Mucilage strongly binds aluminum but does not prevent roots from aluminum injury in Zea mays

Abstract

The possible role of mucilage in protecting roots from aluminum (Al) injury was investigated in Zea mays L. (cv. Golden Cross Bantam), focusing on binding of Al with mucilage and effects of mucilage on Al toxicity. Al was bound to mucilage after the treatment of roots with 10-50 μM Al for 1 h and 30 μM Al for 30, 60, 90 and 120 min. Using molecular sieve chromatography (Sephadex G-100), Al was co-eluted with a high molecular mass sugar and a low molecular mass sugar. The difficulty in desorbing Al from mucilage with organic acids confirmed the strong binding strength of Al by mucilage revealed by 27 Al nuclear magnetic resonance (NMR). Al could not be desorbed completely by succinic, malic, oxalic and citric acid at a molar ratio of 1:1. It could only be completely removed by oxalic acid at a molar ratio of 20:1 (oxalate:Al). Bioassay experiments showed that cell viability and callose formation were unaffected by Al bound to mucilage. However, mucilage deprived roots had only 0.21-0.59 nmol apex -1 higher Al content than control roots after treatment with 30 μM Al for 1, 1.5 and 2 h. Moreover, inhibition of root elongation by 5 μM Al for 6, 12, 24 and 36 h was independent of the presence or absence of mucilage prior to the Al treatment. These results indicate that although mucilage affects the accumulation of Al by roots, it does not confer Al resistance to Z. mays root apices.