Clogging up the plumbing – A specific gene is regulating the formation of water-transporting vessels, In plants

Vascular development in plants is controlled by a newly
identified gene regulator that can block the formation
of water-transporting vessels

Every vascular plant contains an extensive network
of xylem and phloem, specialized tissues that respectively
transport water and nutrients throughout the plant body.
Untangling the processes that determine how these two
types of vasculature develop has proven challenging,
but a team led by Taku Demura of the RIKEN
important novel regulator of xylem formation.
Several years ago, Demura and colleagues identified
a family of seven VASCULAR-RELATED NAC-
DOMAIN (VND) transcription factors; one of these,
VND7, appears to activate a number of genes related
to xylem development2. “The data suggest that VND7
likely functions as the principal regulator of vessel
differentiation,” Demura says. However, the activity
of this factor appears to depend closely on the proteins
with which it partners, and his team has subsequently
focused on identifying these co-regulators.

In their most recent screen, the researchers identified
VNI2, a novel transcriptional regulator that physically
interacts with VND7 and whose expression appears to
correlate closely with vascular development in both root
and stem tissue. However, although both VND7 and VNI2
are categorized as ‘NAC domain’ proteins, VNI2 exhibited
one surprising difference from other members of its family.
“It is known that most of the NAC transcription factors are
transcriptional activators,” says Demura. “In contrast,
VNI2 is a transcriptional repressor.”
Indeed, VNI2 appears to act primarily as an inhibitor
of vascular development (Fig. 1), and plants overexpressing
this factor exhibited profound defects in xylem formation.
These abnormalities were highly similar to those observed
in plants overexpressing modified, inhibitory variants of
VND7, further supporting a partnership between these
two factors. In parallel, Demura and colleagues determined
that VNI2 specifically represses several genes known to be
induced by VND7 in the course of xylem differentiation.
These findings indicate that the VNI2–VND7 complex
contributes directly to the timing and localization of vascular
development, although this is most likely not the sole purpose
of this repressor. “Our paper shows that VNI2 is expressed in
various other cell types in addition to xylem vessels, and we
want to know its other functions,” says Demura. Accordingly,
their initial protein–protein interaction data suggest that VNI2
might pair with other, non-xylem-specific NAC proteins, whose
functional characteristics remain enigmatic.
“We still need to study the VND genes [more closely],” says
Demura, “for a better understanding of xylem cell differentiation.
Since xylem cells are a major source of lignocellulosic biomass,
such knowledge could be applied to potential renewable materials
and biofuels.”