Abstract
Xylem vessels are indispensable tissues in vascular plants that transport water and minerals. The differentiation of xylem vessel cells is characterized by secondary cell wall deposition and programmed cell death. These processes are initiated by a specific set of transcription factors, called VASCULAR-RELATED NAC-DOMAIN (VND) family proteins, through the direct and/or indirectly induction of genes required for secondary cell wall deposition and programmed cell death. In this study, we explored novel regulatory factors for xylem vessel cell differentiation in Arabidopsis thaliana. We tested the effects of cellular stress inducers on VND7-induced differentiation of xylem vessel cells with the VND7–VP16–GR system, in which VND7 activity is post-translationally induced by dexamethasone application. We established that the histone deacetylase (HDAC) inhibitors trichostatin A (TSA) and sirtinol inhibited VND7-induced xylem vessel cell differentiation. The inhibitory effects of TSA and sirtinol treatment were detected only when they were added at the same time as the dexamethasone application, suggesting that TSA and sirtinol mainly influence the early stages of xylem vessel cell differentiation. Expression analysis revealed that these HDAC inhibitors downregulated VND7-downstream genes, including both direct and indirect targets of transcriptional activation. Notably, the HDAC inhibitors upregulated the transcript levels of negative regulators of xylem vessel cells, OVATE FAMILY PROTEIN1 (OFP1), OFP4, and MYB75, which are known to form a protein complex with BEL1-LIKE HOMEODOMAIN6 (BLH6) to repress gene transcription. The KDB system, another in vitro induction system of ectopic xylem vessel cells, demonstrated that TSA and sirtinol also inhibited ectopic formation of xylem vessel cells, and this inhibition was partially suppressed in knat7-1, bhl6-1, knat7-1 bhl6-1, and quintuple ofp1 ofp2 ofp3 ofp4 ofp5 mutants. Thus, the negative effects of HDAC inhibitors on xylem vessel cell differentiation are mediated, at least partly, by the abnormal upregulation of the transcriptional repressor complex OFP1/4–MYB75–KNAT7–BLH6. Collectively, our findings suggest that active regulation of histone deacetylation by HDACs is involved in xylem vessel cell differentiation via the OFP1/4–MYB75–KNAT7–BLH6 complex.
Highlights
Xylem vessels are important tissues in vascular plants that transport water and minerals
To identify novel factors affecting VND7-induced xylem vessel cell differentiation, we examined the effects of known cellular stress inducers (50 chemicals; Supplementary Table 1) on ectopic differentiation of xylem vessel cells in VND7– VP16–glucocorticoid receptor (GR) (Supplementary Figure 1A)
trichostatin A (TSA) and sirtinol, which are histone deacetylase (HDAC) inhibitors (Grozinger et al, 2001; Chang and Pikaard, 2005; Bourque et al, 2011; Liu et al, 2017; Mengel et al, 2017; Ueda et al, 2017), were among the five chemicals that substantially reduced the ratio of SCW deposition (Supplementary Figure 3)
Summary
Xylem vessels are important tissues in vascular plants that transport water and minerals. Overexpression of VND genes induces ectopic SCW thickening and programmed cell death (Kubo et al, 2005; Yamaguchi et al, 2008, 2010a; Zhou et al, 2014; Endo et al, 2015), while the artificial suppression of VND function suppresses the differentiation of xylem vessel cells (Kubo et al, 2005; Yamaguchi et al, 2010b) These results suggest that the VND family functions as a master switch for xylem vessel cell differentiation. MYB46 and MYB83 induce the expression of SCW-related genes such as CesA, which is targeted by VND7 (Ko et al, 2012; Zhong and Ye, 2012), and form a so-called feedforward loop with VND7 (Taylor-Teeples et al, 2015; Turco et al, 2019)
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