Golgi-localized exo-β1,3-galactosidases involved in AGP modification and root cell expansion in Arabidopsis

Plant arabinogalactan proteins (AGPs) are a diverse group of cell surface- and wall-associated glycoproteins. Functionally important AGP glycans are synthesized in the Golgi apparatus, but the relationships between their glycosylation, processing, and functionality are poorly understood. Here we report the identification and functional characterization of two Golgi-localized exo-β-1,3-galactosidases from the glycosyl hydrolase 43 (GH43) family in Arabidopsis thaliana . GH43 loss of function mutants exhibit root cell expansion defects in sugar-containing growth media. This root phenotype is associated with an increase in the extent of AGP cell wall association, as demonstrated by Yariv phenylglycoside dye quantification and comprehensive microarray polymer profiling of sequentially extracted cell walls. Recombinant GH43 characterization showed that the exo-β-1,3-galactosidase activity of GH43s is hindered by β-1,6 branches on β-1,3-galactans. In line with this steric hindrance, the recombinant GH43s did not release galactose from cell wall extracted glycoproteins or AGP rich gum arabic. These results show that Arabidopsis GH43s are involved in AGP glycan biosynthesis in the Golgi, and suggest their exo-β-1,3-galactosidase activity influences AGP and cell wall matrix interactions, thereby adjusting cell wall extensibility.