Glutamate Receptor-Like Channel3.3 Is Involved in Mediating Glutathione-Triggered Cytosolic Calcium Transients, Transcriptional Changes, and Innate Immunity Responses in Arabidopsis

Abstract

The tripeptide reduced glutathione (GSH; g-glutamate [Glu]-cysteine [Cys]-glycine) is a major endogenous antioxidant in both animal and plant cells. It also functions as a neurotransmitter mediating communication among neurons in the central nervous system of animals through modulating specific ionotropic Glu receptors (GLRs) in the membrane. Little is known about such signaling roles in plant cells. Here, we report that transient rises in cytosolic calcium triggered by exogenous GSH in Arabidopsis (Arabidopsis thaliana) leaves were sensitive to GLR antagonists and abolished in loss-of-function atglr3.3 mutants. Like the GSH biosynthesis-defective mutant PHYTOALEXIN DEFICIENT2, atglr3.3 showed enhanced susceptibility to the bacterial pathogen Pseudomonas syringae pv tomato DC3000. Pathogen-induced defense marker gene expression was also decreased in atglr3.3 mutants. Twenty-seven percent of genes that were rapidly responsive to GSH treatment of seedlings were defense genes, most of which were dependent on functional AtGLR3.3, while GSH suppressed pathogen propagation through the AtGLR3.3-dependent pathway. Eight previously identified putative AtGLR3.3 ligands, GSH, oxidized glutathione, alanine, asparagine, Cys, Glu, glycine, and serine, all elicited the AtGLR3.3-dependent cytosolic calcium transients, but only GSH and Cys induced the defense response, with the Gluinduced AtGLR3.3-dependent transcription response being much less apparent than that triggered by GSH. Together, these observations suggest that AtGLR3.3 is required for several signaling effects mediated by extracellular GSH, even though these effects may not be causally related.

Arabidopsis Glutamate Receptor Homolog3.5 Modulates Cytosolic Ca2+ Level to Counteract Effect of Abscisic Acid in Seed Germination

“…Since the discovery of GLR genes in plants (Lam et al, 1998), many studies have demonstrated that Ca 2+ levels mediated by AtGLRs affect myriad cellular processes (Kim et al, 2001;Meyerhoff et al, 2005;Cho et al, 2009;Michard et al, 2011;Li et al, 2013;Mousavi et al, 2013;Vincill et al, 2013). To our knowledge, this is the first report demonstrating that cytoplasmic Ca 2+ dynamics modulated by a plant GLR play a pivotal role in seed germination.…”

“…In plants, several gene families are thought to encode Ca 2+ -permeable channels (Ward et al, 2009;Hedrich, 2012), among them Arabidopsis (Arabidopsis thaliana) glutamate receptor homologs (AtGLRs) share high structural similarity to ionotropic Glu receptor Ca 2+ channels in animals (Lam et al, 1998), and consist of 20 gene family members (Lacombe et al, 2001). Studies have shown that these plant GLRs participate in diverse physiological and developmental procedures, including pollen tube morphogenesis (Michard et al, 2011), root development (Kang et al, 2004;Li et al, 2006;Vincill et al, 2013), stomatal closure regulation (Cho et al, 2009), and various stress responses (Kim et al, 2001;Meyerhoff et al, 2005;Li et al, 2013;Mousavi et al, 2013), which are mostly dependent on Ca 2+ . Despite the importance of AtGLR-related Ca 2+ modulation in these processes, the function and molecular mechanism of Ca 2+ signaling and AtGLRs in seed germination, the early plant developmental event that greatly impacts the plant's entire life and agricultural performance (Rajjou et al, 2012), is largely unknown.…”

The Raf-like kinase ILK1 and the high affinity K+ transporter HAK5 are required for Innate Immunity and Abiotic Stress Response

“…ILK1's kinase-independent role in promoting rapid flg22-induced PM depolarization may be due to the interaction between transport systems. For example, HAK5 transport could be influencing other channels at the PM such as the Glu receptor-like channel GLR3.3, which promotes Ca 2+ influx during immunity (Qi et al, 2006;Li et al, 2013).…”

Inter-subunit interactions between Glutamate-Like Receptors inArabidopsis

“…16 The predicted membrane topology and orientation of the protein as a tetramer, with the ATD and LBD exposed to the external side of the membrane, are considered to be conserved in plant GLRs. 17 Understanding the function of plant GLRs has previously been hampered by gene redundancy and toxicity although genetic, pharmacological, and electrophysiological approaches suggested diverse physiological roles such as carbon/nitrogen balance regulation, 18,19 stomatal opening, 20 pollen tube growth, 21 plant-pathogen interaction, [22][23][24] responses to wound, 25,26 and lateral root formation. 12 Recently, 2 of the Arabidopsis GLRs, AtGLR3.4 and 1.4, were shown to function as amino acid gated channels when expressed in heterologous systems, 13,27 further confirming the functional commonality of plant and mammalian glutamate receptors.…”