Complex sentence: No enzymatic activity has been found for expansin and in particular, no glucanase activity: they don't hydrolyze the matrix polysaccharides; the only definitive assay for expansin activity is thus to measure wall stress relaxation or wall extension.
Split into simpler sentences: No enzymatic activity has been found for expansin and in particular, no glucanase activity: they don't hydrolyze the matrix polysaccharides. The only definitive assay for expansin activity is to measure wall stress relaxation or wall extension.