Gallic acid and hydrolysable tannins are formed in birch leaves from an intermediate compound of the shikimate pathway

Abstract

Gallic acid is the starting material for hydrolysable tannin synthesis, but the mechanism of its formation in higher plants has not been known. To elucidate the pathway of gallic acid synthesis in the leaves of mountain birch ( Betula pubescens ssp. czerepanovii), we studied the effects of glyphosate (N-[phosphonomethyl]-glycine) on the levels of individual hydrolysable tannins. Glyphosate is known to block 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, an enzyme of the shikimate pathway, thus reducing the synthesis of aromatic amino acids and phenolics derived from phenylalanine. However, when birch leaves were treated with glyphosate (1 mM, 72 h), the contents of individual hydrolysable tannins increased about twofold compared to control leaves. Our findings indicate that the immediate precursor of gallic acid may be an intermediate compound of the shikimate pathway before EPSP, most probably 3-dehydroshikimic acid. Consistently with this assumption, a highly active enzyme dehydroshikimate dehydrogenase (DSDG), which catalyses the direct conversion of 3-dehydroshikimic acid into gallic acid, was found in birch leaves. This new enzyme was partially purified to study its properties. It was found that DSDG is an NADP-dependent enzyme with a pH optimum at about 10.0. The enzyme showed an affinity for NADP + approximately 60-fold (K m=0.008 mM) that for dehydroshikimate (K m=0.49 mM). The intracellular organisation of the shikimate pathway and the hydrolysable tannin pathway are discussed.