An aluminum-resistance mechanism in eucalyptus camaldulensis: Complexation between aluminum and oenothein B in presence of organic acids in vitro

A hydrolyzable tannin, oenothein B, has recently been identified as a novel type of Al-binding and detoxifying ligand in a highly aluminum (Al)-resistant tree, Eucalyptus camaldulensis. Oenothein B is mainly localized in the root symplast, where the known ligands, citrate and oxalate, are also present. In this study, to investigate whether oenothein B can form complexes with Al in the presence of citrate and oxalate, we analyzed the chemical form of Al in multiple-ligand mixtures containing Al, oenothein B, citrate, and oxalate in vitro at concentrations mimicking those in the symplast of E. camaldulensis roots. In the mixture, 27% of total Al was detected in the insoluble fraction, and 28% in the high-molecular mass (> 10 kDa) soluble fraction. This means that more than half of Al in the mixture interacts with oenothein B, because among the three ligands only oenothein B can predominantly form insoluble or high molecular mass soluble complexes with Al. The²⁷Al-nuclear magnetic resonance analyses of the mixture showed that low-molecular mass soluble Al was not mainly composed of simple Al–citrate or Al–oxalate complexes. These results support our hypothesis that oenothein B contributes to internal detoxification of Al in the symplast of E. camaldulensis roots.