Enigmatic Apiogalacturonans – What Do We Know About This Group of Pectic Polysaccharides?

Abstract

Abstract The plant cell wall consists of various polysaccharide groups contributing to its functions both structurally and physiologically. Within the most complex group of cell wall polysaccharides, the pectins, one subgroup contains the unusual branched‐chain pentose apiose as major decorating sugar. That polysaccharide, named apiogalacturonan, was first described in the 1960s and subsequent studies tried to resolve different aspects such as structure, biosynthesis, and function. One aspect of the research was its taxonomical distribution, which seemed to be restricted to only a few genera within the whole plant kingdom. The physiological reason for that as well as the genetic background is currently unknown. Some progress was achieved during the last years in shedding more light on the biosynthesis of the activated sugar but to a fundamental understanding of apiogalacturonan biosynthesis much more work is to be done. The aim of this article is to present the different aspects of the performed research studies, to draw conclusions for the current state of research, and to propose future directions to further extend the knowledge on these unusual macromolecules.