Alternative splicing-mediated targeting of the Arabidopsis GLUTAMATE RECEPTOR3.5 to mitochondria affects organelle morphology

Ion Channels in Plant Bioenergetic Organelles, Chloroplasts and Mitochondria: From Molecular Identification to Function

Recent technical advances in electrophysiological measurements, organelle-targeted fluorescence imaging, and organelle proteomics have pushed the research of ion transport a step forward in the case of the plant bioenergetic organelles, chloroplasts and mitochondria, leading to the molecular identification and functional characterization of several ion transport systems in recent years. Here we focus on channels that mediate relatively high-rate ion and water flux and summarize the current knowledge in this field, focusing on targeting mechanisms, proteomics, electrophysiology, and physiological function. In addition, since chloroplasts evolved from a cyanobacterial ancestor, we give an overview of the information available about cyanobacterial ion channels and discuss the evolutionary origin of chloroplast channels. The recent molecular identification of some of these ion channels allowed their physiological functions to be studied using genetically modified Arabidopsis plants and cyanobacteria. The view is emerging that alteration of chloroplast and mitochondrial ion homeostasis leads to organelle dysfunction, which in turn significantly affects the energy metabolism of the whole organism. Clear-cut identification of genes encoding for channels in these organelles, however, remains a major challenge in this rapidly developing field. Multiple strategies including bioinformatics, cell biology, electrophysiology, use of organelle-targeted ion-sensitive probes, genetics, and identification of signals eliciting specific ion fluxes across organelle membranes should provide a better understanding of the physiological role of organellar channels and their contribution to signaling pathways in plants in the future.

Glutamate functions in stomatal closure in Arabidopsis and fava bean

Guard cells are indispensable for higher plants because they control gas exchange and water balance to maintain photosynthetic activity. The signaling processes that govern their movement are controlled by several factors, such as abscisic acid (ABA), blue light, pathogen-associated molecular patterns (PAMPs), and carbon dioxide. Herein, we demonstrated that the amino acid glutamate (Glu), a well-known mammalian neurotransmitter, functions as a novel signaling molecule in stomatal closure in both Arabidopsis and fava bean (Vicia faba L.). Pharmacological and electrophysiological analyses provided important clues for the participation of Glu-receptors, Ca(2+), and protein phosphorylation during the signaling process. Genetic analyses using Arabidopsis ABA-deficient (aba2-1) and ABA-insensitive (abi1-1 and abi2-1) mutants showed that ABA is not required for Glu signaling. However, loss-of-function of the Arabidopsis gene encoding Slow Anion Channel-Associated 1 (SLAC1) and Calcium-Dependent Protein Kinase 6 (CPK6) impaired the Glu response. Moreover, T-DNA knockout mutations of the Arabidopsis Glu receptor-like gene (GLR), GLR3.5, lost their sensitivity to Glu-dependent stomatal closure. Our results strongly support functional Glu-signaling in stomatal closure and the crucial roles of GLRs in this signaling process.

The EF-Hand Ca2+ Binding Protein MICU Choreographs Mitochondrial Ca2+ Dynamics in Arabidopsis

Plant organelle function must constantly adjust to environmental conditions, which requires dynamic coordination. Ca(2+) signaling may play a central role in this process. Free Ca(2+) dynamics are tightly regulated and differ markedly between the cytosol, plastid stroma, and mitochondrial matrix. The mechanistic basis of compartment-specific Ca(2+) dynamics is poorly understood. Here, we studied the function of At-MICU, an EF-hand protein of Arabidopsis thaliana with homology to constituents of the mitochondrial Ca(2+) uniporter machinery in mammals. MICU binds Ca(2+) and localizes to the mitochondria in Arabidopsis. In vivo imaging of roots expressing a genetically encoded Ca(2+) sensor in the mitochondrial matrix revealed that lack of MICU increased resting concentrations of free Ca(2+) in the matrix. Furthermore, Ca(2+) elevations triggered by auxin and extracellular ATP occurred more rapidly and reached higher maximal concentrations in the mitochondria of micu mutants, whereas cytosolic Ca(2+) signatures remained unchanged. These findings support the idea that a conserved uniporter system, with composition and regulation distinct from the mammalian machinery, mediates mitochondrial Ca(2+) uptake in plants under in vivo conditions. They further suggest that MICU acts as a throttle that controls Ca(2+) uptake by moderating influx, thereby shaping Ca(2+) signatures in the matrix and preserving mitochondrial homeostasis. Our results open the door to genetic dissection of mitochondrial Ca(2+) signaling in plants.

Signalling via glutamate and GLRs in Arabidopsis thaliana

The genome of Arabidopsis thaliana (L. Heynh.) contains 20 coding sequences for homologues of animal ionotropic glutamate receptors. These glutamate receptor-like receptors act as sensors and mediators of a multitude of exogenous as well as endogenous signals and are found in all analysed plant species. Their molecular structure clearly indicates a function as integral membrane proteins with a ligand-gated ion channel activity. Altered gene expressions and the occurrence of mRNA splice variants confer a high flexibility on the gene as well as on the RNA level. An individual glutamate receptor of A. thaliana is able to bind two different ligands (most probable amino acids and their derivatives), whereas a functional receptor complex is likely to consist of four single proteins. These features enable an immense number of sensitivities against various local and temporal stimuli. This review encompasses the last 15 years of research concerning glutamate signalling and glutamate receptors in plants. It is aimed at summarising their major characteristics and involvements to obtain a broader and farer reaching perspective of these fundamental components of plant signal transduction.