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 ...

In plants and animals, small peptide ligands that signal in cell-cell communication have been suggested to be a crucial component of development. A bioassay of single-cell transdifferentation demonstrates that a dodecapeptide with two hydroxyproline residues is the functional product of genes from the CLE family, which includes CLAVATA3 in Arabidopsis. The dodecapeptide suppresses xylem cell development at a concentration of 10(-11) M and promotes cell division. An application, corresponding to all 26 Arabidopsis CLE protein family members, of synthetic dodecapeptides reveals two counteracting signaling pathways involved in stem cell fate.

Land plants evolved a long-distance transport system of water and nutrients composed of the xylem and phloem, both of which are generated from the procambium-and cambium-comprising vascular stem cells. However, little is known about the molecular mechanism of cell communication governing xylem-phloem patterning. Here, we show that a dodecapeptide (HEVHypSGHypN-PISN; Hyp, 4-hydroxyproline), TDIF (tracheary element differentiation inhibitory factor), is secreted from the phloem and suppresses the differentiation of vascular stem cells into xylem cells through a leucine-rich repeat receptor-like kinase (LRR-RLK). TDIF binds in vitro specifically to the LRR-RLK, designated TDR (putative TDIF receptor), whose expression is restricted to procambial cells. However, the combined analysis of TDIF with a specific antibody and the expression profiles of the promoters of two genes encoding TDIF revealed that TDIF is synthesized mainly in, and secreted from, the phloem and its neighboring cells. The observation that TDIF is capable of promoting proliferation of procambial cells while suppressing xylem differentiation suggests that this small peptide functions as a phloem-derived, non-cellautonomous signal that controls stem cell fate in the procambium. Our results indicate that we have discovered a cell communication system governing phloem-xylem cross-talk.CLV3/ESR-related (CLE) ͉ leucine-rich repeat receptor-like kinase ͉ phloem ͉ procambium ͉ xylem

The indeterminate nature of plant growth and development depends on the stem cell system found in meristems. The Arabidopsis thaliana vascular meristem includes procambium and cambium. In these tissues, cell-cell signaling, mediated by a ligand-receptor pair made of the TDIF (for tracheary element differentiation inhibitory factor) peptide and the TDR/PXY (for TDIF RECEPTOR/ PHLOEM INTERCALATED WITH XYLEM) membrane protein kinase, promotes proliferation of procambial cells and suppresses their xylem differentiation. Here, we report that a WUSCHEL-related HOMEOBOX gene, WOX4, is a key target of the TDIF signaling pathway. WOX4 is expressed preferentially in the procambium and cambium, and its expression level was upregulated upon application of TDIF in a TDR-dependent manner. Genetic analyses showed that WOX4 is required for promoting the proliferation of procambial/cambial stem cells but not for repressing their commitment to xylem differentiation in response to the TDIF signal. Thus, at least two intracellular signaling pathways that diverge after TDIF recognition by TDR might regulate independently the behavior of vascular stem cells. Detailed observations in loss-of-function mutants revealed that TDIF-TDR-WOX4 signaling plays a crucial role in the maintenance of the vascular meristem organization during secondary growth.