Abstract
Given the importance of wood in many industrial applications, much research has focused on wood formation, especially lignin biosynthesis. However, the mechanisms governing the regulation of lignin biosynthesis in the rubber tree (Hevea brasiliensis) remain to be elucidated. Here, we gained insight into the mechanisms of rubber tree lignin biosynthesis using reaction wood (wood with abnormal tissue structure induced by gravity or artificial mechanical treatment) as an experimental model. We performed transcriptome analysis of rubber tree mature xylem from tension wood (TW), opposite wood (OW), and normal wood (NW) using RNA sequencing (RNA-seq). A total of 214, 1,280, and 32 differentially expressed genes (DEGs) were identified in TW vs. NW, OW vs. NW, and TW vs. OW, respectively. GO and KEGG enrichment analysis of DEGs from different comparison groups showed that zeatin biosynthesis, plant hormone signal transduction, phenylpropanoid biosynthesis, and plant–pathogen interaction pathways may play important roles in reaction wood formation. Sixteen transcripts involved in phenylpropanoid biosynthesis and 129 transcripts encoding transcription factors (TFs) were used to construct a TF–gene regulatory network for rubber tree lignin biosynthesis. Among them, MYB, C2H2, and NAC TFs could regulate all the DEGs involved in phenylpropanoid biosynthesis. Overall, this study identified candidate genes and TFs likely involved in phenylpropanoid biosynthesis and provides novel insights into the mechanisms regulating rubber tree lignin biosynthesis.
Highlights
The rubber tree (Hevea brasiliensis) is a deciduous perennial tropical tree native to the Amazon basin that produces natural rubber as well as rubber wood
Promoter analysis showed that MYB, C2H2, and NAC transcription factors (TFs) families could bind to the promoter sequence of all differentially expressed genes (DEGs) involved in phenylpropanoid biosynthesis (Supplementary Table S2). These results suggest that these three TF families are likely key players in rubber tree reaction wood formation by regulating numerous downstream genes involved in phenylpropanoid biosynthesis
TFs from the MYB and NAC families could regulate all DEGs involved in phenylpropanoid biosynthesis and might play pivotal roles in rubber tree reaction wood formation
Summary
The rubber tree (Hevea brasiliensis) is a deciduous perennial tropical tree native to the Amazon basin that produces natural rubber as well as rubber wood. Given the increasing economic and application value of rubber wood, improving wood productivity and quality has become the focus of rubber tree breeding (Priyadarshan, 2017). As a crucial secondary metabolite of the phenylpropanoid biosynthesis pathway, lignin had profound effects on plant growth and development (Liu et al, 2018; Wang et al, 2019). It is extensively involved in the material transport of vascular bundles, cell wall structural integrity, stem strength, mechanical support, response to pathogens and other environmental stresses (You et al, 2013; Wang et al, 2019). The regulation of lignin synthesis affects lignin accumulation, but the growth and development of the entire plant
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