Abstract
One of the most prominent features of xylem conducting cells is the deposition of secondary walls. In Arabidopsis, secondary wall biosynthesis in the xylem conducting cells, vessels, has been shown to be regulated by two VASCULAR-RELATED NAC-DOMAIN (VND) genes, VND6 and VND7. In this report, we have investigated the roles of five additional Arabidopsis VND genes, VND1 to VND5, in regulating secondary wall biosynthesis in vessels. The VND1 to VND5 genes were shown to be specifically expressed in vessels but not in interfascicular fibers in stems. The expression of VND4 and VND5 was also seen specifically in vessels in the secondary xylem of the root-hypocotyl region. When overexpressed, VND1 to VND5 were able to activate the expression of secondary wall-associated transcription factors and genes involved in secondary wall biosynthesis and programmed cell death. As a result, many normally parenchymatous cells in leaves and stems acquired thickened secondary walls in the VND1 to VND5 overexpressors. In contrast, dominant repression of VND3 function resulted in reduced secondary wall thickening in vessels and a collapsed vessel phenotype. In addition, VND1 to VND5 were shown to be capable of rescuing the secondary wall defects in the fibers of the snd1 nst1 double mutant when expressed under the SND1 promoter. Furthermore, transactivation analysis revealed that VND1 to VND5 could activate expression of the GUS reporter gene driven by the secondary wall NAC binding element (SNBE). Together, these results demonstrate that VND1 to VND5 possess functions similar to that of the SND1 secondary wall NAC and are transcriptional regulators of secondary wall biosynthesis in vessels.
Highlights
Vascular plants have two specialized tissues, xylem and phloem, for the transport of water and food throughout the plant body
At least 10 independent transgenic lines showing GUS staining for each construct were examined and all of them had vessel-specific expression albeit with different GUS staining intensity. These results demonstrate that the VND1 to VND5 genes are expressed in vessels, an expression pattern similar to that of VND6 and VND7
Previous studies have demonstrated that five Arabidopsis secondary wall NACs (SWNs) genes, namely SND1, NST1, NST2, VND6 and VND7, are transcriptional regulators of secondary wall biosynthesis in various secondary wall-forming cell types [3,4]
Summary
Vascular plants have two specialized tissues, xylem and phloem, for the transport of water and food throughout the plant body. The conducting cells in the xylem are tracheary elements that are further grouped into two types, tracheids and vessels. Tracheids are the first type to evolve and are found in seedless vascular plants and gymnosperms, whereas vessels were proposed to have evolved from tracheids and are found in angiosperms [1]. Xylem formation involves a series of complex developmental events, including differentiation of procambial/cambial cells into xylem mother cells, cell elongation, secondary wall thickening and programmed cell death. One of the beststudied events of xylem formation is secondary wall thickening in which a cascade of transcription factors has been revealed to be involved
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