Abstract

Tree bark is a highly specialized array of tissues that plays important roles in plant protection and development. Bark tissues develop from two lateral meristems; the phellogen (cork cambium) produces the outermost stem-environment barrier called the periderm, while the vascular cambium contributes with phloem tissues. Although bark is diverse in terms of tissues, functions and species, it remains understudied at higher resolution. We dissected the stem of silver birch (Betula pendula) into eight major tissue types, and characterized these by a combined transcriptomics and metabolomics approach. We further analyzed the varying bark types within the Betulaceae family. The two meristems had a distinct contribution to the stem transcriptomic landscape. Furthermore, inter- and intraspecies analyses illustrated the unique molecular profile of the phellem. We identified multiple tissue-specific metabolic pathways, such as the mevalonate/betulin biosynthesis pathway, that displayed differential evolution within the Betulaceae. A detailed analysis of suberin and betulin biosynthesis pathways identified a set of underlying regulators and highlighted the important role of local, small-scale gene duplication events in the evolution of metabolic pathways. This work reveals the transcriptome and metabolic diversity among bark tissues and provides insights to its development and evolution, as well as its biotechnological applications.

Highlights

  • Tree bark displays a broad morphological diversity

  • Bark consists of tissues outwards of the vascular cambium: the phloem and periderm, the latter comprised of phelloderm, phellogen and phellem

  • Tangential cryosectioning provided a stemwide perspective of bark tissues in B. pendula

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Summary

Introduction

Bark consists of tissues outwards of the vascular cambium: the phloem and periderm, the latter comprised of phelloderm, phellogen and phellem. Bark tissues originate from stem secondary development, which in woody plants takes place in two distinct lateral meristems. The first meristem, vascular cambium, produces the phloem and xylem tissues. Phellem constitutes the outermost barrier between the stem and the environment, while phelloderm is typically limited to a few parenchymatic cell layers. This developmental progression remains poorly dissected in trees, but a recent study in Arabidopsis described how differentiation steps lead to periderm development starting from pericycle cells (Wunderling et al, 2018)

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