and microRNA165 movement between the stele and the endodermis.
This interesting result suggested that regulated callose
biosynthesis, at the PD level, may be essential for control over
cell-to-cell communication and cell fate determination.
Root vascular development
In the root, protophloem initially differentiates from an independent
differentiation locus in the upper hypocotyl; only later
does protophloem differentiation from the root apical meristem
begin. As the root grows, the cellular pattern is established and
maintained by the self-renewal of pluripotent root meristem
cells. Different cell identities are initiated from the stem cells
around the quiescent center (QC): the provascular initials of
the stele, the cortex/endodermal initials, the epidermal/lateral
root initials, and the columella initials. In the Arabidopsis root,
a central vascular cylinder (consisting of xylem, phloem and
procambium) is surrounded by radially symmetric layers of
pericycle, endodermis, cortex and epidermal cells. In this root