VND-INTERACTING2, a NAC Domain Transcription Factor, Negatively Regulates Xylem Vessel Formation in Arabidopsis

Masatoshi Yamaguchi,Taku Demura,Minoru Kubo,Hiroo Fukuda,Nobutaka Mitsuda,Misato Ohtani,Masaru Ohme-Takagi

doi:10.1105/tpc.108.064048

Abstract

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.

Highlights

The plant vascular system, transporting water and nutrients and supporting the plant body, consists of the xylem and phloem, which differentiate from the vascular cambium and procambium
VASCULAR-RELATED NAC-DOMAIN1 (VND1) to VASCULAR-RELATED NAC-DOMAIN7 (VND7) are preferentially expressed in differentiating xylem vessels (Kubo et al, 2005; Yamaguchi et al, 2008), and the overexpression of VND7 and VND6 can induce the ectopic differentiation of two different types of vessel elements: protoxylem vessels, typically with annular or spiral secondary wall thickenings, and metaxylem vessels, typically with reticulate or pitted secondary wall thickenings (Kubo et al, 2005)
Considered together, our findings indicate that VNI2 regulates xylem cell specification as a transcriptional repressor that interacts with VND proteins and possibly with other NAC domain proteins

Summary

Introduction

The plant vascular system, transporting water and nutrients and supporting the plant body, consists of the xylem and phloem, which differentiate from the vascular cambium and procambium. The functional suppression of VND7 and VND6 results in the inhibition of vessel element formation (Kubo et al, 2005; Yamaguchi et al, 2008). These results strongly suggest that VND6 and VND7 act as the key regulators of the differentiation of xylem vessels. The gene encoding XYLEM NAC DOMAIN1 (XND1) is highly expressed in xylem (Zhao et al, 2005), and its overexpression causes extreme dwarfism associated with the complete suppression of vessel secondary wall biosynthesis and programmed cell death, suggesting that XND1 negatively regulates xylem vessel differentiation (Zhao et al, 2008). SND2, MYB46, MYB103, and KNOTTED1-LIKE HOMEODOMAIN PROTEIN7 transcription factors are direct targets of NST3/SND1/ANAC012 and possibly of NST1, VND6, and VND7 (Zhong et al, 2007a, 2008), suggesting the existence of a transcriptional network regulating xylem differentiation

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Results

Conclusion