Land plants evolved xylem vessels to conduct water and nutrients, and to support the plant. Microarray analysis with a newly established Arabidopsis in vitro xylem vessel element formation system and promoter analysis revealed the possible involvement of some plant-specific NAC-domain transcription factors in xylem formation. VASCULAR-RELATED NAC-DOMAIN6 (VND6) and VND7 can induce transdifferentiation of various cells into metaxylem-and protoxylem-like vessel elements, respectively, in Arabidopsis and poplar. A dominant repression of VND6 and VND7 specifically inhibits metaxylem and protoxylem vessel formation in roots, respectively. These findings suggest that these genes are transcription switches for plant metaxylem and protoxylem vessel formation. Xylem vessels, a conductive component of the vascular tissues in plants, are found throughout the plant body. To colonize the land, plants have evolutionarily developed different types of xylem vessels that function in the long-distance transport of water, various nutrients, and signaling molecules throughout their life (Raven et al. 1999). Two types of vessels mature in characteristic positions within protoxylem and metaxylem of the primary xylem tissue that differentiates from the procambium during the early ontogeny of a plant. The protoxylem vessels, which commonly have annular and spiral thickenings, mature before the surrounding organs have elongated. These are frequently destroyed by the extension of the surrounding tissues. The metaxylem vessels, which usually have reticulate and pitted thickenings, mature after the surrounding organs complete their growth. In contrast to protoxylem vessels, they are not destroyed, and constitute the water-conducting tubes of the mature plant (Esau 1977). In Arabidopsis roots, two protoxylem vessels are typically formed at the outermost position of the vascular system, between which three to four metaxylem vessels develop (Supplementary Fig. S1).Recent forward genetic and molecular biological approaches have revealed several aspects of xylem formation that are affected by several key genes (Ye 2002;Fukuda 2004). These genes are related to auxin transport and signaling, and include PINFORMED1 (Gälweiler et al. 1998) and MONOPTEROS (Przemeck et al. 1996) . However, the hierarchical genetic control of differentiation of individual xylem cells is still poorly understood. In this study, we identified VND6 and VND7, which belong to plant-specific transcription factors, NAC-domain proteins that can induce transdifferentiation of various types of cells into metaxylemand protoxylem-like vessel elements, respectively. It is suggested that VND6 and VND7 are transcription switches for plant metaxylem and protoxylem vessel formation, respectively. Results and DiscussionWe have uncovered an expression profile of 9000 genes during xylem vessel element differentiation in an in vitro Zinnia cell culture (Demura et al. 2002). To gain an expression profile of xylem cell-differentiation-related genes in Arabidopsis, we established an in vitro ...

The nucleotide sequences reported in this paper has been submitted to EMBL Data Library under accession numbers D16559 for NgORF13, NCBI GenBank under accession numbers K03313 for RiORF13 of Agrobacterium rhizogenes pRiA4 (agropine-type) and M60490 for RiORF13 of A. rhizogenes pRi8196 (mannopine-type).