Vascular-related NAC-domain 7 directly regulates a broad range of genes for xylem vessel differentiation

Background Xylem functions in conduction of water and minerals throughout the plants, and supports the plant body. One of the features of xylem cells is development of secondary wall structure between plasma membrane and (primary) cell wall. Recently, it is expected that knowledge on xylem development can be utilized for application of improvement of the plant biomass, since most portion of wood, which represents one of important sources of woody biomass, is mainly composed of two types of xylem cells, xylem vessels and fiber cells. Previously we established the in vitro transdifferentiation system, in which Arabidopsis suspension cells could synchronously transdifferentiate into xylem vessel elements. A number of genes whose expression is elevated during the transdifferentiation processes have been isolated by using microarray analysis [1]. We revealed that one of the identified genes, which encoded a NAC domain protein, VND7 (Vascular-Related NAC Domain Protein7), plays a pivotal role in promoting the xylem vessel differentiation [1,2]. Recently, to efficiently obtain xylem vessel elements, we used a glucocorticoid-mediated post-translational induction system [3]. The transgenic Arabidopsis plants exhibited transdifferentiation of most of cells into xylem vessel elements, and the plants died. This induction system worked in poplar trees and in suspension cultures of cells from Arabidopsis and tobacco. These data demonstrate that the induction systems controlling VND7 activity can be used as powerful tools for understanding xylem cell differentiation. Objectives of the research Several studies report that VND7 regulates expression of downstream of some transcription factors, suggesting that existence of transcriptional network regulating xylem vessel differentiation. Here, in order to identify direct target genes of VND7, we performed global transcriptome analysis using Arabidopsis transgenic lines in which VND7 activity could be induced posttranslationally.