A new biochemical marker for aluminium tolerance in plants

Al was shown to elicit the induction of several pathogenesis-related genes, suggesting that a common signalling pathway may be involved in the early response to Al and pathogens. However, we found no evidence of oxidative burst involving either H2O2 or O2- during the first hours of Al exposure distinguishing the early response to Al from a common response to pathogen infection. We identified a strong superoxide dismutase insensitive nitro blue tetrazolium (NBT) reduction activity in the root tips of control plants. This activity was rapidly inhibited by Al exposure in the meristematic/distal transition zones of roots in all species examined. In wheat (Triticum aestivum), the inhibition of NBT reduction occurred in less than 1 min in vivo suggesting that Al either directly blocks an enzyme responsible for NBT reduction, or affects a signal pathway involved in the regulation of this activity. The sensitivity of NBT reduction to KCN and NaN3 suggests that an enzymatic, rather than a chemical reaction is involved. In tolerant plants, the inhibition of NBT reduction caused by Al was reversed within 24 h of exposure. The level of recovery was a function of the degree of Al tolerance. We show that NBT reduction is a simple biochemical marker allowing the rapid identification of tolerant individuals within a segregating population.