The Plant Vascular System: Evolution, Development and FunctionsF

compelling evidence for the hypothesis that phloem-mobile 24nt
sRNA can mediate in epigenetic TGS of specific genomic
loci.
Similar findings were reported for studies on endogenous
inverted repeats that generate double-stranded RNA molecules
(Dunoyer et al. 2010), as well as for transgenic plants expressing
a hairpin-structured gene under the control of a viral
companion-cell-specific promoter (Bai et al. 2011). In this latter
case, this phloem-transmissible TGS event was shown to be
inherited by subsequent progeny. Collectively, these findings
support the notion that phloem-mobile sRNA can serve to regulate
gene expression within developing tissues epigenetically
to allow for adaptation to environmental inputs.
Phloem-mobile miRNA
Although generally considered to act cell-autonomously
(Voinnet 2009), as mentioned above, numerous miRNAs have
been detected in phloem exudates from various plant species,
and the roles played by some of these have recently been
established. In plants, adaptation to changing nutrient availability
in the soil involves both root-to-shoot (see next section)
and shoot-to-root signaling. In the case of phosphate (Pi),
changes in availability within leaves leads to an upregulation in
miR399 production and, subsequently, its entry into the phloem
translocation stream (Lin et al. 2008; Pant et al. 2008). Delivery
of miR399 into the roots results in the cleavage of the target
mRNA encoding for PHO2, a ubiquitin-conjugating E2 enzyme
(UBC24). This gives rise to increased uptake of Pi into these
roots and restoration of Pi levels within the body of the plant.
Loss of PHO2 activity probably allows for an increase in Pi
transporter capacity; i.e., of influx carriers located in the outer
region of the root and xylem parenchyma-located efflux carriers
that function in Pi loading into the transpiration stream (Chiou
and Lin 2011).
Tuber induction in potato is regulated by phloem delivery
of BEL5 transcripts and miR172 (Martin et al. 2009). Vascular
expression of miR172 and its upregulation under tuber-inducing
SD conditions suggested that this miRNA may act as a longdistance
signaling component in the control of potato tuber
induction. Support for this notion was provided by grafting studies
involving P35S:MIR172 stocks grafted to WT potato scions.
Here, tuberization occurred as early as in P35S:MIR172 potato
lines. In contrast, when P35S:MIR172 scions were grafted to
WT stocks, early tuber induction did not occur. Although these
findings are consistent with miR172 serving as a phloem-mobile
signal, it is also possible that it might act through regulation, in
the CCs, of an independent mobile signal.
Phloem-mobile sRNA control over host infection
by parasitic plants
Parasitic plants cause major losses in some regions of the
world (Ejeta 2007). Recent studies have established that host
Insights into Plant Vascular Biology 347
transcripts can enter into parasitic plants (Roney et al. 2007;
David-Schwartz et al. 2008). The pathway for this trafficking
is through the haustoria of the parasite that interconnects its
vascular system to that of the host. This suggested that host
invasion by parasitic plants might be controlled by phloem
delivery of sRNA species designed specifically to target critical
genes involved in the physiology or development of the
parasitic weed (Yoder et al. 2009).
Based on the observation that parasitic broomrape
(Orobanche aegyptiaca) accumulates large quantities of mannitol,
Aly et al. (2009) engineered transgenic tomatoes to
express a hairpin construct to target the mannose 6-phosphate
reductase (M6PR) that functions as a key enzyme in mannitol
biosynthesis. Analysis of tissue from broomrape growing on
these transgenic tomato plants indicated a significant reduction
in both M6PR transcript and mannitol levels. This strategy
gave rise to a level of tomato protection against this plant
parasite.
An alternate control approach based on targeting a parasitic
developmental program involved the development of transgenic
tobacco plants expressing hairpin constructs for two
dodder (Cuscuta pentagona) haustoria-expressed KNOTTEDlike
HOMEOBOX1 (KNOX1) genes. These constructs were
driven by the CC-specific SUC2 promoter and were based
on 3 ′ UTRs that did not display sequence homology to the
related tobacco orthologues, STM and KNAT1–3 (Alakonya
et al. 2012). Defects in haustoria development and connection
to the transgenic tobacco plants were highly correlated with
the presence of KNOX1 siRNA, delivered most likely through
the vascular system, and down-regulation of the C. pentagona
KNOX1 transcript levels. Importantly, dodder plants growing
on these transgenic tobacco plants exhibited greatly reduced
vigor. Collectively, these studies indicate that an effective
control of plant parasitism may be achieved by targeting a
pyramided combination of parasite genes involved in various
aspects of growth and development.
Root-to-shoot Signaling
Response to abiotic stress
Signals arising within the root system can provide shoots with
an early warning of root conditions, such as water deficiency,
nutrient availability/deficiency, and so forth (Figure 23). The
xylem transports hormones, such as abscisic acid (ABA)
(Bahrun et al. 2002; Jiang and Hartung 2008), ethylene and
cytokinin (CK) (Takei et al. 2002; Hirose et al. 2008; Kudo
et al. 2010; Ghanem et al. 2011), as well as strigolactones
(SLs) (Gomez-Roldan et al. 2008; Umehara et al. 2008; Brewer
et al. 2013; Ruyter-Spira et al. 2012) from roots to aboveground
tissues. In this section of the review, we will address the role of
these xylem-borne signaling agents.