A Calcium-Conducting Plant Glutamate Receptor, GLR3.4, in Phloem Regulates Lateral Root Initiation

The discovery of twenty Arabidopsis homologs of the mammalian ionotropic glutamate-receptor (iGluR) channels responsible for electrochemical signaling in the central nervous system raised many questions about their physiological roles in plants. The plant glutamate receptor-like (GLR) proteins are similar in general respects to iGluRs but different enough in key regions to preclude any assumptions about function. One critical open question is whether or not GLRs are ligand-gated Ca²⁺-permeable channels. Patch clamp analysis of human embryonic kidney (HEK) cells expressing GLR3.4 showed that asparagine, serine, and to a lesser extent glycine gated a sustained transmembrane Ca²⁺ current. Cytoplasmic Ca²⁺ concentration transiently increased in HEK cells expressing GLR3.4 following amino acid treatment. Thus, GLR3.4 encodes an amino acid-gated, non-desensitizing, Ca²⁺-permeable channel. What physiological roles such channels play in plants is another pressing question. In roots, GLR3.4 and the related GRL3.2 protein were present primarily in the phloem, especially at the specialized end walls of adjoining sieve tube members. Mutations in GLR3.2 or GLR3.4 caused a large overproduction and aberrant placement of lateral root primordia. The related GLR3.3 gene was also expressed in roots but it was not observed to accumulate at sieve plates, and its mutation did not affect lateral root primordia. These results support the hypothesis that apoplastic amino acids acting through specific GLR channels influence lateral root development via Ca²⁺ signaling in the phloem.