Oxalate, germin, and the extracellular matrix of higher plants

The article assembles and elaborates evidence which indicates that an 'old' enzyme, oxalate oxidase, and an even 'older' substrate, calcium oxalate, have significant and previously uncontemplated roles in the biochemistry of the extracellular matrix (ECM) of higher plants. These possibilities emerged by chance, but not really by chance, when computerized comparisons of amino acid sequences inevitably led to the discovery that germin, long known to be a protein marker of the onset of growth in germinating wheat, and later known to be an ECM protein, is an oxalate oxidase [J. Biol. Chem. 268, 12239-12242 (1993)]. Dissolution of calcium oxalate, and germin-induced degradation of the resulting soluble oxalate, can release Ca2+ and H2O2, both of which are known to have central roles in the biochemistry of the ECM in higher plants. It is therefore timely to survey the implications of the recent finding that germin is an oxalate oxidase in the context of how oxalate may participate not only in the biochemistry of the ECM, but in the development of higher plants. The findings about oxalate, as a source of H2O2, are a complement to Varner's contemporaneous advocacy of a central role for H2O2 in the development, differentiation, vascularization and signaling processes of higher plants.