Cell-wall antigens in mesophyll cells and mesophyll-derived protoplasts of sugar beet: possible implication in protoplast recalcitrance?

We have investigated the possible relation between plant cell-wall constituents and the recalcitrance of the cell to regenerate organs and whole plants in vitro. A temporal and spatial expression of several carbohydrate epitopes was observed both within leaf tissue used for protoplast isolation and within new walls reformed by recalcitrant mesophyll protoplasts of sugar beet ( Beta vulgaris L.); these include four pectic epitopes, one xyloglucan (rhamnogalacturonan I) epitope, two carbohydrate motifs of arabinogalactan proteins (AGPs) and callose. The walls of mesophyll cells and newly formed walls of protoplasts were similar with respect to the presence of large amounts of pectins recognized by JIM7 antibodies, the scarcity of JIM5-pectins and the complete absence of LM5-responding pectin molecules. Their main differences were the significantly higher accumulation of LM6-recognizing pectins and the very conspicuous greater accumulation of AGPs and callose in walls deposited by protoplasts than in those synthesized by donor cells.