Hydrogen peroxide affects ion channels in lily pollen grain protoplasts

Reactive oxygen species- and nitric oxide-dependent regulation of ion and metal homeostasis in plants

Deterioration and impoverishment of soil, caused by environmental pollution and climate change, result in reduced crop productivity. To adapt to hostile soils, plants have developed a complex network of factors involved in stress sensing, signal transduction, and adaptive responses. The chemical properties of reactive oxygen species (ROS) and reactive nitrogen species (RNS) allow them to participate in integrating the perception of external signals by fine-tuning protein redox regulation and signal transduction, triggering specific gene expression. Here, we update and summarize progress in understanding the mechanistic basis of ROS and RNS production at the subcellular level in plants and their role in the regulation of ion channels/transporters at both transcriptional and post-translational levels. We have also carried out an in silico analysis of different redox-dependent modifications of ion channels/transporters and identified cysteine and tyrosine targets of nitric oxide in metal transporters. Further, we summarize possible ROS- and RNS-dependent sensors involved in metal stress sensing, such as kinases and phosphatases, as well as some ROS/RNS-regulated transcription factors that could be involved in metal homeostasis. Understanding ROS- and RNS-dependent signaling events is crucial to designing new strategies to fortify crops and improve plant tolerance of nutritional imbalance and metal toxicity.

Receptor for Activated C Kinase1B (OsRACK1B) Impairs Fertility in Rice through NADPH-Dependent H2O2 Signaling Pathway

The scaffold protein receptor for Activated C Kinase1 (RACK1) regulates multiple aspects of plants, including seed germination, growth, environmental stress responses, and flowering. Recent studies have revealed that RACK1 is associated with NADPH-dependent reactive oxygen species (ROS) signaling in plants. ROS, as a double-edged sword, can modulate several developmental pathways in plants. Thus, the resulting physiological consequences of perturbing the RACK1 expression-induced ROS balance remain to be explored. Herein, we combined molecular, pharmacological, and ultrastructure analysis approaches to investigate the hypothesized connection using T-DNA-mediated activation-tagged RACK1B overexpressed (OX) transgenic rice plants. In this study, we find that OsRACK1B-OX plants display reduced pollen viability, defective anther dehiscence, and abnormal spikelet morphology, leading to partial spikelet sterility. Microscopic observation of the mature pollen grains from the OX plants revealed abnormalities in the exine and intine structures and decreased starch granules in the pollen, resulting in a reduced number of grains per locule from the OX rice plants as compared to that of the wild-type (WT). Histochemical staining revealed a global increase in hydrogen peroxide (H₂O₂) in the leaves and roots of the transgenic lines overexpressing OsRACK1B compared to that of the WT. However, the elevated H₂O₂ in tissues from the OX plants can be reversed by pre-treatment with diphenylidonium (DPI), an NADPH oxidase inhibitor, indicating that the source of H₂O₂ could be, in part, NADPH oxidase. Expression analysis showed a differential expression of the NADPH/respiratory burst oxidase homolog D (RbohD) and antioxidant enzyme-related genes, suggesting a homeostatic mechanism of H₂O₂ production and antioxidant enzyme activity. BiFC analysis demonstrated that OsRACK1B interacts with the N-terminal region of RbohD in vivo. Taken together, these data indicate that elevated OsRACK1B accumulates a threshold level of ROS, in this case H₂O₂, which negatively regulates pollen development and fertility. In conclusion, we hypothesized that an optimal expression of RACK1 is critical for fertility in rice plants.

The Balance between Different ROS on Tobacco Stigma during Flowering and Its Role in Pollen Germination

The concept of ROS as an important factor controlling pollen germination and tube growth has become generally accepted in the last decade. However, the relationship between various ROS and their significance for the success of in vivo germination and fertilization remained unexplored. For the present study, we collected Nicotiana tabacum stigma exudate on different stages of stigma maturity before and after pollination. Electron paramagnetic resonance (EPR) and colorimetric analysis were used to assess levels of O^•₂⁻ and H₂O₂ on stigma. Superoxide dismutase activity in the stigma tissues at each stage was evaluated zymographically. As the pistil matured, the level of both ROS decreased markedly, while the activity of SOD increased, and, starting from the second stage, the enzyme was represented by two isozymes: Fe SOD and Cu/Zn SOD, which was demonstrated by the in-gel inhibitory analysis. Selective suppression of Cu/Zn SOD activity shifted the ROS balance, which was confirmed by EPR. This shift markedly reduced the rate of pollen germination in vivo and the fertilization efficiency, which was estimated by the seed set. This result showed that hydrogen peroxide is a necessary component of stigma exudate, accelerates germination and ensures successful reproduction. A decrease in O^•₂⁻ production due to NADPH oxidase inhibition, although it slowed down germination, did not lead to a noticeable decrease in the seed set. Thus, the role of the superoxide radical can be characterized as less important.

Potassium signaling in plant abiotic responses: Crosstalk with calcium and reactive oxygen species/reactive nitrogen species

Potassium ion (K⁺) has been regarded as an essential signaling in plant growth and development. K⁺ transporters and channels at transcription and protein levels have been made great progress. K⁺ can enhance plant abiotic stress resistance. Meanwhile, it is now clear that calcium (Ca²⁺), reactive oxygen species (ROS), and reactive nitrogen species (RNS) act as signaling molecules in plants. They regulate plant growth and development and mediate K⁺ transport. However, the interaction of K⁺ with these signaling molecules remains unclear. K⁺ may crosstalk with Ca²⁺ and ROS/RNS in abiotic stress responses in plants. Also, there are interactions among K⁺, Ca²⁺, and ROS/RNS signaling pathways in plant growth, development, and abiotic stress responses. They regulate ion homeostasis, antioxidant system, and stress resistance-related gene expression in plants. Future work needs to focus on the deeper understanding of molecular mechanism of crosstalk among K⁺, Ca²⁺, and ROS/RNS under abiotic stress.

Hydrogen peroxide in tobacco stigma exudate affects pollen proteome and membrane potential in pollen tubes

ROS are known to be accumulated in stigmas of different species and can possibly perform different functions important for plant reproduction. Here we tested the assumption that one of their functions is to control membrane potential and provoke synthesis of unique proteins in germinating pollen. We used spectrofluorometry and spectrophotometry to detect H2 O2 in stigma exudate, quantitative fluorescent microscopy of pollen tubes and flow cytometry of pollen protoplasts to reveal effects on membrane potential, and a label-free quantification approach to study pollen proteome changes after H2 O2 treatment. We found that in both growing pollen tubes and pollen protoplasts exudate causes plasmalemma hyperpolarization similar to that provoked by H2 O2 . This effect is abolished by catalase treatment and the ROS quencher, MnTMPP. Inhibitory analysis indicates probable participation of Ca2+ - and K+ -conducting channels in the observed hyperpolarization. For a deeper understanding of pollen response, we analysed proteome alterations in H2 O2 -treated pollen grains. We found 50 unique proteins and 20 differently accumulated proteins that are mainly involved in cell metabolism, energetics, protein synthesis and folding. Observed hyperpolarization and proteome alterations agree well with previously reported stimulation of pollen germination by H2 O2 and sensitivity of Ca2+ - and K+ -conducting channels to this ROS. Thus, H2 O2 is one of the active substances in tobacco stigma exudate that stimulates various physiological processes in germinating pollen.

Oxygen radicals and cytoplasm zoning in growing lily pollen tubes

Differential modulation of ROS content of the microenvironment (O ¯/MnTMPP/OH·) affects growth speed and morphology in lily pollen tubes. Oxygen radicals influence ionic zoning: membrane potential and pH gradients. Recently, redox-regulation of tip growth has been extensively studied, but differential sensitivity of growing cells to particular ROS and their subcellular localization is still unclear. Here, we used specific dyes to provide mapping of H₂O₂ and O^·₂¯ in short and long pollen tubes. We found apical accumulation of H₂O₂ and H₂O₂-producing organelles in the shank that were not colocalized with O^·₂¯-producing mitochondria. Differential modulation of ROS content of the germination medium affected both growth speed and pollen tube morphology. Oxygen radicals affected ionic zoning: membrane potential and pH gradients. OH· caused depolarization all along the tube while O^·₂¯ provoked hyperpolarization and cytoplasm alkalinization. O^·₂¯accelerated growth and reduced tube diameter, indicating that this ROS can be considered as pollen tube growth stimulator. Serious structural disturbances were observed upon exposure to OH· and ROS quencher MnTMPP: pollen tube growth slowed down and ballooned tips formed in both cases, but OH· affected membrane transport and organelle distribution as well. OH·, thus, can be considered as a negative regulator of pollen tube growth. Pollen tubes, in turn, are able to reduce OH· concentration, which was assessed by electron paramagnetic resonance spectroscopy (EPR).

cROStalk for Life: Uncovering ROS Signaling in Plants and Animal Systems, from Gametogenesis to Early Embryonic Development

This review explores the role of reactive oxygen species (ROS)/Ca2+ in communication within reproductive structures in plants and animals. Many concepts have been described during the last years regarding how biosynthesis, generation products, antioxidant systems, and signal transduction involve ROS signaling, as well as its possible link with developmental processes and response to biotic and abiotic stresses. In this review, we first addressed classic key concepts in ROS and Ca2+ signaling in plants, both at the subcellular, cellular, and organ level. In the plant science field, during the last decades, new techniques have facilitated the in vivo monitoring of ROS signaling cascades. We will describe these powerful techniques in plants and compare them to those existing in animals. Development of new analytical techniques will facilitate the understanding of ROS signaling and their signal transduction pathways in plants and mammals. Many among those signaling pathways already have been studied in animals; therefore, a specific effort should be made to integrate this knowledge into plant biology. We here discuss examples of how changes in the ROS and Ca2+ signaling pathways can affect differentiation processes in plants, focusing specifically on reproductive processes where the ROS and Ca2+ signaling pathways influence the gametophyte functioning, sexual reproduction, and embryo formation in plants and animals. The study field regarding the role of ROS and Ca2+ in signal transduction is evolving continuously, which is why we reviewed the recent literature and propose here the potential targets affecting ROS in reproductive processes. We discuss the opportunities to integrate comparative developmental studies and experimental approaches into studies on the role of ROS/ Ca2+ in both plant and animal developmental biology studies, to further elucidate these crucial signaling pathways.

The Role of Calcium/Calcium-Dependent Protein Kinases Signal Pathway in Pollen Tube Growth

Pollen tube (PT) growth as a key step for successful fertilization is essential for angiosperm survival and especially vital for grain yield in cereals. The process of PT growth is regulated by many complex and delicate signaling pathways. Among them, the calcium/calcium-dependent protein kinases (Ca2+/CPKs) signal pathway has become one research focus, as Ca2+ ion is a well-known essential signal molecule for PT growth, which can be instantly sensed and transduced by CPKs to control myriad biological processes. In this review, we summarize the recent progress in understanding the Ca2+/CPKs signal pathway governing PT growth. We also discuss how this pathway regulates PT growth and how reactive oxygen species (ROS) and cyclic nucleotide are integrated by Ca2+ signaling networks.

ROS and Ions in Cell Signaling during Sexual Plant Reproduction

Pollen grain is a unique haploid organism characterized by two key physiological processes: activation of metabolism upon exiting dormancy and polar tube growth. In gymnosperms and flowering plants, these processes occur in different time frames and exhibit important features; identification of similarities and differences is still in the active phase. In angiosperms, the growth of male gametophyte is directed and controlled by its microenvironment, while in gymnosperms it is relatively autonomous. Recent reviews have detailed aspects of interaction between angiosperm female tissues and pollen such as interactions between peptides and their receptors; however, accumulated evidence suggests low-molecular communication, in particular, through ion exchange and ROS production, equally important for polar growth as well as for pollen germination. Recently, it became clear that ROS and ionic currents form a single regulatory module, since ROS production and the activity of ion transport systems are closely interrelated and form a feedback loop.

Reactive Oxygen Species as Mediators of Gametophyte Development and Double Fertilization in Flowering Plants

Reactive oxygen species (ROS) are toxic by-products of aerobic metabolism. In plants, they also function as important signaling molecules that regulate biotic and abiotic stress responses as well as plant growth and development. Recent studies have implicated ROS in various aspects of plant reproduction. In male gametophytes, ROS are associated with germline development as well as the developmentally associated programmed cell death of tapetal cells necessary for microspore development. ROS have a role in regulation of female gametophyte patterning and maintenance of embryo sac polarity. During pollination, ROS play roles in the generation of self-incompatibility response during pollen-pistil interaction, pollen tube growth, pollen tube burst for sperm release and fertilization. In this mini review, we provide an overview of ROS production and signaling in the context of plant reproductive development, from female and male gametophyte development to fertilization.

Redox-regulation of ion homeostasis in growing lily pollen tubes

The pollen tube is characterized by cytoplasm compartmentalization typical for cells with polar growth. This concept includes "ion zoning", i.e. gradient distribution of ionic currents across the plasma membrane and free inorganic ions in the cytoplasm. One of the putative mechanisms for maintaining "ion zoning" is indicated by the sensitivity of the ion transport systems to reactive oxygen species (ROS). Here we test the possibility of redox regulation of ionic gradients and membrane potential (MP) gradient in growing pollen tubes using quantitative fluorescence microscopy. ROS quencher MnTMPP and exogenic H₂O₂ cause different alterations of intracellular Ca²⁺ gradient, pH gradient and MP gradient during short-term exposure. MnTMPP significantly shifts the gradients of Ca²⁺ and MP at low concentrations while high concentration cause growth alterations (ballooned tips) and cytoplasm acidification. H₂O₂ at 0,5 and 1 mM affects ion homeostasis as well (MP, Ca²⁺, pH) but doesn't decrease viability or alter shape of the tubes. Here we present original quantitative data on the interconnection between ROS and ion transport during tip growth.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

Extracellular ATP promoted pollen germination and tube growth of Nicotiana tabacum through promoting K+ and Ca2+ absorption

Extracellular ATP (eATP) plays an essential role in plant growth, development, and stress tolerance. Here, we report that eATP participated in Nicotiana tabacum pollen germination (PG) and pollen tube growth (PTG) by regulating K+ and Ca2+ influx. Exogenous ATP or ADP effectively promoted PG and PTG in a dose-dependent manner; weakly hydrolysable ATP analog (ATPγS) showed a similar effect. AMP, adenosine, adenine, and phosphate did not affect PG or PTG. Within a certain range, higher concentrations of K+ or Ca2+ in the medium increased the effect of ATP in promoting PG and PTG. However, in mediums containing K+ or Ca2+ concentrations above this range, the effect of ATP was reversed, resulting in PG and PTG inhibition. Ca2+ chelators (EGTA), Ca2+ channel blockers, and K+ channel blockers suppressed ATP-promoted PG and PTG. Results from a patch clamp showed that ATP activated a K+ and Ca2+ influx in pollen protoplasts. These results suggest that, as an apoplastic signal, eATP may be involved in PG and PTG via regulating Ca2+ and K+ absorption.

The role of reactive oxygen species in pollen germination in Picea pungens (blue spruce)

Endogenous ROS, including those produced by NADPH oxidase, are required for spruce pollen germination and regulate membrane potential in pollen tubes; [Formula: see text] and H ₂ O ₂ are unevenly distributed along the tube. Recently, the key role of reactive oxygen species (ROS) in plant reproduction has been decisively demonstrated for angiosperms. This paper is dedicated to the involvement of ROS in pollen germination of gymnosperms, which remained largely unknown. We found that ROS are secreted from pollen grains of blue spruce during the early stage of activation. The localization of different ROS in pollen tube initials and pollen tubes demonstrated the accumulation of H₂O₂ in pollen tube apex. Colocalization with mitochondria-derived [Formula: see text] showed that H₂O₂ is produced in mitochondria and amyloplasts in addition to its apical gradient in the cytosol. The necessity of intracellular ROS and, particularly, [Formula: see text] for pollen germination was demonstrated using different antioxidants. ·OH and extracellular ROS, on the contrary, were found to be not necessary for germination. Exogenous hydrogen peroxide did not affect the germination efficiency but accelerated pollen tube growth in a concentration-dependent manner. The optical measurements of membrane potential showed that in spruce pollen tubes there is a gradient which is controlled by H⁺-ATPase, potassium- and calcium-permeable channels, anion channels and ROS, as demonstrated by inhibitory analysis. An important role of NADPH oxidase in the regulation of ROS balance in particular, and in germination in general, has been demonstrated by inhibiting the enzyme, which leads to the reduction in ROS release, depolarization of pollen tube plasma membrane, and blocking of pollen germination.

Calcium imaging in Arabidopsis pollen cells using G-CaMP5

Calcium (Ca²⁺ ) signaling has been implicated in pollen germination and pollen tube growth. To date, however, we still know very little about how exactly Ca²⁺ signaling links to various physiological subcellular processes during pollen germination and pollen tube growth. Given that Ca²⁺ signaling is tightly related to the cytosolic concentration and dynamics of Ca²⁺ , it is vital to trace the dynamic changes in Ca²⁺ levels in order to decode Ca²⁺ signaling. Here, we demonstrate that G-CaMP5 serves well as an indicator for monitoring cytosolic Ca²⁺ dynamics in pollen cells. Using this probe, we show that cytosolic Ca²⁺ changes dramatically during pollen germination, and, as reported previously, Ca²⁺ forms a tip-focused gradient in the pollen tube and undergoes oscillation in the tip region during pollen tube growth. In particular, using G-CaMP5 allowed us to capture the dynamic changes in the cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ) in pollen tubes in response to various exogenous treatments. Our data suggest that G-CaMP5 is a suitable probe for monitoring the dynamics of [Ca²⁺ ]_cyt in pollen cells.

Spatio-temporal Aspects of Ca2+ Signalling: Lessons from Guard Cells and Pollen Tubes

Changes in cytosolic Ca2+ concentration ([Ca2+]cyt) serve to transmit information in eukaryotic cells. The involvement of this second messenger in plant cell growth as well as osmotic- and water relations is well established. After almost 40 years of intense research on the coding and decoding of plant Ca2+ signals, numerous proteins involved in Ca2+ action have been identified. However, we are still far from understanding the complexity of Ca2+ networks. New in vivo Ca2+ imaging techniques combined with molecular genetics allow visualisation of spatio-temporal aspects of Ca2+ signalling. In parallel, cell biology together with protein biochemistry and electrophysiology are able to dissect information processing by this second messenger in space and time. Here we focus on the time-resolved changes in cellular events upon Ca2+ signals, concentrating on the two best-studied cell types, pollen tubes and guard cells. We put their signalling networks side by side, compare them with those of other cell types and discuss rapid signalling in the context of Ca2+ transients and oscillations to regulate ion homeostasis.

ROS-Activated Ion Channels in Plants: Biophysical Characteristics, Physiological Functions and Molecular Nature

Ion channels activated by reactive oxygen species (ROS) have been found in the plasma membrane of charophyte Nitella flixilis, dicotyledon Arabidopsis thaliana, Pyrus pyrifolia and Pisum sativum, and the monocotyledon Lilium longiflorum. Their activities have been reported in charophyte giant internodes, root trichoblasts and atrichoblasts, pollen tubes, and guard cells. Hydrogen peroxide and hydroxyl radicals are major activating species for these channels. Plant ROS-activated ion channels include inwardly-rectifying, outwardly-rectifying, and voltage-independent groups. The inwardly-rectifying ROS-activated ion channels mediate Ca2+-influx for growth and development in roots and pollen tubes. The outwardly-rectifying group facilitates K⁺ efflux for the regulation of osmotic pressure in guard cells, induction of programmed cell death, and autophagy in roots. The voltage-independent group mediates both Ca2+ influx and K⁺ efflux. Most studies suggest that ROS-activated channels are non-selective cation channels. Single-channel studies revealed activation of 14.5-pS Ca2+ influx and 16-pS K⁺ efflux unitary conductances in response to ROS. The molecular nature of ROS-activated Ca2+ influx channels remains poorly understood, although annexins and cyclic nucleotide-gated channels have been proposed for this role. The ROS-activated K⁺ channels have recently been identified as products of Stellar K⁺ Outward Rectifier (SKOR) and Guard cell Outwardly Rectifying K⁺ channel (GORK) genes.

Effects of Ni2+ and Cu2+ on K+ and H+ currents in lily pollen protoplasts

Heavy metals affect plant development and reproduction if they are present in excessive amounts, a situation that is becoming increasingly common. Pollen is a convenient object for pollution assessment as it is in most cases a 2- or 3-cellular organism exposed to the environment. At the same time, pollen is a key stage in the life cycle of seed plants; pollen viability and efficiency of germination are crucial for reproductive success and crop yield. In the present study we reveal for the first time, to our knowledge, targets for heavy metals (Cu2+ and Ni2+) in the pollen grain plasma membrane using the patch-clamp technique. Ni2+ dramatically decreases K+ current in pollen grain protoplasts, whereas Cu2+ does not alter the current density. Instead, Cu2+ strongly enhances H+ current driven by H+-ATPase, whereas Ni2+ fails to affect this current. The short-term treatment with Cu2+ also leads to reactive oxygen species (ROS) accumulation in pollen grain protoplasts but intracellular pH and membrane potential remain unchanged. Ni2+ had no significant effect on ROS content or membrane potential. Thus, plasmalemma K+ channels in pollen grains are sensitive to Ni2+ and H+-ATPase is sensitive to Cu2+, possibly, in a ROS-mediated way. Both metals leave pollen viable since membrane potential is maintained at the control level.