Arabidopsis mutants used to study the effects of S(+)-beta-methyl-alpha, beta-diaminopropionic acid (BMAA), a cycad-derived glutamate receptor agonist

To begin to understand the molecular mechanisms that regulate plant glutamate receptors, a forward genetic approach to isolate mutants resistant to the effects of BMAA, an animal glutamate receptor agonist, was developed. Because of the neurotoxic properties attributed to BMAA on the animal iGluRs, characterization of these mutants may provide new insights into the general functioning of glutamate receptors. BMAA affects Arabidopsis seedlings by inducing elongation of the hypocotyl. But BMAA insensitive (bim) mutants are resistant to BMAA and have short hypocotyls. The bim mutants were subgrouped into three Classes, based on their darkgrown morphology: Class I bim mutants have an etiolated phenotype, Class II bim mutants have short hypocotyls and closed cotyledons, and Class III bim mutants are constitutively photomorphogenic with short hypocotyls and open cotyledons. This study focuses on three bim mutants from Class II. The identity of the mutants was revealed through positional cloning, genetic complementation analysis, and gene sequencing. Positional cloning identified CHITINASE-LIKE gene (CTL1) and genetic complementation identified PROCUSTE (PRC1). The inability of these mutants to elongate their hypocotyl is likely due to cell wall defects. In order to understand the link between these cell wall mutants and BMAA effects, transcript levels for the identified genes were quantified under different conditions. PROCUSTE and CHITINASE-LIKE genes were found to be repressed by light, but when BMAA is added there is a slight induction. In the dark, the presence of BMAA causes a significant repression of the transcript levels when compared to those in the absence of BMAA. I provide a model to describe the effect of BMAA on PRC1 and CTL1 in wild type. A separate series of experiments described in the last chapter provides data indicating that BMAA may alter ion sensitivity in wild-type Arabidopsis seedlings.