The cell biology of secondary cell wall biosynthesis

In a new review, Meents and colleagues look at the creation of the secondary cell wall (SCWs), the feature that form the architecture of terrestrial plant biomass. This strong and rigid wall sits inside the primary cell wall, where it provides physical support for the plant and reinforces conduits for long-distance transport in the xylem. Although only a subset of cells in any given plant form SCWs, in many woody plants, such as trees, the majority of the plant’s mass is composed of SCWs in the form of fibres, tracheids and vessels.

Secondary cell walls in primary and secondary stem growth. Illustrative examples of SCWs in water-conducting cells (vessels, tracheids) and supportive fibres. (A and B) SCWs in primary growth. (A) SCWs in monocot primary growth exemplified in a cross-section of a grass stem internode where vascular bundles with large metaxylem vessels are encased in SCW-rich sclerenchyma (e.g. Brachypodium). (B) SCWs in eudicot primary growth illustrated in a stem cross-section prior to onset of secondary thickening (e.g. Brassica). SCWs are found in the vascular vessels and fibres, which are continuous with the thick interfascicular fibres. (C and D) SCWs in secondary growth, marked by the presence of the vascular cambium. (C) Gymnosperm secondary growth showing thick SCWs in the water-conducting and supportive tracheids of the secondary xylem (e.g. Pinus). (D) Angiosperm secondary growth showing SCWs in the water-conducting vessels and the supportive fibres (e.g. Populus).

The cells that synthesise a strong, thick secondary cell wall around their protoplast must undergo a dramatic commitment to cellulose, hemicellulose and lignin production. As the authors have noted in previous work, the walls form rapidly. Cellulose, hemicellulose and lignin are deposited in precise and characteristic patterns depending on their physiological function. Thinking about the secondary cell wall in a cell biology context helps us to see how these diverse biosynthetic processes are interconnected, relying on many of the same organelles, such as the Golgi and the microtubule-lined secondary cell wall domains of the plasma membrane. SCW biosynthesis requires the coordination of plasma membrane cellulose synthases, hemicellulose production in the Golgi and lignin polymer deposition in the apoplast. Additionally, author Lacey Samuels has shown that it’s not just from inside the cell, neighbouring cells can contribute to lignification.

The bioenergy research world has promoted the study of diverse taxa, such as grasses and poplar, and advanced our understanding of secondary cell wall biosynthetic proteins and their products. Learning how these proteins are arranged and controlled by the cell, and how they interact in the Golgi, at the plasma membrane and in the secondary cell wall, may provide some unexpected insights that will contribute to the exploitation of this carbon-rich renewable resource.