A family of peptide signaling molecules (AtPeps) and their plasma membrane receptor AtPepR1 are known to act in pathogen-defense signaling cascades in plants. Little is currently known about the molecular mechanisms that link these signaling peptides and their receptor, a leucine-rich repeat receptor-like kinase, to downstream pathogen-defense responses. We identify some cellular activities of these molecules that provide the context for a model for their action in signaling cascades. AtPeps activate plasma membrane inwardly conducting Ca ²⁺ permeable channels in mesophyll cells, resulting in cytosolic Ca ²⁺ elevation. This activity is dependent on their receptor as well as a cyclic nucleotide-gated channel (CNGC2). We also show that the leucine-rich repeat receptor-like kinase receptor AtPepR1 has guanylyl cyclase activity, generating cGMP from GTP, and that cGMP can activate CNGC2-dependent cytosolic Ca ²⁺ elevation. AtPep-dependent expression of pathogen-defense genes ( PDF1.2 , MPK3 , and WRKY33 ) is mediated by the Ca ²⁺ signaling pathway associated with AtPep peptides and their receptor. The work presented here indicates that extracellular AtPeps, which can act as danger-associated molecular patterns, signal by interaction with their receptor, AtPepR1, a plasma membrane protein that can generate cGMP. Downstream from AtPep and AtPepR1 in a signaling cascade, the cGMP-activated channel CNGC2 is involved in AtPep- and AtPepR1-dependent inward Ca ²⁺ conductance and resulting cytosolic Ca ²⁺ elevation. The signaling cascade initiated by AtPeps leads to expression of pathogen-defense genes in a Ca ²⁺ -dependent manner.

Caesium (Cs⁺) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs⁺ is chemically similar to potassium (K⁺), and much is known about K⁺ transport mechanisms, it is not clear through which K⁺ transport mechanisms Cs⁺ is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K⁺ and Cs⁺ uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K⁺ deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K⁺ deprivation. Under K⁺-deficient conditions in the presence of Cs⁺, Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs⁺ accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K⁺in planta, AtHAK5 also transports Cs⁺. Further experiments showed that AtHAK5 mediated Cs⁺ uptake into yeast cells and that, although the K⁺ deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH₄⁺in planta, Cs⁺ uptake by yeast was stimulated by low concentrations of NH₄⁺. Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs⁺ than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K⁺ and Cs⁺ under conditions of low K⁺ availability.

The amino acids glutamate (Glu) and glycine (Gly) trigger large, rapid rises in cytosolic Ca2+ concentration and a concomitant rise in membrane potential (depolarization) in plants. The possibility that plant homologs of neuronal ionotropic glutamate receptors mediate these neuron-like ionic responses was tested in Arabidopsis (Arabidopsis thaliana) seedlings using a combination of Ca2+ measurements, electrophysiology, and reverse genetics. The membrane depolarization triggered by Glu was greatly reduced or completely blocked in some conditions by mutations in GLR3.3, one of the 20 GLR genes in Arabidopsis. The same mutations completely blocked the associated rise in cytosolic Ca2+. These results genetically demonstrate the participation of a glutamate receptor in the rapid ionic responses to an amino acid. The GLR3.3-independent component of the depolarization required Glu concentrations above 25 μ m, did not display desensitization, and was strongly suppressed by increasing extracellular pH. It is suggested to result from H+-amino acid symport. Six amino acids commonly present in soils (Glu, Gly, alanine, serine, asparagine, and cysteine) as well as the tripeptide glutathione (γ-glutamyl-cysteinyl-Gly) were found to be strong agonists of the GLR3.3-mediated responses. All other amino acids induced a small depolarization similar to the non-GLR, putative symporter component and in most cases evoked little or no Ca2+ rise. From these results it may be concluded that sensing of six amino acids in the rhizosphere and perhaps extracellular peptides is coupled to Ca2+ signaling through a GLR-dependent mechanism homologous to a fundamental component of neuronal signaling.

Background: Reaumuria trigyna is an endangered small shrub endemic to desert regions in Inner Mongolia. This dicotyledonous recretohalophyte has unique morphological characteristics that allow it to tolerate the stress imposed by semi-desert saline soil. However, it is impossible to explore the mechanisms underlying this tolerance without detailed genomic information. Fortunately, newly developed high-throughput sequencing technologies are powerful tools for de novo sequencing to gain such information for this species. Results: Two sequencing libraries prepared from control (C21) and NaCl-treated samples (T43) were sequenced using short reads sequencing technology (Illumina) to investigate changes in the R. trigyna transcriptome in response to salt stress. Among 65340 unigenes, 35495 (52.27%) were annotated with gene descriptions, conserved domains, gene ontology terms, and metabolic pathways with a cut-off E-value of 10^-5. These included 44 Gene Ontology (GO) terms, 119 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and 25 Clusters of Orthologous Groups families. By comparing the transcriptomes from control and NaCl-treated plants, 5032 genes showed significantly differences in transcript abundance under salt stress (false discovery rate ≤ 0.001 and |log₂Ratio| ≥ 1). These genes were significantly enriched in 29 KEGG pathways and 26 GO terms. The transcription profiles indicated that genes related to ion transport and the reactive oxygen species scavenging system were relevant to the morphological and physiological characteristics of this species. The expression patterns of 30 randomly selected genes resulted from quantitative real-time PCR were basically consistent with their transcript abundance changes identified by RNA-seq. Conclusions: The present study identified potential genes involved in salt tolerance of R. trigyna. The globally sequenced genes covered a considerable proportion of the R. trigyna transcriptome. These data represent a genetic resource for the discovery of genes related to salt tolerance in this species, and may be a useful source of reference sequences for closely related taxa. These results can also further our understanding of salt tolerance in other halophytes surviving under sodic stress.

Arabidopsis CNGC2 is a Ca2+-permeable channel that mediates Ca2+ influx into mesophyll cells near minor veins but has no direct role in the hypersensitive respo

The tripeptide reduced glutathione (GSH; γ-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 Glu-induced 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.