Rhamnogalacturonan-II, a pectic polysaccharide in the walls of growing plant cell, forms a dimer that is covalently cross-linked by a borate ester. - O'Neill, ,M..A.,.; Warrenfeltz, ,D.,.; Kates, ,K.,.; Pellerin, ,P.,.; Doco, ,T.,.; Darvill, ,A..G.,.; and Albersheim, ,P.. (J. Biol. Chem., 1996)