Storming the barricades of rhamnogalacturonan-II synthesis and function.

Despite its low abundance, rhamnogalacturonan-II (RG-II) is an essential structural component of the cell wall and is present in a highly conserved molecular configuration across all plants. RG-II is a branched pectin domain that contains 13 different sugars linked by over 20 different bond types, and uniquely among pectins it can be covalently dimerized via borate diesters. RG-II is hypothesized to crosslink the pectin matrix, controlling cell wall architecture and porosity, but has resisted detailed analyses due to its compositional complexity and the lethality of RG-II-deficient mutants. Here, we highlight how biochemical dissection, genetic engineering, chemical inhibitors, and high-resolution imaging have enabled recent leaps in our understanding of RG-II structure, synthesis, localization, dimerization, and function, pointing out new questions and research directions that have been enabled by these advances.