The trafficking of the cellulose synthase complex in higher plants

Abstract

Background: Cellulose is an important constituent of plant cell walls in a biological context, and is also a material commonly utilized by mankind in the pulp and paper, timber, textile and biofuel industries. The biosynthesis of cellulose in higher plants is a function of the cellulose synthase complex (CSC). The CSC, a large transmembrane complex containing multiple cellulose synthase proteins, is believed to be assembled in the Golgi apparatus, but is thought only to synthesize cellulose when it is localized at the plasma membrane, where CSCs synthesize and extrude cellulose directly into the plant cell wall. Therefore, the delivery and endocytosis of CSCs to and from the plasma membrane are important aspects for the regulation of cellulose biosynthesis.

Scope: Recent progress in the visualization of CSC dynamics in living plant cells has begun to reveal some of the routes and factors involved in CSC trafficking. This review highlights the most recent major findings related to CSC trafficking, provides novel perspectives on how CSC trafficking can influence the cell wall, and proposes potential avenues for future exploration.

Keywords: Plant cell wall; actin; cellulose synthase; endocytosis; exocytosis; microtubule; trafficking; trans-Golgi network.

Fig. 1.

An overview of the localization, trafficking pathways and proposed trafficking mechanisms of CSCs. The schematic diagram shows the many cell wall-related trafficking compartments and pathways that have been proposed. Pectin and hemicellulose are synthesized in the Golgi apparatus, and the polymers are secreted to the wall, whereas cellulose is exclusively synthesized by CSCs that are located at the plasma membrane. The CSC is thought to be assembled in the Golgi apparatus, which is responsible for the actin-dependent cell-wide distribution of CSCs. CSCs are believed to be secreted through the TGN/EE and may be partitioned into a specific domain of the TGN/EE. The mechanisms for trafficking of CSCs away from the TGN are unknown, but might involve clathrin-mediated vesicle formation. The mechanism of the fusion of CSC-containing vesicles at the plasma membrane is also unknown, but it is known that these events occur at the cortical microtubules. We speculate that the exocyst complex might be involved in the fusion event. Plasma membrane-localized CSCs move bi-directionally along cortical microtubules and are believed to be propelled by the force of cellulose polymerization. CSI proteins mediate the CSC–microtubule interaction. The endocytosis of CSCs have been shown to occur through AP2/clathrin-mediated endocytosis, and newly endocytosed material is known to be rapidly trafficked to the TGN/EE where recycling of CSCs might occur. CSC and CSI-containing cortical MASCs could be responsible for secretion, endocytosis and/or recycling, and we propose that a kinesin motor might be involved in the motility of MASCs at microtubule-depolymerizing ends.