Selective element deposits in maize colonized by a heavy metal tolerance conferring arbuscular mycorrhizal fungus

The Glomus isolate Brl from the zinc violet, Viola calaminaria (DC.) Lej., confers heavy metal tolerance to plants including maize, alfalfa, barley and others (see accompanying paper). In the present study, the bulk analysis of maize grown in two different heavy metal soils in greenhouse experiments indicated that roots and shoots contained considerably lower heavy metal concentrations when maize was colonized with the isolate Brl compared with plants grown with a common Glomus strain or to non-colonized controls. Essential elements like K, P and Mg were enriched in roots in Brl colonized maize. Since arbuscular mycorrhizal (AM) plana grew much faster until flower and seed formation and had an approximately 25-fold higher dry weight than the controls, a massive acquisition of essential elements has happened. Data from three different microbeam techniques indicated distinct differences in the cellular distribution of heavy metals and essential elements in AM colonized roots compared with the non-mycorrhizal controls. SIMS images showed a selective enrichment of Mg, Ca, Fe, Ni and Zn in the inner cortical cell region containing the fungal structures (arbuscules) and a lower concentration of the heavy metals Fe, Zn and Ni in the stele than in the cortex. EDXA measurements indicated a selective enrichment of Mg and K in the stele. The data from SIMS and LAMMA suggested Al to be more or less evenly distributed in the root cells. The present investigation appears to be the first comprehensive approach to map elemental distribution within root tissues in AM colonized and control maize by three different methods of microbeam analysis. Since the microbeam techniques had to be applied near the detection limit of the methods, the data obtained by the three different approaches were not always uniform. However, the combination of these three techniques showed that the growth of maize in the heavy metal soil was at least partly due to a selective immobilization of heavy metals within those root tissues containing the fungal cells. The measurements also indicated that AM fungi might cope with heavy metal toxicity for each metal individually.