The Plant Vascular System: Evolution, Development and FunctionsF

oots, CK again increases in the xylem transpiration stream
(Rahayu et al. 2005; Ruffel et al. 2011). Interestingly, transzeatin-type
CK moves in the xylem, and isopentenyl-type CK
is present in the phloem translocation stream. This suggests
that these structural variations carry specific information from
the root-to-shoot and shoot-to-root, respectively (Hirose et al.
2008; Werner and Schmülling 2009).
As discussed above, phosphate acquisition by the root system
involves phloem-mobile signals from the shoot (Figure 24).
In terms of the root-to-shoot component of this phosphate
signaling network, it has been suggested that the level of
phosphate in the xylem transpiration stream may serve as one
component in this signaling pathway (Bieleski 1973, Poirier
et al. 1991; Burleigh and Harrison 1999; Hamburger et al. 2002;
Lai et al. 2007; Stefanovic et al. 2007; Chiou and Lin 2011;
Thibaud et al. 2010). Studies on the growth of Arabidopsis roots
being exposed to low phosphate conditions identified the tip of
the primary root, including the meristem region and root cap,
as the site that may sense local phosphate availability (Linkohr
et al. 2002; Svistoonoff 2007). However, currently, there is no
evidence for the existence of a phosphate sensor or receptor.
Both CK and SLs have also been considered to function in
xylem transmission of root phosphate status (Martin et al. 2000;
Franco-Zorrilla et al. 2005; Kohlen et al. 2011). Plants grown
under limiting phosphate conditions have repressed levels of
trans-zeatin-type CK in their xylem sap (Martin et al. 2000) and,
under these conditions, expression of the CK receptor CRE1
is similarly decreased (Franco-Zorrilla et al. 2002, 2005). In
many plant species, the SLs are up-regulated upon exposure
to phosphate deficiency conditions. Grafting studies have indicated
that SLs produced in the root can move to the shoot
(Beveridge et al. 1994; Napoli 1996; Turnbull et al. 2002). In
such studies, WT rootstocks grafted to mutant scions lacking
the ability to produce SLs were able to restore WT branching
patterns in these scions. Thus, xylem-transported SLs can
contribute to the regulation of shoot architectural responses
to phosphate-limiting conditions (Kohlen et al. 2011). Collectively,
these findings suggest that the levels of phosphate, CK
and SLs in the xylem transpiration stream play an important
role in coordinating vegetative growth with phosphate nutrient
availability (Rouached et al. 2011).
Xylem signaling in plant-symbiotic associations
The interaction of nitrogen-fixing bacteria (Rhizobia) is generally
confined to legumes, whereas most flowering plants
establish symbiotic associations with arbuscular mycorrhizal
(AM) fungi for phosphate acquisition. In both types of plantsymbiont
association, there is a significant metabolic cost to
the plant host. Thus, there is a need for the plant to ensure
that the cost-benefit ratio remains favorable. To this end,
Insights into Plant Vascular Biology 349
Figure 24. Long-distance signaling in response to phosphate
deficiency conditions.
Phosphate (Pi) availability in the soil solution is transmitted through
the xylem transpiration stream (blue lines) which passes predominantly
to the mature source leaves. Pi per se, and/or other root-toshoot
signals (1), including cytokinin and strigolactones, are thought
to be involved in this nutrient signal transduction pathway. When
roots encounter low levels of available Pi, changes in these xylemborne
signaling components are decoded in the leaves (2), resulting
in the activation of Pi deficiency responsive pathways. Outputs from
this Pi-stress response pathway, including miR399 21-nucleotide
silencing agents and other potential signaling components (3) are
loaded into the phloem (red lines). Phloem-mobile signals move
down to the root where they enter different target receiver cells
to mediate an increase in Pi uptake (4) and alter root architecture
(4’). The miR399 signal targets PHOSPHATE2 (PHO2) transcripts
to derepress Pi transporter activity. A different set of phloemmobile
signals are likely delivered to developing leaves (5) and
the shoot apex (6) to regulate growth and development in order to
survive under the Pi-stress condition. This long-distance signaling
network operates to ensure that the root system integrates its
physiological activities to optimize growth conditions within the
shoot.