Abstract
As the most abundant biopolymer on Earth, cellulose is a key structural component of the plant cell wall. Cellulose is produced at the plasma membrane by cellulose synthase (CesA) complexes (CSCs), which are assembled in the endomembrane system and trafficked to the plasma membrane. While several proteins that affect CesA activity have been identified, components that regulate CSC assembly and trafficking remain unknown. Here we show that STELLO1 and 2 are Golgi-localized proteins that can interact with CesAs and control cellulose quantity. In the absence of STELLO function, the spatial distribution within the Golgi, secretion and activity of the CSCs are impaired indicating a central role of the STELLO proteins in CSC assembly. Point mutations in the predicted catalytic domains of the STELLO proteins indicate that they are glycosyltransferases facing the Golgi lumen. Hence, we have uncovered proteins that regulate CSC assembly in the plant Golgi apparatus.
Highlights
As the most abundant biopolymer on Earth, cellulose is a key structural component of the plant cell wall
Using the pfam-based co-expression tool FamNet, we found that the pfam domain of unknown function (DUF)[288] was co-expressed with the pfam cellulose synthase (CesA) (Supplementary Fig. 1a)
Microarray data suggested that STL1 and STL2 have similar expression profiles, and are active in cells that are expanding or producing secondary cell walls (Supplementary Fig. 1c), which we confirmed with transgenic Arabidopsis plants expressing pSTL1:STL1-green fluorescent protein (GFP) (Supplementary Fig. 1d–i)
Summary
As the most abundant biopolymer on Earth, cellulose is a key structural component of the plant cell wall. While several proteins that affect CesA activity have been identified, components that regulate CSC assembly and trafficking remain unknown. The bulk of plant cell wall polysaccharides are synthesized in the Golgi and secreted to the apoplast, with the exception of cellulose that is synthesized at the plasma membrane by cellulose synthase (CesA) complexes (CSCs)[2,3]. Apart from the CesA proteins, several components that impact on cellulose synthesis have been identified, including the GPI-anchored protein COBRA, the endo-glucanase KORRIGAN, the chitinase-like proteins (CTL1 and 2) and the recently discovered companion of CesA1 and 2 (refs 16–21) Most of these proteins affect the activity of the CSCs once they have reached the plasma membrane[3]. We further demonstrate that the STLs affect CSC assembly, which impacts on cellulose synthesis efficiency
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