Exogenous L-glutamate (L-Glu) has been shown to be able to elicit major changes in Arabidopsis root architecture at micromolar concentrations. The root response, which is strongly genotype-dependent, is specific to L-Glu and involves both inhibition of primary root growth and stimulation of root branching behind the primary root tip. The L-Glu appears to be sensed directly at the root tip, where it inhibits meristematic activity. An intriguing and still unanswered question is whether members of the family of Glu receptor-like genes (GLRs) have a role in mediating this response. A pharmacological approach described here, using agonists and antagonists of mammalian ionotropic Glu receptors, has failed to
resolve the issue. Progress towards identifying the genes involved in the root response to L-Glu is likely to come through the application of forward and reverse genetics, in combination with quantitative trait loci (QTL) mapping.