Wood is formed by the successive addition of secondary xylem, which consists of cells with a conspicuously thickened secondary wall composed mainly of lignin and cellulose. Several genes involved in lignin and cellulose biosynthesis have been characterized, but the factors that regulate the formation of secondary walls in woody tissues remain to be identified. In this study, we show that plant-specific transcription factors, designated NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST3, are key regulators of the formation of secondary walls in woody tissues of Arabidopsis thaliana. In nst1-1 nst3-1 double knockout plants, the secondary wall thickenings in interfascicular fibers and secondary xylem, except for vascular vessels, were completely suppressed without affecting formation of cells destined to be woody tissues. Conversely, as shown previously for NST1, overexpression of NST3 induced ectopic secondary wall thickenings in various aboveground tissues. Furthermore, the expression of chimeric repressors derived from NST1 and NST3 suppressed secondary wall thickenings in the presumptive interfascicular fibers. Because putative orthologs of NST1 and NST3 are present in the genome of poplar, our results suggest that they are also key regulators of the formation of secondary walls in woody plants and could be used as a tool for the genetic engineering of wood and its derivatives.

In plants, secondary wall thickenings play important roles in various biological processes, although the factors regulating these processes remain to be characterized. We show that expression of chimeric repressors derived from NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2 in Arabidopsis thaliana resulted in an anther dehiscence defect due to loss of secondary wall thickening in anther endothecium. Plants with double, but not single, T-DNA-tagged lines for NST1 and NST2 had the same anther-indehiscent phenotype as transgenic plants that expressed the individual chimeric repressors, indicating that NST1 and NST2 are redundant in regulating secondary wall thickening in anther walls. The activity of the NST2 promoter was particularly strong in anther tissue, while that of the NST1 promoter was detected in various tissues in which lignified secondary walls develop. Ectopic expression of NST1 or NST2 induced ectopic thickening of secondary walls in various aboveground tissues. Epidermal cells with ectopic thickening of secondary walls had structural features similar to those of tracheary elements. However, among genes involved in the differentiation of tracheary elements, only those related to secondary wall synthesis were clearly upregulated. None of the genes involved in programmed cell death were similarly affected. Our results suggest NAC transcription factors as possible regulators of secondary wall thickening in various tissues.

Many multicellular organisms have remarkable capability to regenerate new organs after wounding. As a first step of organ regeneration, adult somatic cells often dedifferentiate to reacquire cell proliferation potential, but mechanisms underlying this process remain unknown in plants. Here we show that an AP2/ERF transcription factor, WOUND INDUCED DEDIFFERENTIATION 1 (WIND1), is involved in the control of cell dedifferentiation in Arabidopsis. WIND1 is rapidly induced at the wound site, and it promotes cell dedifferentiation and subsequent cell proliferation to form a mass of pluripotent cells termed callus. We further demonstrate that ectopic overexpression of WIND1 is sufficient to establish and maintain the dedifferentiated status of somatic cells without exogenous auxin and cytokinin, two plant hormones that are normally required for cell dedifferentiation. In vivo imaging of a synthetic cytokinin reporter reveals that wounding upregulates the B-type ARABIDOPSIS RESPONSE REGULATOR (ARR)-mediated cytokinin response and that WIND1 acts via the ARR-dependent signaling pathway to promote cell dedifferentiation. This study provides novel molecular insights into how plants control cell dedifferentiation in response to wounding.

Coordination of the maintenance of the undifferentiated fate of cells in the shoot meristem and the promotion of cellular differentiation in plant organs is essential for the development of plant shoots. CINCINNATA-like (CIN-like) TEOSINTE BRANCHED1, CYCLOIDEA, and PCF (TCP) transcription factors are involved in this coordination via the negative regulation of CUP-SHAPED COTYLEDON (CUC) genes, which regulate the formation of shoot meristems and the specification of organ boundaries. However, the molecular mechanism of the action of CIN-like TCPs is poorly understood. We show here that TCP3, a model of CIN-like TCPs of Arabidopsis thaliana, directly activates the expression of genes for miR164, ASYM-METRIC LEAVES1 (AS1), INDOLE-3-ACETIC ACID3/SHORT HYPOCOTYL2 (IAA3/SHY2), and SMALL AUXIN UP RNA (SAUR) proteins. Gain of function of these genes suppressed the formation of shoot meristems and resulted in the fusion of cotyledons, whereas their loss of function induced ectopic expression of CUC genes in leaves. Our results indicate that miR164, AS1, IAA3/SHY2, and SAUR partially but cooperatively suppress the expression of CUC genes. Since CIN-like TCP genes were revealed to act dose dependently in the differentiation of leaves, we propose that evolutionarily diverse CIN-like TCPs have important roles in the signaling pathways that generate different leaf forms, without having any lethal effects on shoots.

SUMMARYThe Arabidopsis thaliana NAC domain transcription factor, VASCULAR-RELATED NAC-DOMAIN7 (VND7), acts as a key regulator of xylem vessel differentiation. 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 post-translationally. This analysis identified 63 putative direct target genes of VND7, which encode a broad range of proteins, such as transcription factors, IRREGULAR XYLEM proteins and proteolytic enzymes, known to be closely associated with xylem vessel formation. Recombinant VND7 protein binds to several promoter sequences present in candidate direct target genes: specifically, in the promoter of XYLEM CYSTEINE PEPTIDASE1, two distinct regions were demonstrated to be responsible for VND7 binding. We also found that expression of VND7 restores secondary cell wall formation in the fiber cells of inflorescence stems of nst1 nst3 double mutants, as well as expression of NAC SECONDARY WALL THICKENING PROMOTING FACTOR3 (NST3, however, the vessel-type secondary wall deposition was observed only as a result of VND7 expression. These findings indicated that VND7 upregulates, directly and/or indirectly, many genes involved in a wide range of processes in xylem vessel differentiation, and that its target genes are partially different from those of NSTs.

Constituting approximately 10% of flowering plant species, orchids (Orchidaceae) display unique flower morphologies, possess an extraordinary diversity in lifestyle, and have successfully colonized almost every habitat on Earth 1-3 . Here we report the draft genome sequence of Apostasia shenzhenica 4 , a representative of one of two genera that form a sister lineage to the rest of the Orchidaceae, providing a reference for inferring the genome content and structure of the most recent common ancestor of all extant orchids and improving our understanding of their origins and evolution. In addition, we present transcriptome data for representatives of Vanilloideae, Cypripedioideae and Orchidoideae, and novel thirdgeneration genome data for two species of Epidendroideae, covering all five orchid subfamilies. A. shenzhenica shows clear evidence of a whole-genome duplication, which is shared by all orchids and occurred shortly before their divergence. Comparisons between A. shenzhenica and other orchids and angiosperms also permitted the reconstruction of an ancestral orchid gene toolkit. We identify new gene families, gene family expansions and contractions, and changes within MADS-box gene classes, which control a diverse suite of developmental processes, during orchid evolution. This study sheds new light on the genetic mechanisms underpinning key orchid innovations, including the development of the labellum and gynostemium, pollinia, and seeds without endosperm, as well as the evolution of epiphytism; reveals relationships between the Orchidaceae subfamilies; and helps clarify the evolutionary history of orchids within the angiosperms.

The Arabidopsis thaliana NAC domain transcription factor VASCULAR-RELATED NAC-DOMAIN7 (VND7) acts as a master regulator of xylem vessel differentiation. To understand the mechanism by which VND7 regulates xylem vessel differentiation, we used a yeast two-hybrid system to screen for proteins that interact with VND7 and identified cDNAs encoding two NAC domain proteins, VND-INTERACTING1 (VNI1) and VNI2. Binding assays demonstrated that VNI2 effectively interacts with VND7 and the VND family proteins, VND1-5, as well as with other NAC domain proteins at lower affinity. VNI2 is expressed in both xylem and phloem cells in roots and inflorescence stems. The expression of VNI2 overlaps with that of VND7 in elongating vessel precursors in roots. VNI2 contains a predicted PEST motif and a C-terminally truncated VNI2 protein, which lacks part of the PEST motif, is more stable than full-length VNI2. Transient reporter assays showed that VNI2 is a transcriptional repressor and can repress the expression of vessel-specific genes regulated by VND7. Expression of C-terminally truncated VNI2 under the control of the VND7 promoter inhibited the normal development of xylem vessels in roots and aerial organs. These data suggest that VNI2 regulates xylem cell specification as a transcriptional repressor that interacts with VND proteins and possibly also with other NAC domain proteins.

Orchids make up about 10% of all seed plant species, have great economical value, and are of specific scientific interest because of their renowned flowers and ecological adaptations. Here, we report the first draft genome sequence of a lithophytic orchid, Dendrobium catenatum. We predict 28,910 protein-coding genes, and find evidence of a whole genome duplication shared with Phalaenopsis. We observed the expansion of many resistance-related genes, suggesting a powerful immune system responsible for adaptation to a wide range of ecological niches. We also discovered extensive duplication of genes involved in glucomannan synthase activities, likely related to the synthesis of medicinal polysaccharides. Expansion of MADS-box gene clades ANR1, StMADS11, and MIKC*, involved in the regulation of development and growth, suggests that these expansions are associated with the astonishing diversity of plant architecture in the genus Dendrobium. On the contrary, members of the type I MADS box gene family are missing, which might explain the loss of the endospermous seed. The findings reported here will be important for future studies into polysaccharide synthesis, adaptations to diverse environments and flower architecture of Orchidaceae.