Clogging up the plumbing

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 Biomass Engineering Program in Wako has now uncovered an important novel regulator of xylem formation1.

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, 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.”

The corresponding author for this highlight is based at the Cellulose Production Research Team, RIKEN Biomass Engineering Program

Journal information

1. Yamaguchi, M., Ohtani, M., Mitsuda, N., Kubo, M., Ohme-Takagi, M., Fukuda, H. & Demura, T. VND-INTERACTING2, a NAC domain transcription factor, negatively regulates xylem vessel formation in Arabidopsis. Plant Cell 22, 1249–1263 (2010).

2. Kubo, M., Udagawa, M., Nishikubo, N. Horiguchi, G., Yamaguchi, M., Ito, J., Mimura, T., Fukuda, H. & Demura, T. Transcription switches for protoxylem and metaxylem vessel formation. Genes & Development 19, 1855–1860 (2005).