Expression of plant cyclic nucleotide-gated cation channels in yeast

The chimeric cyclic nucleotide-gated ion channel ATCNGC11/12 constitutively induces programmed cell death in a Ca2+ dependent manner

The hypersensitive response (HR) involves programmed cell death (PCD) in response to pathogen infection. To investigate the pathogen resistance signaling pathway, we previously identified the Arabidopsis mutant cpr22, which displays constitutive activation of multiple defense responses including HR like cell death. The cpr22 mutation has been identified as a 3 kb deletion that fuses two cyclic nucleotide-gated ion channel (CNGC)-encoding genes, ATCNGC11 and ATCNGC12, to generate a novel chimeric gene, ATCNGC11/12. In this study, we conducted a characterization of cell death induced by transient expression of ATCNGC11/12 in Nicotiana benthamiana. Electron microscopic analysis of this cell death showed similar characteristics to PCD, such as plasma membrane shrinkage and vesicle formation. The hallmark of animal PCD, fragmentation of nuclear DNA, was also observed in ATCNGC11/12-induced cell death. The development of cell death was significantly suppressed by caspase-1 inhibitors, suggesting the involvement of caspases in this process. Recently, vacuolar processing enzyme (VPE) was isolated as the first plant caspase-like protein, which is involved in HR development. In VPE-silenced plants development of cell death induced by ATCNGC11/12 was much slower and weaker compared to control plants, suggesting the involvement of VPE as a caspase in ATCNGC11/12-induced cell death. Complementation analysis using a Ca2+ uptake deficient yeast mutant demonstrated that the ATCNGC11/12 channel is permeable to Ca2+. Additionally, calcium channel blockers such as GdCl3 inhibited ATCNGC11/12-induced HR formation, whereas potassium channel blockers did not. Taken together, these results indicate that the cell death that develops in the cpr22 mutant is indeed PCD and that the chimeric channel, ATCNGC11/12, is at the point of, or up-stream of the calcium signal necessary for the development of HR.

A cyclic nucleotide-gated channel is essential for polarized tip growth of pollen

Ion signals are critical to regulating polarized growth in many cell types, including pollen in plants and neurons in animals. Genetic evidence presented here indicates that pollen tube growth requires cyclic nucleotide-gated channel (CNGC) 18. CNGCs are nonspecific cation channels found in plants and animals and have well established functions in excitatory signal transduction events in animals. In Arabidopsis, male sterility was observed for two cngc18 null mutations. CNGC18 is expressed primarily in pollen, as indicated from a promoter::GUS (beta-glucuronidase) reporter analysis and expression profiling. The underlying cause of sterility was identified as a defect in pollen tube growth, resulting in tubes that were kinky, short, often thin, and unable to grow into the transmitting tract. Expression of a GFP-tagged CNGC18 in mutant pollen provided complementation and evidence for asymmetric localization of CNGC18 to the plasma membrane at the growing tip, starting at the time of pollen grain germination. Heterologous expression of CNGC18 in Escherichia coli resulted in a time- and concentration-dependent accumulation of more Ca2+. Thus, CNGC18 provides a mechanism to directly transduce a cyclic nucleotide (cNMP) signal into an ion flux that can produce a localized signal capable of regulating the pollen tip-growth machinery. These results identify a CNGC that is essential to an organism's life cycle and raise the possibility that CNGCs have a widespread role in regulating cell-growth dynamics in both plant and animals.

The grateful dead: calcium and cell death in plant innate immunity

Plant cells sensing pathogenic microorganisms evoke defence systems that can confer resistance to infection. This innate immune reaction can include triggering of basal defence responses as well as programmed cell death, or hypersensitive response (HR). In both cases (basal defence and HR), pathogen perception is translated into elevated cytosolic Ca(2+) (mediated by plasma membrane and intracellular channels) as an early step in a signalling cascade. Cyclic nucleotide-gated channels contribute to this influx of Ca(2+) into the cell. The molecular nature of other transport proteins contributing to the Ca(2+) elevation is unclear. Pathogen recognition occurs at two levels: the perception of pathogen-associated molecular pattern (PAMP) molecules widely present in microorganisms, and an interaction between pathogen avirulence gene products (if present) and corresponding plant R (resistance) gene products. The Ca(2+) elevation occurring in response to PAMP perception or R gene interactions could occur due to phosphorylation events, G-protein signalling and/or an increase in cyclic nucleotides. Downstream from the initial Ca(2+) rise, the signalling cascade includes: activation of calmodulin and protein kinases, and nitric oxide and reactive oxygen species generation. Some of these downstream events amplify the Ca(2+) signal by further activation of Ca(2+) transporters.

Roles of ion channels and transporters in guard cell signal transduction

Stomatal complexes consist of pairs of guard cells and the pore they enclose. Reversible changes in guard cell volume alter the aperture of the pore and provide the major regulatory mechanism for control of gas exchange between the plant and the environment. Stomatal movement is facilitated by the activity of ion channels and ion transporters found in the plasma membrane and vacuolar membrane of guard cells. Progress in recent years has elucidated the molecular identities of many guard cell transport proteins, and described their modulation by various cellular signal transduction components during stomatal opening and closure prompted by environmental and endogenous stimuli.

Death don't have no mercy and neither does calcium: Arabidopsis CYCLIC NUCLEOTIDE GATED CHANNEL2 and innate immunity

Plant innate immune response to pathogen infection includes an elegant signaling pathway leading to reactive oxygen species generation and resulting hypersensitive response (HR); localized programmed cell death in tissue surrounding the initial infection site limits pathogen spread. A veritable symphony of cytosolic signaling molecules (including Ca(2+), nitric oxide [NO], cyclic nucleotides, and calmodulin) have been suggested as early components of HR signaling. However, specific interactions among these cytosolic secondary messengers and their roles in the signal cascade are still unclear. Here, we report some aspects of how plants translate perception of a pathogen into a signal cascade leading to an innate immune response. We show that Arabidopsis thaliana CYCLIC NUCLEOTIDE GATED CHANNEL2 (CNGC2/DND1) conducts Ca(2+) into cells and provide a model linking this Ca(2+) current to downstream NO production. NO is a critical signaling molecule invoking plant innate immune response to pathogens. Plants without functional CNGC2 lack this cell membrane Ca(2+) current and do not display HR; providing the mutant with NO complements this phenotype. The bacterial pathogen-associated molecular pattern elicitor lipopolysaccharide activates a CNGC Ca(2+) current, which may be linked to NO generation due to buildup of cytosolic Ca(2+)/calmodulin.

Arabidopsis plasma membrane protein crucial for Ca2+ influx and touch sensing in roots

Plants can sense and respond to mechanical stimuli, like animals. An early mechanism of mechanosensing and response is speculated to be governed by as-yet-unidentified sensory complexes containing a Ca(2+)-permeable, stretch-activated (SA) channel. However, the components or regulators of such complexes are poorly understood at the molecular level in plants. Here, we report the molecular identification of a plasma membrane protein (designated Mca1) that correlates Ca(2+) influx with mechanosensing in Arabidopsis thaliana. MCA1 cDNA was cloned by the functional complementation of lethality of a yeast mid1 mutant lacking a putative Ca(2+)-permeable SA channel component. Mca1 was localized to the yeast plasma membrane as an integral membrane protein and mediated Ca(2+) influx. Mca1 also increased [Ca(2+)](cyt) upon plasma membrane distortion in Arabidopsis. The growth of MCA1-overexpressing plants was impaired in a high-calcium but not a low-calcium medium. The primary roots of mca1-null plants failed to penetrate a harder agar medium from a softer one. These observations demonstrate that Mca1 plays a crucial role in a Ca(2+)-permeable SA channel system that leads to mechanosensing in Arabidopsis. We anticipate our findings to be a starting point for a deeper understanding of the molecular mechanisms of mechanotransduction in plants.

Cyclic nucleotide-gated channels in plants

Until recently the role of cyclic nucleotide monophosphates (cNMPs) in plants had been controversial, with equivocal data about their concentrations, biosynthetic and degrading enzymes, and cellular targets. This review discusses the current knowledge in this field, with focus on the largest class of cNMP targets in plant cells, the cyclic nucleotide-gated channels (CNGCs). Aspects of structure and function are addressed, with reference to studies in heterologous systems and in planta. The picture emerging, albeit still fragmented, is of proteins with diverse functions in the control of ion homeostasis, development, and defense against biotic and abiotic threats.

Characterization of plant phenotypes associated with loss-of-function of AtCNGC1, a plant cyclic nucleotide gated cation channel

Of the 57 cation channel genes in the Arabidopsis genome, over a third encode cyclic nucleotide gated cation channels (CNGCs). CNGCs are ion channels regulated by cytosolic signaling molecules (cyclic nucleotides, calmodulin, and Ca(2+)), and which conduct Ca(2+) as well as K(+) and in some cases Na(+). Little is currently known about the role CNGCs may play in plant growth and development. Here, we examined the hypothesis that an Arabidopsis thaliana genotype containing a null mutation in one of the CGNC genes (AtCNGC1) would display cation uptake-related growth phenotype differences from wild type (WT) plants. We determined that AtCNGC1 protein is primarily expressed in the roots of Arabidopsis seedlings. Seedlings lacking this protein had slightly (6-22%) lower shoot Ca(2+) than WT plants. Primary roots of Atcngc1 mutant seedlings grew faster than roots of WT plants, and had larger angles of gravicurvature and less nitric oxide generation upon gravistimulation. We conclude that channels formed (at least in part) by AtCNGC1 contribute (along with other channels) to Ca(2+) uptake into plants, and that Ca(2+) uptake into roots through AtCNGC1 affects some aspects of growth in the primary root of Arabidopsis seedlings.