Phosphorylation of plant actin-depolymerising factor by calmodulin-like domain protein kinase

ZmADF5, a Maize Actin-Depolymerizing Factor Conferring Enhanced Drought Tolerance in Maize

Drought stress is seriously affecting the growth and production of crops, especially when agricultural irrigation still remains quantitatively restricted in some arid and semi-arid areas. The identification of drought-tolerant genes is important for improving the adaptability of maize under stress. Here, we found that a new member of the actin-depolymerizing factor (ADF) family; the ZmADF5 gene was tightly linked with a consensus drought-tolerant quantitative trait locus, and the significantly associated signals were detected through genome wide association analysis. ZmADF5 expression could be induced by osmotic stress and the application of exogenous abscisic acid. Its overexpression in Arabidopsis and maize helped plants to keep a higher survival rate after water-deficit stress, which reduced the stomatal aperture and the water-loss rate, as well as improved clearance of reactive oxygen species. Moreover, seventeen differentially expressed genes were identified as regulated by both drought stress and ZmADF5, four of which were involved in the ABA-dependent drought stress response. ZmADF5-overexpressing plants were also identified as sensitive to ABA during the seed germination and seedling stages. These results suggested that ZmADF5 played an important role in the response to drought stress.

Melatonin-mediated CcARP1 alters F-actin dynamics by phosphorylation of CcADF9 to balance root growth and salt tolerance in pigeon pea

As a multifunctional hormone-like molecule, melatonin exhibits a pleiotropic role in plant salt stress tolerance. While actin cytoskeleton is essential to plant tolerance to salt stress, it is unclear if and how actin cytoskeleton participates in the melatonin-mediated alleviation of plant salt stress. Here, we report that melatonin alleviates salt stress damage in pigeon pea by activating a kinase-like protein, which interacts with an actin-depolymerizing factor. Cajanus cajan Actin-Depolymerizing Factor 9 (CcADF9) has the function of severing actin filaments and is highly expressed under salt stress. The CcADF9 overexpression lines (CcADF9-OE) showed a reduction of transgenic root length and an increased sensitivity to salt stress. By using CcADF9 as a bait to screen an Y2H library, we identified actin depolymerizing factor-related phosphokinase 1 (ARP1), a novel protein kinase that interacts with CcADF9. CcARP1, induced by melatonin, promotes salt resistance of pigeon pea through phosphorylating CcADF9, inhibiting its severing activity. The CcARP1 overexpression lines (CcARP1-OE) displayed an increased transgenic root length and resistance to salt stress, whereas CcARP1 RNA interference lines (CcARP1-RNAi) presented the opposite phenotype. Altogether, our findings reveal that melatonin-induced CcARP1 maintains F-actin dynamics balance by phosphorylating CcADF9, thereby promoting root growth and enhancing salt tolerance.

Bio-positive effects of ionizing radiation on pollen: The role of ROS

The concept of 'hormesis' is defined as a dose-response relationship whereby low doses of various toxic substances or physical stressors trigger bio-positive effects in diverse biological systems, whereas high doses cause inhibition of cellular performance (e.g. growth, viability). The two-sided phenomenon of specific low-dose stimulation and high-dose inhibition imposed by a 'hormetic-factor' has been well documented in toxicology and pharmacology. Multitudinous factors have been identified that correspondingly cause hormetic effects in diverse taxa of animals, fungi, and plants. This study particularly aims to elucidate the molecular basis for stimulatory implications of ionizing radiation (IR) on plant male gametophytes (pollen). Beyond that, this analysis impacts general research on cell growth, plant breeding, radiation protection, and, in a wider sense, medical treatment. For this purpose, IR-related data were surveyed and discussed in connection with the present knowledge about pollen physiology. It is concluded that IR-induced reactive oxygen species (ROS) have a key role here. Moreover, it is hypothesized that IR-exposure shifts the ratio between diverse types of ROS in the cell. The interrelation between ROS, intracellular Ca2+-gradient, NADPH oxidases, ROS-scavengers, actin dynamics, and cell wall properties are most probably involved in IR-hormesis of pollen germination and tube growth. Modulation of gene expression, phytohormone signalling, and cellular antioxidant capacity are also implicated in IR-hormesis.

Exploring the Role of the Plant Actin Cytoskeleton: From Signaling to Cellular Functions

The plant actin cytoskeleton is characterized by the basic properties of dynamic array, which plays a central role in numerous conserved processes that are required for diverse cellular functions. Here, we focus on how actins and actin-related proteins (ARPs), which represent two classical branches of a greatly diverse superfamily of ATPases, are involved in fundamental functions underlying signal regulation of plant growth and development. Moreover, we review the structure, assembly dynamics, and biological functions of filamentous actin (F-actin) from a molecular perspective. The various accessory proteins known as actin-binding proteins (ABPs) partner with F-actin to finely tune actin dynamics, often in response to various cell signaling pathways. Our understanding of the significance of the actin cytoskeleton in vital cellular activities has been furthered by comparison of conserved functions of actin filaments across different species combined with advanced microscopic techniques and experimental methods. We discuss the current model of the plant actin cytoskeleton, followed by examples of the signaling mechanisms under the supervision of F-actin related to cell morphogenesis, polar growth, and cytoplasmic streaming. Determination of the theoretical basis of how the cytoskeleton works is important in itself and is beneficial to future applications aimed at improving crop biomass and production efficiency.

Actin cytoskeleton in the control of vesicle transport, cytoplasmic organization, and pollen tube tip growth

Pollen tubes extend rapidly via tip growth. This process depends on a dynamic actin cytoskeleton, which has been implicated in controlling organelle movements, cytoplasmic streaming, vesicle trafficking, and cytoplasm organization in pollen tubes. In this update review, we describe the progress in understanding the organization and regulation of the actin cytoskeleton and the function of the actin cytoskeleton in controlling vesicle traffic and cytoplasmic organization in pollen tubes. We also discuss the interplay between ion gradients and the actin cytoskeleton that regulates the spatial arrangement and dynamics of actin filaments and the organization of the cytoplasm in pollen tubes. Finally, we describe several signaling components that regulate actin dynamics in pollen tubes.

Activation of actin-depolymerizing factor by CDPK16-mediated phosphorylation promotes actin turnover in Arabidopsis pollen tubes

As the stimulus-responsive mediator of actin dynamics, actin-depolymerizing factor (ADF)/cofilin is subject to tight regulation. It is well known that kinase-mediated phosphorylation inactivates ADF/cofilin. Here, however, we found that the activity of Arabidopsis ADF7 is enhanced by CDPK16-mediated phosphorylation. We found that CDPK16 interacts with ADF7 both in vitro and in vivo, and it enhances ADF7-mediated actin depolymerization and severing in vitro in a calcium-dependent manner. Accordingly, the rate of actin turnover is reduced in cdpk16 pollen and the amount of actin filaments increases significantly at the tip of cdpk16 pollen tubes. CDPK16 phosphorylates ADF7 at Serine128 both in vitro and in vivo, and the phospho-mimetic mutant ADF7S128D has enhanced actin-depolymerizing activity compared to ADF7. Strikingly, we found that failure in the phosphorylation of ADF7 at Ser128 impairs its function in promoting actin turnover in vivo, which suggests that this phospho-regulation mechanism is biologically significant. Thus, we reveal that CDPK16-mediated phosphorylation up-regulates ADF7 to promote actin turnover in pollen.

Functional non-equivalence of pollen ADF isovariants in Arabidopsis

ADF/cofilin is a central regulator of actin dynamics. We previously demonstrated that two closely related Arabidopsis class IIa ADF isovariants, ADF7 and ADF10, are involved in the enhancement of actin turnover in pollen, but whether they have distinct functions remains unknown. Here, we further demonstrate that they exhibit distinct functions in regulating actin turnover both in vitro and in vivo. We found that ADF7 binds to ADP-G-actin with lower affinity, and severs and depolymerizes actin filaments less efficiently in vitro than ADF10. Accordingly, in pollen grains, ADF7 more extensively decorates actin filaments and is less freely distributed in the cytoplasm compared to ADF10. We further demonstrate that ADF7 and ADF10 show distinct intracellular localizations during pollen germination, and they have non-equivalent functions in promoting actin turnover in pollen. We thus propose that cooperation and labor division of ADF7 and ADF10 enable pollen cells to achieve exquisite control of the turnover of different actin structures to meet different cellular needs.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

Global survey of alternative splicing and gene modules associated with fertility regulation in a thermosensitive genic male sterile wheat

Thermosensitive genic male sterile (TGMS) wheat lines are the core of two-line hybrid systems. Understanding the mechanism that regulates male sterility in TGMS wheat lines is helpful for promoting wheat breeding. Several studies have obtained information regarding the mechanisms associated with male sterility at the transcriptional level, but it is not clear how the post-transcriptional process of alternative splicing might contribute to controlling male sterility. In this study, we performed genome-wide analyses of alternative splicing during the meiosis stage in TGMS line BS366 using PacBio and RNA-Seq hybrid sequencing. Cytological observations indicated that cytoskeleton assembly in pollen cells, calcium deposition in pollen and tapetal cells, and vesicle transport in tapetal cells were deficient in BS366. According to our cytological findings, 49 differentially spliced genes were isolated. Moreover, 25 long non-coding RNA targets and three bHLH transcription factors were identified. Weighted gene co-expression network analysis detected four candidate differentially spliced genes that had strong co-relation with the seed setting percentage, which is the direct representation of male sterility in BS366. In this study, we obtained comprehensive data regarding the alternative splicing-mediated regulation of male sterility in TGMS wheat. The candidates identified may provide the molecular basis for an improved understanding of male sterility.

Arabidopsis calcium-dependent protein kinase 3 regulates actin cytoskeleton organization and immunity

Pattern-triggered immunity and effector-triggered immunity are two primary forms of innate immunity in land plants. The molecular components and connecting nodes of pattern-triggered immunity and effector-triggered immunity are not fully understood. Here, we report that the Arabidopsis calcium-dependent protein kinase CPK3 is a key regulator of both pattern-triggered immunity and effector-triggered immunity. In vitro and in vivo phosphorylation assays, coupled with genetic and cell biology-based analyses, show that actin-depolymerization factor 4 (ADF4) is a physiological substrate of CPK3, and that phosphorylation of ADF4 by CPK3 governs actin cytoskeletal organization associated with pattern-triggered immunity. CPK3 regulates stomatal closure induced by flg22 and is required for resistance to Pst DC3000. Our data further demonstrates that CPK3 is required for resistance to Pst DC3000 carrying the effector AvrPphB. These results suggest that CPK3 is a missing link between cytoskeleton organization, pattern-triggered immunity and effector-triggered immunity.

Remodelling of the actin cytoskeleton occurs during plant immune responses to pathogens. Here Lu et al. show that this process requires the calcium-dependent kinase CPK3 which phosphorylates actin depolymerizing factor 4 and is required for both PAMP and effector-triggered immunity in Arabidopsis.

Role of a complex of two proteins in alleviating sodium ion stress in an economic crop

An economically valuable woody plant species tree bean (Cajanus cajan (L.) Millsp.) is predominantly cultivated in tropical and subtropical areas and is regarded as an important food legume (or pulse) crop that is facing serious sodium ion stress. NAM (N-acetyl-5-methoxytryptamine) has been implicated in abiotic and biotic stress tolerance in plants. However, the role of NAM in sodium ion stress tolerance has not been determined. In this study, the effect of NAM was investigated in the economically valuable woody plant species, challenged with stress at 40 mM sodium ion for 3 days. NAM-treated plants (200 μM) had significantly higher fresh weight, average root length, significantly reduced cell size, increased cell number, and increased cytoskeleton filaments in single cells. The expression pattern of one of 10 Tree bean Dynamic Balance Movement Related Protein (TbDMP), TbDMP was consistent with the sodium ion-stress alleviation by NAM. Using TbDMP as bait, Dynamic Balance Movement Related Kinase Protein (TbDBK) was determined to interact with TbDMP by screening the tree bean root cDNA library in yeast. Biochemical experiments showed that NAM enhanced the interaction between the two proteins which promoted resist sodium ion stress resistance. This study provides evidence of a pathway through which the skeleton participates in NAM signaling.

Overexpression of MdCPK1a gene, a calcium dependent protein kinase in apple, increase tobacco cold tolerance via scavenging ROS accumulation

Calcium-dependent protein kinases (CDPKs) are important calcium receptors, which play a crucial part in the process of sensing and decoding intracellular calcium signals during plant development and adaptation to various environmental stresses. In this study, a CDPK gene MdCPK1a, was isolated from apple (Malus×domestica) which contains 1701bp nucleotide and encodes a protein of 566 amino acid residues, and contains the conserved domain of CDPKs. The transient expression and western blot experiment showed that MdCPK1a protein was localized in the nucleus and cell plasma membrane. Ectopic expression of MdCPK1a in Nicotiana benthamiana increased the resistance of the tobacco plants to salt and cold stresses. The mechanism of MdCPK1a regulating cold resistance was further investigated. The overexpressed MdCPK1a tobacco plants had higher survival rates and longer root length than wild type (WT) plants under cold stress, and the electrolyte leakages (EL), the content of malondialdehyde (MDA) and reactive oxygen species (ROS) were lower, and accordingly, antioxidant enzyme activities, such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were higher, suggesting the transgenic plants suffered less chilling injury than WT plants. Moreover, the transcript levels of ROS-scavenging and stress-related genes were higher in the transgenic plants than those in WT plants whether under normal conditions or cold stress. The above results suggest that the improvement of cold tolerance in MdCPK1a-overexpressed plants was due to scavenging ROS accumulation and modulating the expression of stress-related genes.

Villin controls the formation and enlargement of punctate actin foci in pollen tubes

Self-incompatibility (SI) in the poppy Papaver rhoeas triggers dramatic alterations in actin within pollen tubes. However, how these actin alterations are mechanistically achieved remains largely unexplored. Here, we used treatment with the Ca2+ ionophore A23187 to mimic the SI-induced elevation in cytosolic Ca2+ and trigger formation of the distinctive F-actin foci. Live-cell imaging revealed that this remodeling involves F-actin fragmentation and depolymerization, accompanied by the rapid formation of punctate actin foci and subsequent increase in their size. We established that actin foci are generated and enlarged from crosslinking of fragmented actin filament structures. Moreover, we show that villins associate with actin structures and are involved in this actin reorganization process. Notably, we demonstrate that Arabidopsis VILLIN5 promotes actin depolymerization and formation of actin foci by fragmenting actin filaments, and controlling the enlargement of actin foci via bundling of actin filaments. Our study thus uncovers important novel insights about the molecular players and mechanisms involved in forming the distinctive actin foci in pollen tubes.

Actin Depolymerizing Factor Modulates Rhizobial Infection and Nodule Organogenesis in Common Bean

Actin plays a critical role in the rhizobium-legume symbiosis. Cytoskeletal rearrangements and changes in actin occur in response to Nod factors secreted by rhizobia during symbiotic interactions with legumes. These cytoskeletal rearrangements are mediated by diverse actin-binding proteins, such as actin depolymerization factors (ADFs). We examined the function of an ADF in the Phaseolus vulgaris-rhizobia symbiotic interaction (PvADFE). PvADFE was preferentially expressed in rhizobia-inoculated roots and nodules. PvADFE promoter activity was associated with root hairs harbouring growing infection threads, cortical cell divisions beneath root hairs, and vascular bundles in mature nodules. Silencing of PvADFE using RNA interference increased the number of infection threads in the transgenic roots, resulting in increased nodule number, nitrogen fixation activity, and average nodule diameter. Conversely, overexpression of PvADFE reduced the nodule number, nitrogen fixation activity, average nodule diameter, as well as NODULE INCEPTION (NIN) and EARLY NODULIN2 (ENOD2) transcript accumulation. Hence, changes in ADFE transcript levels affect rhizobial infection and nodulation, suggesting that ADFE is fine-tuning these processes.

Proteins with calmodulin-like domains: structures and functional roles

The appearance of modular proteins is a widespread phenomenon during the evolution of proteins. The combinatorial arrangement of different functional and/or structural domains within a single polypeptide chain yields a wide variety of activities and regulatory properties to the modular proteins. In this review, we will discuss proteins, that in addition to their catalytic, transport, structure, localization or adaptor functions, also have segments resembling the helix-loop-helix EF-hand motifs found in Ca2+-binding proteins, such as calmodulin (CaM). These segments are denoted CaM-like domains (CaM-LDs) and play a regulatory role, making these CaM-like proteins sensitive to Ca2+ transients within the cell, and hence are able to transduce the Ca2+ signal leading to specific cellular responses. Importantly, this arrangement allows to this group of proteins direct regulation independent of other Ca2+-sensitive sensor/transducer proteins, such as CaM. In addition, this review also covers CaM-binding proteins, in which their CaM-binding site (CBS), in the absence of CaM, is proposed to interact with other segments of the same protein denoted CaM-like binding site (CLBS). CLBS are important regulatory motifs, acting either by keeping these CaM-binding proteins inactive in the absence of CaM, enhancing the stability of protein complexes and/or facilitating their dimerization via CBS/CLBS interaction. The existence of proteins containing CaM-LDs or CLBSs substantially adds to the enormous versatility and complexity of Ca2+/CaM signaling.

Actin Cytoskeleton as Actor in Upstream and Downstream of Calcium Signaling in Plant Cells

In plant cells, calcium (Ca²⁺) serves as a versatile intracellular messenger, participating in several fundamental and important biological processes. Recent studies have shown that the actin cytoskeleton is not only an upstream regulator of Ca²⁺ signaling, but also a downstream regulator. Ca²⁺ has been shown to regulates actin dynamics and rearrangements via different mechanisms in plants, and on this basis, the upstream signaling encoded within the Ca²⁺ transient can be decoded. Moreover, actin dynamics have also been proposed to act as an upstream of Ca²⁺, adjust Ca²⁺ oscillations, and establish cytosolic Ca²⁺ ([Ca²⁺]_cyt) gradients in plant cells. In the current review, we focus on the advances in uncovering the relationship between the actin cytoskeleton and calcium in plant cells and summarize our current understanding of this relationship.

An actin-depolymerizing factor from the halophyte smooth cordgrass, Spartina alterniflora (SaADF2), is superior to its rice homolog (OsADF2) in conferring drought and salt tolerance when constitutively overexpressed in rice

Actin-depolymerizing factors (ADFs) maintain the cellular actin network dynamics by regulating severing and disassembly of actin filaments in response to environmental cues. An ADF isolated from a monocot halophyte, Spartina alterniflora (SaADF2), imparted significantly higher level of drought and salinity tolerance when expressed in rice than its rice homologue OsADF2. SaADF2 differs from OsADF2 by a few amino acid residues, including a substitution in the regulatory phosphorylation site serine-6, which accounted for its weak interaction with OsCDPK6 (calcium-dependent protein kinase), thus resulting in an increased efficacy of SaADF2 and enhanced cellular actin dynamics. SaADF2 overexpression preserved the actin filament organization better in rice protoplasts under desiccation stress. The predicted tertiary structure of SaADF2 showed a longer F-loop than OsADF2 that could have contributed to higher actin-binding affinity and rapid F-actin depolymerization in vitro by SaADF2. Rice transgenics constitutively overexpressing SaADF2 (SaADF2-OE) showed better growth, relative water content, and photosynthetic and agronomic yield under drought conditions than wild-type (WT) and OsADF2 overexpressers (OsADF2-OE). SaADF2-OE preserved intact grana structure after prolonged drought stress, whereas WT and OsADF2-OE presented highly damaged and disorganized grana stacking. The possible role of ADF2 in transactivation was hypothesized from the comparative transcriptome analyses, which showed significant differential expression of stress-related genes including interacting partners of ADF2 in overexpressers. Identification of a complex, differential interactome decorating or regulating stress-modulated cytoskeleton driven by ADF isoforms will lead us to key pathways that could be potential target for genome engineering to improve abiotic stress tolerance in agricultural crops.

Natural and targeted isovariants of the rice actin depolymerizing factor 2 can alter its functional and regulatory binding properties

Actin depolymerizing factors (ADFs) are ubiquitous actin-binding proteins that play essential roles in maintaining cellular actin dynamics by depolymerizing/severing F-actin. Plant ADF isoforms show functional divergence via differential biochemical and cellular properties. We have shown previously that ADF2 of rice (OsADF2) and smooth cordgrass (SaADF2) displayed contrasting biochemical properties and stress response in planta. As a proof-of-concept that amino acid variances contribute to such functional difference, single amino acid mutants of OsADF2 were generated based on its sequence differences with SaADF2. Biochemical studies showed that the single-site amino acid mutations altered actin binding, depolymerizing, and severing properties of OsADF2. Phosphosensitive mutations, such as serine-6>threonine, changed the regulatory phosphorylation efficiency of ADF₂ variants. The N-terminal mutations had greater effect on the phosphorylation pattern of OsADF2, whereas C-terminal mutations affected actin binding and severing. The presence of introduced mutations in isovariants of monocot ADF suggests that these residues are significant control points regulating their functional divergence, including abiotic stress response.

Apple S-RNase interacts with an actin-binding protein, MdMVG, to reduce pollen tube growth by inhibiting its actin-severing activity at the early stage of self-pollination induction

In S-RNase-mediated self-incompatibility, S-RNase secreted from the style destroys the actin cytoskeleton of the self-pollen tubes, eventually halting their growth, but the mechanism of this process remains unclear. In vitro biochemical assays revealed that S-RNase does not bind or sever filamentous actin (F-actin). In apple (Malus domestica), we identified an actin-binding protein containing myosin, villin and GRAM (MdMVG), that physically interacts with S-RNase and directly binds and severs F-actin. Immunofluorescence assays and total internal reflection fluorescence microscopy indicated that S-RNase inhibits the F-actin-severing activity of MdMVG in vitro. In vivo, the addition of S-RNase to self-pollen tubes increased the fluorescence intensity of actin microfilaments and reduced the severing frequency of microfilaments and the rate of pollen tube growth in self-pollination induction in the presence of MdMVG overexpression. By generating 25 single-, double- and triple-point mutations in the amino acid motif E-E-K-E-K of MdMVG via mutagenesis and testing the resulting mutants with immunofluorescence, we identified a triple-point mutant, MdMVG^{(E167A/E171A/K185A)} , that no longer has F-actin-severing activity or interacts with any of the four S-haplotype S-RNases, indicating that all three amino acids (E167, E171 and K185) are essential for the severing activity of MdMVG and its interaction with S-RNases. We conclude that apple S-RNase interacts with MdMVG to reduce self-pollen tube growth by inhibiting its F-actin-severing activity.

Simultaneous imaging and functional studies reveal a tight correlation between calcium and actin networks

Tip-growing cells elongate in a highly polarized manner via focused secretion of flexible cell-wall material. Calcium has been implicated as a vital factor in regulating the deposition of cell-wall material. However, deciphering the molecular and mechanistic calcium targets in vivo has remained challenging. Here, we investigated intracellular calcium dynamics in the moss Physcomitrella patens, which provides a system with an abundant source of genetically identical tip-growing cells, excellent cytology, and a large molecular genetic tool kit. To visualize calcium we used a genetically encoded cytosolic FRET probe, revealing a fluctuating tipward gradient with a complex oscillatory profile. Wavelet analysis coupled with a signal-sifting algorithm enabled the quantitative comparison of the calcium behavior in cells where growth was inhibited mechanically, pharmacologically, or genetically. We found that cells with suppressed growth have calcium oscillatory profiles with longer frequencies, suggesting that there is a feedback between the calcium gradient and growth. To investigate the mechanistic basis for this feedback we simultaneously imaged cytosolic calcium and actin, which has been shown to be essential for tip growth. We found that high cytosolic calcium promotes disassembly of a tip-focused actin spot, while low calcium promotes assembly. In support of this, abolishing the calcium gradient resulted in dramatic actin accumulation at the tip. Together these data demonstrate that tipward calcium is quantitatively linked to actin accumulation in vivo and that the moss P. patens provides a powerful system to uncover mechanistic links between calcium, actin, and growth.

Plant actin depolymerizing factor: actin microfilament disassembly and more

ACTIN DEPOLYMERIZING FACTOR (ADF) is a conserved protein among eukaryotes. The main function of ADF is the severing and depolymerizing filamentous actin (F-actin), thus regulating F-actin organization and dynamics and contributing to growth and development of the organisms. Mammalian genomes contain only a few ADF genes, whereas angiosperm plants have acquired an expanding number of ADFs, resulting in the differentiation of physiological functions. Recent studies have revealed functions of ADFs in plant growth and development, and various abiotic and biotic stress responses. In biotic stress responses, ADFs are involved in both susceptibility and resistance, depending on the pathogens. Furthermore, recent studies have highlighted a new role of ADF in the nucleus, possibly in the regulation of gene expression. In this review, I will summarize the current status of plant ADF research and discuss future research directions.

Genome-Wide Identification, Characterization and Expression Profiling of ADF Family Genes in Solanum lycopersicum L

The actin depolymerizing factor (ADF) proteins have growth, development, defense-related and growth regulatory functions in plants. The present study used genome-wide analysis to investigate ADF family genes in tomato. Eleven tomato ADF genes were identified and differential expression patterns were found in different organs. SlADF6 was preferentially expressed in roots, suggesting its function in root development. SlADF1, SlADF3 and SlADF10 were predominately expressed in the flowers compared to the other organs and specifically in the stamen compared to other flower parts, indicating their potential roles in pollen development. The comparatively higher expression of SlADF3 and SlADF11 at early fruit developmental stages might implicate them in determining final fruit size. SlADF5 and SlADF8 had relatively higher levels of expression five days after the breaker stage of fruit development, suggesting their possible role in fruit ripening. Notably, six genes were induced by cold and heat, seven by drought, five by NaCl, and four each by abscisic acid (ABA), jasmonic acid (JA) and wounding treatments. The differential expression patterns of the SlADF genes under different types of stresses suggested their function in stress tolerance in tomato plants. Our results will be helpful for the functional characterization of ADF genes during organ and fruit development of tomato under different stresses.

Computational Study of the Binding Mechanism of Actin-Depolymerizing Factor 1 with Actin in Arabidopsis thaliana

Actin is a highly conserved protein. It plays important roles in cellular function and exists either in the monomeric (G-actin) or polymeric form (F-actin). Members of the actin-depolymerizing factor (ADF)/cofilin protein family bind to both G-actin and F-actin and play vital roles in actin dynamics by manipulating the rates of filament polymerization and depolymerization. It has been reported that the S6D and R98A/K100A mutants of actin-depolymerizing factor 1 (ADF1) in Arabidopsis thaliana decreased the binding affinity of ADF for the actin monomer. To investigate the binding mechanism and dynamic behavior of the ADF1–actin complex, we constructed a homology model of the AtADF1–actin complex based on the crystal structure of AtADF1 and the twinfilin C-terminal ADF-H domain in a complex with a mouse actin monomer. The model was then refined for subsequent molecular dynamics simulations. Increased binding energy of the mutated system was observed using the Molecular Mechanics Generalized Born Surface Area and Poisson–Boltzmann Surface Area (MM-GB/PBSA) methods. To determine the residues that make decisive contributions to the ADF1 actin-binding affinity, per-residue decomposition and computational alanine scanning analyses were performed, which provided more detailed information on the binding mechanism. Root-mean-square fluctuation and principal component analyses confirmed that the S6D and R98A/K100A mutants induced an increased conformational flexibility. The comprehensive molecular insight gained from this study is of great importance for understanding the binding mechanism of ADF1 and G-actin.

CASEIN KINASE1-LIKE PROTEIN2 Regulates Actin Filament Stability and Stomatal Closure via Phosphorylation of Actin Depolymerizing Factor

The opening and closing of stomata are crucial for plant photosynthesis and transpiration. Actin filaments undergo dynamic reorganization during stomatal closure, but the underlying mechanism for this cytoskeletal reorganization remains largely unclear. In this study, we identified and characterized Arabidopsis thaliana casein kinase 1-like protein 2 (CKL2), which responds to abscisic acid (ABA) treatment and participates in ABA- and drought-induced stomatal closure. Although CKL2 does not bind to actin filaments directly and has no effect on actin assembly in vitro, it colocalizes with and stabilizes actin filaments in guard cells. Further investigation revealed that CKL2 physically interacts with and phosphorylates actin depolymerizing factor 4 (ADF4) and inhibits its activity in actin filament disassembly. During ABA-induced stomatal closure, deletion of CKL2 in Arabidopsis alters actin reorganization in stomata and renders stomatal closure less sensitive to ABA, whereas deletion of ADF4 impairs the disassembly of actin filaments and causes stomatal closure to be more sensitive to ABA Deletion of ADF4 in the ckl2 mutant partially recues its ABA-insensitive stomatal closure phenotype. Moreover, Arabidopsis ADFs from subclass I are targets of CKL2 in vitro. Thus, our results suggest that CKL2 regulates actin filament reorganization and stomatal closure mainly through phosphorylation of ADF.

Comprehensive analyses of the annexin gene family in wheat

Background

Annexins are an evolutionarily conserved multigene family of calcium-dependent phospholipid binding proteins that play important roles in stress resistance and plant development. They have been relatively well characterized in model plants Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), but nothing has been reported in hexaploid bread wheat (Triticum aestivum) and barely (Hordeum vulgare), which are the two most economically important plants.

Results

Based on available genomic and transcriptomic data, 25 and 11 putative annexin genes were found through in silico analysis in wheat and barley, respectively. Additionally, eight and 11 annexin genes were identified from the draft genome sequences of Triticum urartu and Aegilops tauschii, progenitor for the A and D genome of wheat, respectively. By phylogenetic analysis, annexins in these four species together with other monocots and eudicots were classified into six different orthologous groups. Pi values of each of Ann1–12 genes among T. aestivum, T. urartu, A. tauschii and H. vulgare species was very low, with the exception of Ann2 and Ann5 genes. Ann2 gene has been under positive selection, but Ann6 and Ann7 have been under purifying selection among the four species in their evolutionary histories. The nucleotide diversities of Ann1–12 genes in the four species were 0.52065, 0.59239, 0.60691 and 0.53421, respectively. No selective pressure was operated on annexin genes in the same species. Gene expression patterns obtained by real-time PCR and re-analyzing the public microarray data revealed differential temporal and spatial regulation of annexin genes in wheat under different abiotic stress conditions such as salinity, drought, cold and abscisic acid. Among those genes, TaAnn10 is specifically expressed in the anther but fails to be induced by low temperature in thermosensitive genic male sterile lines, suggesting that specific down-regulation of TaAnn10 is associated with conditional male sterility in wheat.

Conclusions

This study analyzed the size and composition of the annexin gene family in wheat and barley, and investigated differential tissue-specific and stress responsive expression profiles of the gene family in wheat. These results provided significant information for understanding the diverse roles of plant annexins and opened a new avenue for functional studies of cold induced male sterility in wheat.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2750-y) contains supplementary material, which is available to authorized users.

Signaling to actin stochastic dynamics

Advances in microscopy techniques applied to living cells have dramatically transformed our view of the actin cytoskeleton as a framework for cellular processes. Conventional fluorescence imaging and static analyses are useful for quantifying cellular architecture and the network of filaments that support vesicle trafficking, organelle movement, and response to biotic stress. However, new imaging techniques have revealed remarkably dynamic features of individual actin filaments and the mechanisms that underpin their construction and turnover. In this review, we briefly summarize knowledge about actin and actin-binding proteins in plant systems. We focus on the quantitative properties of the turnover of individual actin filaments, highlight actin-binding proteins that participate in actin dynamics, and summarize the current genetic evidence that has been used to dissect specific aspects of the stochastic dynamics model. Finally, we describe some signaling pathways in which recent data implicate changes in actin filament dynamics and the associated cytoplasmic responses.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

TaADF7, an actin-depolymerizing factor, contributes to wheat resistance against Puccinia striiformis f. sp. tritici

The actin cytoskeleton is involved in plant defense responses; however, the role of the actin-depolymerizing factor (ADF) family, which regulates actin cytoskeletal dynamics, in plant disease resistance, is largely unknown. Here, we characterized a wheat (Triticum aestivum) ADF gene, TaADF7, with three copies located on chromosomes 1A, 1B, and 1D, respectively. All three copies encoded the same protein, although there were variations in 19 nucleotide positions in the open reading frame. Transcriptional expression of the three TaADF7 copies were all sharply elevated in response to avirulent Puccinia striiformis f. sp. tritici (Pst) infection, with similar expression patterns. TaADF7 regulated the actin cytoskeletal dynamics by targeting the actin cytoskeleton to execute actin binding/severing activities. When the TaADF7 copies were all silenced by virus-induced gene silencing, the growth of Pst hypha increased and sporadic urediniospores were observed, as compared with control plants, upon inoculation with avirulent Pst. In addition, the accumulation of reactive oxygen species (ROS) and the hypersensitive response (HR) were greatly weakened, whereas cytochalasin B partially rescued the HR in TaADF7 knock-down plants. Together, these findings suggest that TaADF7 is likely to contribute to wheat resistance against Pst infection by modulating the actin cytoskeletal dynamics to influence ROS accumulation and the HR.

ACTIN DEPOLYMERIZING FACTOR4 regulates actin dynamics during innate immune signaling in Arabidopsis

Conserved microbe-associated molecular patterns (MAMPs) are sensed by pattern recognition receptors (PRRs) on cells of plants and animals. MAMP perception typically triggers rearrangements to actin cytoskeletal arrays during innate immune signaling. However, the signaling cascades linking PRR activation by MAMPs to cytoskeleton remodeling are not well characterized. Here, we developed a system to dissect, at high spatial and temporal resolution, the regulation of actin dynamics during innate immune signaling in plant cells. Within minutes of MAMP perception, we detected changes to single actin filament turnover in epidermal cells treated with bacterial and fungal MAMPs. These MAMP-induced alterations phenocopied an ACTIN DEPOLYMERIZING FACTOR4 (ADF4) knockout mutant. Moreover, actin arrays in the adf4 mutant were unresponsive to a bacterial MAMP, elf26, but responded normally to the fungal MAMP, chitin. Together, our data provide strong genetic and cytological evidence for the inhibition of ADF activity regulating actin remodeling during innate immune signaling. This work is the first to directly link an ADF/cofilin to the cytoskeletal rearrangements elicited directly after pathogen perception in plant or mammalian cells.

The actin cytoskeleton in root hairs: all is fine at the tip

Filamentous actin forms characteristic bundles in plant cells that facilitate cytoplasmic streaming. In contrast, networks of actin exhibiting fast turnover are found especially near sites of rapid cell expansion. These networks may serve various functions including delivering and retaining vesicles while preventing penetration of organelles into the area where cell growth occurs thereby allowing fast turnover of vesicles to and from the plasma membrane. Root hairs elongate by polarized growth at their tips and the local accumulation of fine F-actin near the tip has provided valuable insight into the organization of these networks. Here we will sequentially focus on the role of the actin cytoskeleton in root hair tip growth and on how activities of different actin binding proteins in the apical part of growing root hairs contribute to build the fine F-actin configuration that correlates with tip growth.

Arabidopsis CDPK6 phosphorylates ADF1 at N-terminal serine 6 predominantly

KEY MESSAGE

We found that Arabidopsis AtADF1 was phosphorylated by AtCDPK6 at serine 6 predominantly and the phosphoregulation plays a key role in the regulation of ADF1-mediated depolymerization of actin filaments.

ABSTRACT

Since actin-depolymerizing factor (ADF) is highly conserved among eukaryotes, it is one of the key modulators for actin organization. In plants, ADF is directly involved in the depolymerization of actin filaments, and therefore important for F-actin-dependent cellular activities. The activity of ADF is tightly controlled through a number of molecular mechanisms, including phosphorylation-mediated inactivation of ADF. To investigate Arabidopsis ADF1 phosphoregulation, we generated AtADF1 phosphorylation site-specific mutants. Using transient expression and stable transgenic approaches, we analyzed the ADF1 phosphorylation mutants in the regulation of actin filament organizations in plant cells. By in vitro phosphorylation assay, we showed that AtADF1 is phosphorylated by AtCDPK6 at serine 6 predominantly. Chemically induced expression of AtCDPK6 can negatively regulate the wild-type AtADF1 in depolymerizing actin filaments, but not those of the mutants AtADF1(S6A) and AtADF1(S6D). These results demonstrate a regulatory function of Arabidopsis CDPK6 in the N-terminal phosphorylation of AtADF1.

Nicotiana tabacum actin-depolymerizing factor 2 is involved in actin-driven, auxin-dependent patterning

Polar transport of auxin has been identified as a central element of pattern formation. To address the underlying cellular mechanisms, we use the tobacco cell line (Nicotiana tabacum L. cv. Bright Yellow 2; BY-2) as model. We showed previously that cell divisions within a cell file are synchronized by polar auxin flow, linked to the organization of actin filaments (AF) which, in turn, is modified via actin-binding proteins (ABPs). From a preparatory study for disturbed division synchrony in cell lines overexpressing different ABPs, we identified the actin depolymerizing factor 2 (ADF2). A cell line overexpressing GFP-NtADF2 was specifically affected in division synchrony. The cell division pattern could be rescued by addition of Phosphatidylinositol 4,5-bisphosphate (PIP2) or by phalloidin. These observations allow to draw first conclusions on the pathway linking auxin signalling via actin reorganization to synchronized cell division placing the regulation of cortical actin turnover by ADF2 into the focus.

Ca2+-dependent protein kinase11 and 24 modulate the activity of the inward rectifying K+ channels in Arabidopsis pollen tubes

Potassium (K(+)) influx into pollen tubes via K(+) transporters is essential for pollen tube growth; however, the mechanism by which K(+) transporters are regulated in pollen tubes remains unknown. Here, we report that Arabidopsis thaliana Ca(2+)-dependent protein kinase11 (CPK11) and CPK24 are involved in Ca(2+)-dependent regulation of the inward K(+) (K(+)in) channels in pollen tubes. Using patch-clamp analysis, we demonstrated that K(+)in currents of pollen tube protoplasts were inhibited by elevated [Ca(2+)]cyt. However, disruption of CPK11 or CPK24 completely impaired the Ca(2+)-dependent inhibition of K(+)in currents and enhanced pollen tube growth. Moreover, the cpk11 cpk24 double mutant exhibited similar phenotypes as the corresponding single mutants, suggesting that these two CDPKs function in the same signaling pathway. Bimolecular fluorescence complementation and coimmunoprecipitation experiments showed that CPK11 could interact with CPK24 in vivo. Furthermore, CPK11 phosphorylated the N terminus of CPK24 in vitro, suggesting that these two CDPKs work together as part of a kinase cascade. Electrophysiological assays demonstrated that the Shaker pollen K(+)in channel is the main contributor to pollen tube K(+)in currents and acts as the downstream target of the CPK11-CPK24 pathway. We conclude that CPK11 and CPK24 together mediate the Ca(2+)-dependent inhibition of K(+)in channels and participate in the regulation of pollen tube growth in Arabidopsis.

Control of the actin cytoskeleton in root hair development

The development of root hair includes four stages: bulge site selection, bulge formation, tip growth, and maturation. The actin cytoskeleton is involved in all of these stages and is organized into distinct arrangements in the different stages. In addition to the actin configuration, actin isoforms also play distinct roles in the different stages. The actin cytoskeleton is regulated by actin-binding proteins, such as formin, Arp2/3 complex, profilin, actin depolymerizing factor, and villin. Some upstream signals, i.e. calcium, phospholipids, and small GTPase regulate the activity of these actin-binding proteins to produce the proper actin configuration. We constructed a working model on how the actin cytoskeleton is controlled by actin-binding proteins and upstream signaling in root hair development based on the current literature: at the tip of hairs, actin polymerization appears to be facilitated by Arp2/3 complex that is activated by small GTPase, and profilin that is regulated by phosphatidylinositol 4,5-bisphosphate. Meanwhile, actin depolymerization and turnover are likely mediated by villin and actin depolymerizing factor, which are stimulated by calcium. At the shank, actin cables are produced by formin and villin. Under the complicated interaction, the actin cytoskeleton is controlled spatially and temporally during root hair development.

Holding back the microfilament--structural insights into actin and the actin-monomer-binding proteins of apicomplexan parasites

Parasites from the phylum Apicomplexa are responsible for several major diseases of man, including malaria and toxoplasmosis. These highly motile protozoa use a conserved actomyosin-based mode of movement to power tissue traversal and host cell invasion. The mode termed as 'gliding motility' relies on the dynamic turnover of actin, whose polymerisation state is controlled by a markedly limited number of identifiable regulators when compared with other eukaryotic cells. Recent studies of apicomplexan actin regulator structure-in particular those of the core triad of monomer-binding proteins, actin-depolymerising factor/cofilin, cyclase-associated protein/Srv2, and profilin-have provided new insights into possible mechanisms of actin regulation in parasite cells, highlighting divergent structural features and functions to regulators from other cellular systems. Furthermore, the unusual nature of apicomplexan actin itself is increasingly coming into the spotlight. Here, we review recent advances in understanding of the structure and function of actin and its regulators in apicomplexan parasites. In particular we explore the paradox between there being an abundance of unpolymerised actin, its having a seemingly increased potential to form filaments relative to vertebrate actin, and the apparent lack of visible, stable filaments in parasite cells.

Spatial and temporal expression of actin depolymerizing factors ADF7 and ADF10 during male gametophyte development in Arabidopsis thaliana

The actin cytoskeleton plays a crucial role in many aspects of plant cell development. During male gametophyte development, the actin arrays are conspicuously remodeled both during pollen maturation in the anther and after pollen hydration on the receptive stigma and pollen tube elongation. Remodeling of actin arrays results from the highly orchestrated activities of numerous actin binding proteins (ABPs). A key player in actin remodeling is the actin depolymerizing factor (ADF), which increases actin filament treadmilling rates. We prepared fluorescent protein fusions of two Arabidopsis pollen-specific ADFs, ADF7 and ADF10. We monitored the expression and subcellular localization of these proteins during male gametophyte development, pollen germination and pollen tube growth. ADF7 and ADF10 were differentially expressed with the ADF7 signal appearing in the microspore stage and that of ADF10 only during the polarized microspore stage. ADF7 was associated with the microspore nucleus and the vegetative nucleus of the mature grain during less metabolically active stages, but in germinating pollen grains and elongating pollen tubes, it was associated with the subapical actin fringe. On the other hand, ADF10 was associated with filamentous actin in the developing gametophyte, in particular with the arrays surrounding the apertures of the mature pollen grain. In the shank of elongating pollen tubes, ADF10 was associated with thick actin cables. We propose possible specific functions of these two ADFs based on their differences in expression and localization.

Cold stress contributes to aberrant cytokinesis during male meiosis I in a wheat thermosensitive genic male sterile line

The male sterility of a wheat thermosensitive genic male sterile (TGMS) line is strictly controlled by temperature. When the TGMS line BS366 was exposed to 10 °C from the pollen mother cell stage to the meiosis stage, a few pollen grains were formed and devoid of starch. We report here a large-scale transcriptomic study using the Affymetrix wheat GeneChip to follow gene expression in BS366 line anthers in response to cold stress. Notably, many cytoskeletal signaling components were gradually induced in response to cold stress in BS366 line anthers. However, the cytoskeleton-associated genes that play key roles in the dynamic organization of the cytoskeleton were dramatically repressed. Histological studies revealed that the separation of dyads occurred abnormally during male meiosis I, indicating defective male meiotic cytokinesis. Fluorescence labelling and subcellular histological observations revealed that the phragmoplast was defectively formed and the cell plate was abnormally assembled during meiosis I under cold stress. Based on the transcriptomic analysis and observations of characterized histological changes, our results suggest that cold stress repressed transcription of cytoskeleton dynamic factors and subsequently caused the defective cytokinesis during meiosis I. The results may explain the male sterility caused by low temperature in wheat TGMS lines.

Regulation of actin dynamics by actin-binding proteins in pollen

A dynamic network of polymers, the actin cytoskeleton, co-ordinates numerous fundamental cellular processes. In pollen tubes, organelle movements and cytoplasmic streaming, organization of the tip zone, vesicle trafficking, and tip growth have all been linked to actin-based function. Further, during the self-incompatibility response of Papaver rhoeas, destruction of the cytoskeleton is a primary target implicated in the rapid cessation of pollen tube growth and alterations in actin dynamics are associated with the initiation of programmed cell death. Surprisingly, these diverse cellular processes are accomplished with only a small amount of filamentous actin and a huge pool of polymerizable monomers. These observations hint at incredibly fast and complex actin dynamics in pollen. To understand the molecular mechanisms regulating actin dynamics in plant cells, the abundant actin monomer-binding proteins, a major filament nucleator, a family of bundling and severing proteins, and a modulator of growth at the barbed-end of actin filaments have been characterized biochemically. The activities of these proteins are generally consistent with textbook models for actin turnover. For example, the three monomer-binding proteins, profilin, ADF, and CAP, are thought to function synergistically to enhance turnover and the exchange of subunits between monomer and polymer pools. How individual actin filaments behave in living cells, however, remains largely unexplored. Actin dynamics were examined using variable angle epifluorescence microscopy (VAEM) in expanding hypocotyl epidermal cells. Our observations of single filament behaviour are not consistent with filament turnover by treadmilling, but rather represent a novel property called stochastic dynamics. A new model for the dynamic control of actin filament turnover in plant cells is presented.

A cyclic nucleotide-gated channel is necessary for optimum fertility in high-calcium environments

* Arabidopsis cngc2 plants are hypersensitive to external calcium and exhibit reduced plant size and fertility, especially when they are treated with elevated but physiologically relevant levels of calcium. This report focuses on the role of cyclic nucleotide-gated channel 2 (CNGC2) in plant fertility. * To determine the cause of the reduced fertility, we investigated the flower structure and growth potential of both male and female reproductive organs in cngc2 plants grown in high-calcium conditions. * cngc2 mutants had short stamens that may limit pollen deposition and pistils that were not conducive to pollen tube growth. * Our data indicate that sporophytic, but not gametophytic, defects are the main cause of the observed reduction in seed yield in cngc2 plants, and suggest that correct cyclic nucleotide and calcium signaling are important for cell elongation and pollen tube guidance.

Cloning and characterization of a calcium dependent protein kinase gene associated with cotton fiber development

The gene GhCPK1 encoding a calcium dependent protein kinase was identified from cotton. Transcripts of GhCPK1 accumulated primarily in the elongating fiber, and Arabidopsis plants transformed with GhCPK1 promoter-GUS construct exhibited GUS activity mainly in the developing trichomes, roots, young leaves and sepals. In the bombarded onion epidermal cells, GhCPK1-GFP fusion proteins showed a subcellular distribution in the plasma membrane. In vitro assays indicated that GhCPK1 was a functional calcium-dependent kinase able to undergo autophosphorylation and phosphorylation of the known substrate histone III-S. Together, these results suggest that GhCPK1 may play a role in the calcium signaling events associated with fiber elongation.

Imaging of the Yellow Cameleon 3.6 indicator reveals that elevations in cytosolic Ca2+ follow oscillating increases in growth in root hairs of Arabidopsis

In tip-growing cells, the tip-high Ca(2+) gradient is thought to regulate the activity of components of the growth machinery, including the cytoskeleton, Ca(2+)-dependent regulatory proteins, and the secretory apparatus. In pollen tubes, both the Ca(2+) gradient and cell elongation show oscillatory behavior, reinforcing the link between the two. We report that in growing root hairs of Arabidopsis (Arabidopsis thaliana), an oscillating tip-focused Ca(2+) gradient can be resolved through imaging of a cytosolically expressed Yellow Cameleon 3.6 fluorescence resonance energy transfer-based Ca(2+) sensor. Both elongation of the root hairs and the associated tip-focused Ca(2+) gradient show a similar dynamic character, oscillating with a frequency of 2 to 4 min(-1). Cross-correlation analysis indicates that the Ca(2+) oscillations lag the growth oscillations by 5.3 +/- 0.3 s. However, growth never completely stops, even during the slow cycle of an oscillation, and the concomitant tip Ca(2+) level is always slightly elevated compared with the resting Ca(2+) concentration along the distal shaft, behind the growing tip. Artificially increasing Ca(2+) using the Ca(2+) ionophore A23187 leads to immediate cessation of elongation and thickening of the apical cell wall. In contrast, dissipating the Ca(2+) gradient using either the Ca(2+) channel blocker La(3+) or the Ca(2+) chelator EGTA is accompanied by an increase in the rate of cell expansion and eventual bursting of the root hair tip. These observations are consistent with a model in which the maximal oscillatory increase in cytosolic Ca(2+) is triggered by cell expansion associated with tip growth and plays a role in the subsequent restriction of growth.

Actin depolymerizing factor is essential for viability in plants, and its phosphoregulation is important for tip growth

Actin depolymerizing factor (ADF)/cofilin is important for regulating actin dynamics, and in plants is thought to be required for tip growth. However, the degree to which ADF is necessary has been elusive because of the presence of multiple ADF isoforms in many plant species. In the moss Physcomitrella patens, ADF is encoded by a single, intronless gene. We used RNA interference to demonstrate that ADF is essential for plant viability. Loss of ADF dramatically alters the organization of the F-actin cytoskeleton, and leads to an inhibition of tip growth. We show that ADF is subject to phosphorylation in vivo, and using complementation studies we show that mutations of the predicted phosphorylation site partially rescue plant viability, but with differential affects on tip growth. Specifically, the unphosphorylatable ADF S6A mutant generates small polarized plants with normal F-actin organization, whereas the phosphomimetic S6D mutant generates small, unpolarized plants with a disorganized F-actin cytoskeleton. These data indicate that phosphoregulation at serine 6 is required for full ADF function in vivo, and, in particular, that the interaction between ADF and actin is important for tip growth.

Antagonistic control of powdery mildew host cell entry by barley calcium-dependent protein kinases (CDPKs)

Calcium-dependent protein kinases (CDPKs) are known to play pivotal roles in intracellular signaling during abiotic and biotic stress responses. To unravel potential functions of CDPKs in the course of barley (Hordeum vulgare)-powdery mildew (Blumeria graminis) interactions, we systematically analyzed the HvCDPK gene family. We found that, according to the existence of respective expressed sequence tags, at least nine paralogs are expressed in the barley leaf epidermis, the sole target tissue of powdery mildew fungi. We exemplarily selected two HvCDPKs with known full-length coding sequence for functional analysis. Transient expression of a putative constitutive active variant of one of these (HvCDPK4) in Nicotiana benthamiana triggered kinase-dependent mesophyll cell death in tobacco leaves. In a barley mlo mutant genotype, a constitutive active variant of the second paralog, HvCDPK3, partially compromised the highly effective resistance to B. graminis f. sp. hordei. A similar break of mlo resistance was seen upon expression of the junction domain of HvCDPK4, supposed to act as a dominant inhibitor of CDPK activity. Expression of a constitutive active HvCDPK3 or HvCDPK4 form also compromised penetration resistance to the inappropriate wheat powdery mildew fungus. Collectively, our data provide evidence for antagonistic roles of individual CDPK paralogs in the control of host cell entry during the early phase of powdery mildew pathogenesis.

The function of actin-binding proteins in pollen tube growth

Pollen tube growth is a key step in sexual reproduction of higher plants. The pollen tube is a typical example of tip-growing cells and shows a polarized cytoplasm. To develop and maintain polarized growth, pollen tubes need a carefully regulated actin cytoskeleton. It is well known that actin-binding proteins are responsible for the direct control of dynamic actin filaments and serve as a link between signal transduction pathways and dynamic actin changes in determining cellular architecture. Several of these classes have been identified in pollen tubes and their detailed characterisation is progressing rapidly. Here, we aim to survey what is known about the major actin-binding proteins that affect actin assembly and dynamics, and their higher-order organisation in pollen tube growth.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Calcium-dependent protein kinase isoforms in Petunia have distinct functions in pollen tube growth, including regulating polarity

Calcium is a key regulator of pollen tube growth, but little is known concerning the downstream components of the signaling pathways involved. We identified two pollen-expressed calmodulin-like domain protein kinases from Petunia inflata, CALMODULIN-LIKE DOMAIN PROTEIN KINASE1 (Pi CDPK1) and Pi CDPK2. Transient overexpression or expression of catalytically modified Pi CDPK1 disrupted pollen tube growth polarity, whereas expression of Pi CDPK2 constructs inhibited tube growth but not polarity. Pi CDPK1 exhibited plasma membrane localization most likely mediated by acylation, and we present evidence that suggests this localization is critical to the biological function of this kinase. Pi CDPK2 substantially localized to as yet unidentified internal membrane compartments, and this localization was again, at least partially, mediated by acylation. In contrast with Pi CDPK1, altering the localization of Pi CDPK2 did not noticeably alter the effect of overexpressing this isoform on pollen tube growth. Ca(2+) requirements for Pi CDPK1 activation correlated closely with Ca(2+) concentrations measured in the growth zone at the pollen tube apex. Interestingly, loss of polarity associated with overexpression of Pi CDPK1 was associated with elevated cytosolic Ca(2+) throughout the bulging tube tip, suggesting that Pi CDPK1 may participate in maintaining Ca(2+) homeostasis. These results are discussed in relation to previous models for Ca(2+) regulation of pollen tube growth.