Arabinogalactan-proteins stimulate the organogenesis of guard cell protoplasts-derived callus in sugar beet

Arabinogalactan proteins (AGPs) represent a class of proteoglycans implicated in the development and differentiation of cells and tissues both in planta and in vitro. Here we report that AGP-rich extracts isolated from media of embryogenic and non-embryogenic suspension cultures of sugar beet (Beta vulgaris L.) are able to enhance the organogenesis of guard protoplast-derived callus and to increase the number of shoots formed, in comparison to control cultures. Immunocytochemical detection of carbohydrate antigens in the extracts revealed the presence of epitopes that typify both AGP and pectin, the latter being frequently bound to AGPs or, in some cases, even contributing to the polysaccharide structure of proteoglycan molecules. The most abundant epitopes proved to be those recognized by the JIM13, LM2, and MAC207 antibodies, whereas some others could be found only in relatively small or trace amounts-these included epitopes recognized by JIM16, JIM5, and LM6. Surprisingly, the JIM4- and JIM8-binding epitopes that are expressed in the course of in vitro morphogenetic processes of many species could not be detected at all in sugar beet AGPs. This is the first report of the improvement of sugar beet protoplast-derived callus organogenesis by exogenous AGP-rich extracts, an achievement that will have great impact on the biotechnological applications of protoplast technology in this species.