Pectic substances: From simple pectic polysaccharides to complex pectins—A new hypothetical model

Pectic substances are a group of polysaccharides in plant cell walls and some mucilages, which are endowed with multifunctional properties in muro as well as out of muro, such as the control of cell wall integrity and porosity, the protection of plants against phytopathogens, and gelling, emulsifying, stabilising, thickening and health benefit properties to name a few. The fine structures of distinct pectic elements and complex pectins formed from these are not, however, fully known. It is widely believed that three to four pectic element kinds, namely, homogalacturonan (HG), rhamnogalacturonan-I (RG-I), rhamnogalacturonan-II (RG-II) and/or xylogalacturonan (XGA) are covalently inter-linked to form pectin-complex in muro, but the way different blocks of these pectic polysaccharides are positioned relative to one another in such a macromolecular pectin-complex is still a matter of controversy. This review aims at emphasizing on the depiction of the four possible structural elements of complex pectins and providing an update for them in the light of the latest findings. In addition, it pinpoints evidence for the insufficiency of the two main hypothetical models currently existing and the possibility that such a pectin composite could be more complicated as shown in a new hypothetical model.