Dale Sanders

Calcium at the crossroads of signaling

### Calcium Signals: A Central Paradigm in Stimulus–Response Coupling Cells must respond to an array of environmental and developmental cues. The signaling networks that have evolved to generate appropriate cellular responses are varied and are normally composed of elements that include a

Communicating with Calcium

### Calcium as a Ubiquitous Signal in Plants All living cells use a network of signal transduction pathways to conduct developmental programs, obtain nutrients, control their metabolism, and cope with their environment. A major challenge for cell biologists is to understand the “language” of

The Language of Calcium Signaling

Ca(2+) signals are a core regulator of plant cell physiology and cellular responses to the environment. The channels, pumps, and carriers that underlie Ca(2+) homeostasis provide the mechanistic basis for generation of Ca(2+) signals by regulating movement of Ca(2+) ions between subcellular compartments and between the cell and its extracellular environment. The information encoded within the Ca(2+) transients is decoded and transmitted by a toolkit of Ca(2+)-binding proteins that regulate transcription via Ca(2+)-responsive promoter elements and that regulate protein phosphorylation. Ca(2+)-signaling networks have architectural structures comparable to scale-free networks and bow tie networks in computing, and these similarities help explain such properties of Ca(2+)-signaling networks as robustness, evolvability, and the ability to process multiple signals simultaneously.

The vacuolar Ca2+-activated channel TPC1 regulates germination and stomatal movement

Cytosolic free calcium ([Ca2+]cyt) is a ubiquitous signalling component in plant cells. Numerous stimuli trigger sustained or transient elevations of [Ca2+]cyt that evoke downstream stimulus-specific responses. Generation of [Ca2+]cyt signals is effected through stimulus-induced opening of Ca2+-permeable ion channels that catalyse a flux of Ca2+ into the cytosol from extracellular or intracellular stores. Many classes of Ca2+ current have been characterized electrophysiologically in plant membranes. However, the identity of the ion channels that underlie these currents has until now remained obscure. Here we show that the TPC1 (‘two-pore channel 1’) gene of Arabidopsis thaliana encodes a class of Ca2+-dependent Ca2+-release channel that is known from numerous electrophysiological studies as the slow vacuolar channel. Slow vacuolar channels are ubiquitous in plant vacuoles, where they form the dominant conductance at micromolar [Ca2+]cyt. We show that a tpc1 knockout mutant lacks functional slow vacuolar channel activity and is defective in both abscisic acid-induced repression of germination and in the response of stomata to extracellular calcium. These studies unequivocally demonstrate a critical role of intracellular Ca2+-release channels in the physiological processes of plants.

Mechanisms of Na+ Uptake by Plant Cells

Soil salinity affects vast areas of land globally, with a particularly high impact in some agricultural intensively used soils due to irrigation practice. A diverse range of plants is able to thrive on saline soils but all major crop species are intolerant to salt. Identification of pathways for Na + transport across plant cell membranes has been highlighted as comprising a key gap in our understanding of salt tolerance in plants. During the last few years there have, however, been remarkable advances in this area as Na + permeable ion channels in plant cells have been characterized. This review summarizes the present knowledge regarding Na + transport pathways across plant membranes. In particular, data on selectivity, conductance, abundance and regulation of the major cation uptake channel types have been collected and this information has been integrated into a simple model in order to address the following questions: (i) how much Na + enters the cell through an ensemble of different channel types in saline conditions? (ii) what is the relative contribution of each channel type to the total Na + inward current? (iii) how does modulation of the activity of the different channel types affect the ability of the plasma membrane to discriminate between K + and Na + ? The model calculations underline the importance of voltage-independent non-selective cation channels in Na + -uptake and suggest that future research in the field of salt tolerance in plants should include studies on the regulation of this channel type.

Zinc biofortification of cereals: problems and solutions

The goal of biofortification is to develop plants that have an increased content of bioavailable nutrients in their edible parts. Cereals serve as the main staple food for a large proportion of the world population but have the shortcoming, from a nutrition perspective, of being low in zinc and other essential nutrients. Major bottlenecks in plant biofortification appear to be the root-shoot barrier and--in cereals--the process of grain filling. New findings demonstrate that the root-shoot distribution of zinc is controlled mainly by heavy metal transporting P1B-ATPases and the metal tolerance protein (MTP) family. A greater understanding of zinc transport is important to improve crop quality and also to help alleviate accumulation of any toxic metals.

Phylogenetic and functional analysis of the Cation Diffusion Facilitator (CDF) family: improved signature and prediction of substrate specificity.

BackgroundThe Cation Diffusion Facilitator (CDF) family is a ubiquitous family of heavy metal transporters. Much interest in this family has focused on implications for human health and bioremediation. In this work a broad phylogenetic study has been undertaken which, considered in the context of the functional characteristics of some fully characterised CDF transporters, has aimed at identifying molecular determinants of substrate selectivity and at suggesting metal specificity for newly identified CDF transporters.ResultsRepresentative CDF members from all three kingdoms of life (Archaea, Eubacteria, Eukaryotes) were retrieved from genomic databases. Protein sequence alignment has allowed detection of a modified signature that can be used to identify new hypothetical CDF members. Phylogenetic reconstruction has classified the majority of CDF family members into three groups, each containing characterised members that share the same specificity towards the principally-transported metal, i.e. Zn, Fe/Zn or Mn. The metal selectivity of newly identified CDF transporters can be inferred by their position in one of these groups. The function of some conserved amino acids was assessed by site-directed mutagenesis in the poplar Zn2+ transporter PtdMTP1 and compared with similar experiments performed in prokaryotic members. An essential structural role can be assigned to a widely conserved glycine residue, while aspartate and histidine residues, highly conserved in putative transmembrane domains, might be involved in metal transport. The potential role of group-conserved amino acid residues in metal specificity is discussed.ConclusionIn the present study phylogenetic and functional analyses have allowed the identification of three major substrate-specific CDF groups. The metal selectivity of newly identified CDF transporters can be inferred by their position in one of these groups. The modified signature sequence proposed in this work can be used to identify new hypothetical CDF members.

Roles of Higher Plant K+ Channels

Living organisms maintain a cellular solute composition very different from that of the external environment. This implicitly requires the transport of solutes across the cell membrane, and ion channels are integral membrane proteins that play indispensable roles in such transport. In the past dozen years, radical advances have aided in our understanding of ion channel function and regulation in higher plants. Nowhere are these advances more striking than with respect to K+ channels, where the synergistic application of electrophysiological, cell biological, physiological, and molecular techniques has demonstrated an array of channel types playing diverse but defined roles in plant physiology. The major function of K+ channels in animal cells is that of membrane voltage control and short-term repolarization of the membrane. Although K+ channels in plants share similar roles in the regulation of the membrane voltage, early research on guard cells led to the model that shows that plant K+ channels in addition provide important pathways for long-term physiological K+ uptake and release. An extensive range of recent studies suggests diverse longterm transport functions of plant K+ channels, including participation in osmotically driven movements, solute loading into the xylem, cation nutrition, and, by virtue of the presence of K+ channels at endomembranes, intracel

CNGCs: prime targets of plant cyclic nucleotide signalling?

Cyclic nucleotide-gated channels (CNGCs) are a recently identified family of plant ion channels. They show a high degree of similarity to Shaker-type voltage-gated channels and contain a C-terminal cyclic nucleotide-binding domain with an overlapping calmodulin-binding domain. Heterologously expressed plant CNGCs show activation by cyclic nucleotides and permeability to monovalent and divalent cations. In plants, downstream effectors of cyclic nucleotide signals have so far remained obscure, and CNGCs might be their prime targets. The unique position of CNGCs as ligand-gated Ca(2+)-permeable channels suggests that they function at key sites where cyclic nucleotide and Ca(2+) signalling pathways interact. Such processes include plant defence responses, and two recently characterized Arabidopsis mutants in CNGC genes indeed show altered pathogen responses.

The Saccharomyces cerevisiae CCH1 gene is involved in calcium influx and mating

The yeast Saccharomyces cerevisiae gene CCH1 (ORF YGR217w) shows high homology with animal calcium channel α1‐subunit genes. Knock‐out mutants were constructed of Cch1 and of Mid1 which is known to mediate Ca2+ influx in response to the α‐mating pheromone. Cch1 is involved in calcium influx and the late stage of the mating process. The cch1 mutant sensitivity against the α‐mating pheromone can be reduced by the addition of extra calcium. The product of this gene is likely to interact with the MID1 gene product in Ca influx or its control.