Abstract
MicroRNAs (miRNAs) play critical regulatory roles by acting as sequence specific guide during secondary wall formation in woody and non-woody species. Although thousands of plant miRNAs have been sequenced, there is no comprehensive view of miRNA mediated gene regulatory network to provide profound biological insights into the regulation of xylem development. Herein, we report the involvement of six highly conserved amg-miRNA families (amg-miR166, amg-miR172, amg-miR168, amg-miR159, amg-miR394, and amg-miR156) as the potential regulatory sequences of secondary cell wall biosynthesis. Within this highly conserved amg-miRNA family, only amg-miR166 exhibited strong differences in expression between phloem and xylem tissue. The functional characterization of amg-miR166 targets in various tissues revealed three groups of HD-ZIP III: ATHB8, ATHB15, and REVOLUTA which play pivotal roles in xylem development. Although these three groups vary in their functions, -psRNA target analysis indicated that miRNA target sequences of the nine different members of HD-ZIP III are always conserved. We found that precursor structures of amg-miR166 undergo exhaustive sequence variation even within members of the same family. Gene expression analysis showed three key lignin pathway genes: C4H, CAD, and CCoAOMT were upregulated in compression wood where a cascade of miRNAs was downregulated. This study offers a comprehensive analysis on the involvement of highly conserved miRNAs implicated in the secondary wall formation of woody plants.
Highlights
Tremendous interest has been devoted to better understand the molecular basis of wood formation
This novel discovery is consistent with the findings reported earlier across three 2-year-old A. mangium genotypes: NSW22, AVW22 and WMH16 [12]
Arabidopsis genome contains five class III HD-ZIP genes (ATHB8, PHAVOLUTA/ATHB9, PHABULOSA/ATHB14, CORONA/ATHB15 and REVOLUTA/IFL1) which are targeted by miR166, our study revealed only three different class III HD-ZIP are present in A. mangium
Summary
Tremendous interest has been devoted to better understand the molecular basis of wood formation. Genomics studies in herbaceous and woody species have identified pathway genes playing an interconnected role in lignin biosynthesis These complexities reveal groups of transcriptional factors playing an independent and additive regulatory role in lignin biosynthesis [5]. Important progress has been made into the identification of transcriptional factors that bind the AC elements of monolignol biosynthetic pathway genes, little is known about the transcriptional regulators that are responsible for turning on the secondary wall biosynthetic program [2] This was hampered by the overlapping roles in the transcriptional regulation of lignin biosynthetic pathway with the biosynthesis of other secondary wall components [6]. Such a multilayered regulatory network affects multiple target genes and points to the existence of switches that can be tuned to regulate different cell wall pathways in new spatial and temporal patterns [7]
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