The cell wall pectic rhamnogalacturonan II, an enigma in plant glycobiology

During the colonization of the land habitat from the marine ecosystem, plants were constrained to adapt their physiology to this new environment. The emergence of cell wall rhamnogalacturonan II (RG-II) was one of the most remarkable adaptive innovations that occurred during the evolution of land plants. This complex pectic molecule is composed of a short homogalacturonan domain substituted by four structurally complex side chains containing rare monosaccharides such as d-apiose (d-Api), l-aceric acid (l-AceA), 3-deoxy-d-manno-oct-2-ulosonic acid (d-Kdo) and 3-deoxy-d-lyxo-hept-2-ulosaric acid (d-Dha). In addition, RG-II is dimerized in muro by a borate diester bond that is required for normal plant development. RG-II structure was conserved during evolution of land plants to maintain its capability to perform the borate-mediated dimerization. Although the alteration of RG-II biosynthesis and in muro dimerization are known to deeply affect cell growth and plant development, the biological significance of this polymer is still an enigma for plant biologists, even forty years after its discovery.