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

Phosphorylation of ZmAL14 by ZmSnRK2.2 regulates drought resistance through derepressing ZmROP8 expression

Drought stress has negative effects on crop growth and production. Characterization of transcription factors that regulate the expression of drought-responsive genes is critical for understanding the transcriptional regulatory networks in response to drought, which facilitates the improvement of crop drought tolerance. Here, we identified an Alfin-like (AL) family gene ZmAL14 that negatively regulates drought resistance. Overexpression of ZmAL14 exhibits susceptibility to drought while mutation of ZmAL14 enhances drought resistance. An abscisic acid (ABA)-activated protein kinase ZmSnRK2.2 interacts and phosphorylates ZmAL14 at T38 residue. Knockout of ZmSnRK2.2 gene decreases drought resistance of maize. A dehydration-induced Rho-like small guanosine triphosphatase gene ZmROP8 is directly targeted and repressed by ZmAL14. Phosphorylation of ZmAL14 by ZmSnRK2.2 prevents its binding to the ZmROP8 promoter, thereby releasing the repression of ZmROP8 transcription. Overexpression of ZmROP8 stimulates peroxidase activity and reduces hydrogen peroxide accumulation after drought treatment. Collectively, our study indicates that ZmAL14 is a negative regulator of drought resistance, which can be phosphorylated by ZmSnRK2.2 through the ABA signaling pathway, thus preventing its suppression on ZmROP8 transcription during drought stress response.

Actin-depolymerizing factors 8 and 11 promote root hair elongation at high pH

A root hair is a polarly elongated single-celled structure that derives from a root epidermal cell and functions in uptake of water and nutrients from the surrounding environment. Previous reports have demonstrated that short periods of high pH inhibit root hair extension; but the effects of long-term high-pH treatment on root hair growth are still unclear. Here, we report that the duration of root hair elongation is significantly prolonged with increasing external pH, which counteracts the effect of decreasing root hair elongation rate and ultimately produces longer root hairs, whereas loss of actin-depolymerizing factor 8 and 11 (ADF8/11) function causes shortening of root hair length at high pH (pH 7.4). Accumulation of ADF8/11 at the tips of root hairs is inhibited by high pH, and increasing environmental pH affects the actin filament (F-actin) meshwork at the root hair tip. At high pH, the tip-focused F-actin meshwork is absent in root hairs of the adf8/11 mutant, actin filaments are disordered at the adf8/11 root hair tips, and actin turnover is attenuated. Secretory and recycling vesicles do not aggregate in the apical region of adf8/11 root hairs at high pH. Together, our results suggest that, under long-term exposure to high extracellular pH, ADF8/11 may establish and maintain the tip-focused F-actin meshwork to regulate polar trafficking of secretory/recycling vesicles at the root hair tips, thereby promoting root hair elongation.

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.

Dehydration-responsive cytoskeleton proteome of rice reveals reprograming of key molecular pathways to mediate metabolic adaptation and cell survival

The plant cytoskeletal proteins play a key role that control cytoskeleton dynamics, contributing to crucial biological processes such as cell wall morphogenesis, stomatal conductance and abscisic acid accumulation in repercussion to water-deficit stress or dehydration. Yet, it is still completely unknown which specific biochemical processes and regulatory mechanisms the cytoskeleton uses to drive dehydration tolerance. To better understand the role of cytoskeleton, we developed the dehydration-responsive cytoskeletal proteome map of a resilient rice cultivar. Initially, four-week-old rice plants were exposed to progressive dehydration, and the magnitude of dehydration-induced compensatory physiological responses was monitored in terms of physicochemical indices. The organelle fractionation in conjunction with label-free quantitative proteome analysis led to the identification of 955 dehydration-responsive cytoskeletal proteins (DRCPs). To our knowledge, this is the first report of a stress-responsive plant cytoskeletal proteome, representing the largest inventory of cytoskeleton and cytoskeleton-associated proteins. The DRCPs were apparently involved in a wide array of intra-cellular molecules transportation, organelles positioning, cytoskeleton organization followed by different metabolic processes including amino acid metabolism. These findings presented open a unique view on global regulation of plant cytoskeletal proteome is intimately linked to cellular metabolic rewiring of adaptive responses, and potentially confer dehydration tolerance, especially in rice, and other crop species, in general.

Integrated multi-omics analyses and genome-wide association studies reveal prime candidate genes of metabolic and vegetative growth variation in canola

Genome-wide association studies (GWAS) identified thousands of genetic loci associated with complex plant traits, including many traits of agronomical importance. However, functional interpretation of GWAS results remains challenging because of large candidate regions due to linkage disequilibrium. High-throughput omics technologies, such as genomics, transcriptomics, proteomics and metabolomics open new avenues for integrative systems biological analyses and help to nominate systems information supported (prime) candidate genes. In the present study, we capitalise on a diverse canola population with 477 spring-type lines which was previously analysed by high-throughput phenotyping of growth-related traits and by RNA sequencing and metabolite profiling for multi-omics-based hybrid performance prediction. We deepened the phenotypic data analysis, now providing 123 time-resolved image-based traits, to gain insight into the complex relations during early vegetative growth and reanalysed the transcriptome data based on the latest Darmor-bzh v10 genome assembly. Genome-wide association testing revealed 61 298 robust quantitative trait loci (QTL) including 187 metabolite QTL, 56814 expression QTL and 4297 phenotypic QTL, many clustered in pronounced hotspots. Combining information about QTL colocalisation across omics layers and correlations between omics features allowed us to discover prime candidate genes for metabolic and vegetative growth variation. Prioritised candidate genes for early biomass accumulation include A06p05760.1_BnaDAR (PIAL1), A10p16280.1_BnaDAR, C07p48260.1_BnaDAR (PRL1) and C07p48510.1_BnaDAR (CLPR4). Moreover, we observed unequal effects of the Brassica A and C subgenomes on early biomass production.

SCAB1 coordinates sequential Ca2+ and ABA signals during osmotic stress induced stomatal closure in Arabidopsis

Hyperosmotic stress caused by drought is a detrimental threat to plant growth and agricultural productivity due to limited water availability. Stomata are gateways of transpiration and gas exchange, the swift adjustment of stomatal aperture has a strong influence on plant drought resistance. Despite intensive investigations of stomatal closure during drought stress in past decades, little is known about how sequential signals are integrated during complete processes. Here, we discovered that the rapid Ca2+ signaling and subsequent abscisic acid (ABA) signaling contribute to the kinetics of both F-actin reorganizations and stomatal closure in Arabidopsis thaliana, while STOMATAL CLOSURE-RELATED ACTIN BINDING PROTEIN1 (SCAB1) is the molecular switch for this entire process. During the early stage of osmotic shock responses, swift elevated calcium signaling promotes SCAB1 phosphorylation through calcium sensors CALCIUM DEPENDENT PROTEIN KINASE3 (CPK3) and CPK6. The phosphorylation restrained the microfilament binding affinity of SCAB1, which bring about the F-actin disassembly and stomatal closure initiation. As the osmotic stress signal continued, both the kinase activity of CPK3 and the phosphorylation level of SCAB1 attenuated significantly. We further found that ABA signaling is indispensable for these attenuations, which presumably contributed to the actin filament reassembly process as well as completion of stomatal closure. Notably, the dynamic changes of SCAB1 phosphorylation status are crucial for the kinetics of stomatal closure. Taken together, our results support a model in which SCAB1 works as a molecular switch, and directs the microfilament rearrangement through integrating the sequentially generated Ca2+ and ABA signals during osmotic stress induced stomatal closure.

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.

Plant KASH proteins SINE1 and SINE2 have synergistic and antagonistic interactions with actin-branching and actin-bundling factors

Linker of nucleoskeleton and cytoskeleton (LINC) complexes consist of outer nuclear membrane KASH proteins, interacting in the nuclear envelope lumen with inner nuclear membrane SUN proteins and connecting the nucleus and cytoskeleton. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and abscisic acid (ABA), which requires F-actin reorganization. SINE2 influences actin depolymerization and SINE1 actin repolymerization. The actin-related protein 2/3 (ARP2/3) complex, an actin nucleator, and the plant actin-bundling and -stabilizing factor SCAB1 are involved in stomatal aperture control. Here, we have tested the genetic interaction of SINE1 and SINE2 with SCAB1 and the ARP2/3 complex. We show that SINE1 and the ARP2/3 complex function in the same pathway during ABA-induced stomatal closure, while SINE2 and the ARP2/3 complex play opposing roles. The actin repolymerization defect observed in sine1-1 is partially rescued in scab1-2 sine1-1, while SINE2 is epistatic to SCAB1. In addition, SINE1 and ARP2/3 act synergistically in lateral root development. The absence of SINE2 renders trichome development independent of the ARP2/3 complex. Together, these data reveal complex and differential interactions of the two KASH proteins with the actin-remodeling apparatus and add evidence to the proposed differential role of SINE1 and SINE2 in actin dynamics.

Research progress on the roles of actin-depolymerizing factor in plant stress responses

Actin-depolymerizing factors (ADFs) are highly conserved small-molecule actin-binding proteins found throughout eukaryotic cells. In land plants, ADFs form a small gene family that displays functional redundancy despite variations among its individual members. ADF can bind to actin monomers or polymerized microfilaments and regulate dynamic changes in the cytoskeletal framework through specialized biochemical activities, such as severing, depolymerizing, and bundling. The involvement of ADFs in modulating the microfilaments' dynamic changes has significant implications for various physiological processes, including plant growth, development, and stress response. The current body of research has greatly advanced our comprehension of the involvement of ADFs in the regulation of plant responses to both biotic and abiotic stresses, particularly with respect to the molecular regulatory mechanisms that govern ADF activity during the transmission of stress signals. Stress has the capacity to directly modify the transcription levels of ADF genes, as well as indirectly regulate their expression through transcription factors such as MYB, C-repeat binding factors, ABF, and 14-3-3 proteins. Furthermore, apart from their role in regulating actin dynamics, ADFs possess the ability to modulate the stress response by influencing downstream genes associated with pathogen resistance and abiotic stress response. This paper provides a comprehensive overview of the current advancements in plant ADF gene research and suggests that the identification of plant ADF family genes across a broader spectrum, thorough analysis of ADF gene regulation in stress resistance of plants, and manipulation of ADF genes through genome-editing techniques to enhance plant stress resistance are crucial avenues for future investigation in this field.

Genome-wide identification of actin-depolymerizing factor gene family and their expression patterns under various abiotic stresses in soybean (Glycine max)

The actin-depolymerizing factor (ADF) encoded by a family of genes is highly conserved among eukaryotes and plays critical roles in the various processes of plant growth, development, and stress responses via the remodeling of the architecture of the actin cytoskeleton. However, the ADF family and the encoded proteins in soybean (Glycine max) have not yet been systematically investigated. In this study, 18 GmADF genes (GmADF1 - GmADF18) were identified in the soybean genome and were mapped to 14 different chromosomes. Phylogenetic analysis classified them into four groups, which was confirmed by their structure and the distribution of conserved motifs in the encoded proteins. Additionally, 29 paralogous gene pairs were identified in the GmADF family, and analysis of their Ka/Ks ratios indicated their purity-based selection during the evolutionary expansion of the soybean genome. The analysis of the expression profiles based on the RNA-seq and qRT-PCR data indicated that GmADFs were diversely expressed in different organs and tissues, with most of them responding actively to drought- and salt-induced stresses, suggesting the critical roles played by them in various biological processes. Overall, our study shows that GmADF genes may play a crucial role in response to various abiotic stresses in soybean, and the highly inducible candidate genes could be used for further functional studies and molecular breeding in soybean.

Calcium-dependent protein kinase GhCDPK4 plays a role in drought and abscisic acid stress responses

Drought is an important factor limiting the yield and quality of cotton. In the present study, the gene encoding the cotton calcium-dependent protein kinase GhCDPK4 was identified and characterized in the transcriptome of cotton under PEG-induced drought stress. In RT-qPCR experiments, GhCDPK4 expression was found to be up-regulated under drought and abscisic acid (ABA) stress. Under drought conditions, heterologous overexpression of GhCDPK4 in tobacco showed a better phenotypic status, higher antioxidant enzyme activity, and lower relative electrolyte leakage (REL) and malondialdehyde (MDA) content. Meanwhile, ghcdpk4-silenced cotton plants, which were extremely sensitive to drought, exhibited higher levels of O^2-，H₂O₂, and MDA contents compared to the control. Meanwhile, silenced lines showed impaired stomatal closure under drought stress, resulting in increased water loss from transpiration in silenced lines. GhCDPK4 expression was induced by ABA, and there are five ABA-responsive elements in its promoter. and C2-DOMAIN ABA-RELATED 4(CAR4, Gh_D09G1653) were found to interact and be co-expressed in the GhCDPK4 interaction network. Therefore, GhCDPK4 may reduce the extent of water loss and oxidative damage in cotton under drought by positively regulating ABA-controlled stomatal closure and reactive oxygen species (ROS) scavenging systems. This study demonstrates the great potential of GhCDPK4 in improving drought resistance in crops.

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.

An Overview of the Public Health Challenges in Diagnosing and Controlling Human Foodborne Pathogens

Pathogens found in food are believed to be the leading cause of foodborne illnesses; and they are considered a serious problem with global ramifications. During the last few decades, a lot of attention has been paid to determining the microorganisms that cause foodborne illnesses and developing new methods to identify them. Foodborne pathogen identification technologies have evolved rapidly over the last few decades, with the newer technologies focusing on immunoassays, genome-wide approaches, biosensors, and mass spectrometry as the primary methods of identification. Bacteriophages (phages), probiotics and prebiotics were known to have the ability to combat bacterial diseases since the turn of the 20th century. A primary focus of phage use was the development of medical therapies; however, its use quickly expanded to other applications in biotechnology and industry. A similar argument can be made with regards to the food safety industry, as diseases directly endanger the health of customers. Recently, a lot of attention has been paid to bacteriophages, probiotics and prebiotics most likely due to the exhaustion of traditional antibiotics. Reviewing a variety of current quick identification techniques is the purpose of this study. Using these techniques, we are able to quickly identify foodborne pathogenic bacteria, which forms the basis for future research advances. A review of recent studies on the use of phages, probiotics and prebiotics as a means of combating significant foodborne diseases is also presented. Furthermore, we discussed the advantages of using phages as well as the challenges they face, especially given their prevalent application in food safety.

The rubber tree kinome: Genome-wide characterization and insights into coexpression patterns associated with abiotic stress responses

The protein kinase (PK) superfamily constitutes one of the largest and most conserved protein families in eukaryotic genomes, comprising core components of signaling pathways in cell regulation. Despite its remarkable relevance, only a few kinase families have been studied in Hevea brasiliensis. A comprehensive characterization and global expression analysis of the PK superfamily, however, is currently lacking. In this study, with the aim of providing novel inferences about the mechanisms associated with the stress response developed by PKs and retained throughout evolution, we identified and characterized the entire set of PKs, also known as the kinome, present in the Hevea genome. Different RNA-sequencing datasets were employed to identify tissue-specific expression patterns and potential correspondences between different rubber tree genotypes. In addition, coexpression networks under several abiotic stress conditions, such as cold, drought and latex overexploitation, were employed to elucidate associations between families and tissues/stresses. A total of 1,809 PK genes were identified using the current reference genome assembly at the scaffold level, and 1,379 PK genes were identified using the latest chromosome-level assembly and combined into a single set of 2,842 PKs. These proteins were further classified into 20 different groups and 122 families, exhibiting high compositional similarities among family members and with two phylogenetically close species Manihot esculenta and Ricinus communis. Through the joint investigation of tandemly duplicated kinases, transposable elements, gene expression patterns, and coexpression events, we provided insights into the understanding of the cell regulation mechanisms in response to several conditions, which can often lead to a significant reduction in rubber yield.

SALT OVERLY SENSITIVE 1 is inhibited by clade D Protein phosphatase 2C D6 and D7 in Arabidopsis thaliana

The salt overly sensitive (SOS) pathway is essential for maintaining sodium ion homeostasis in plants. This conserved pathway is activated by a calcium signaling-dependent phosphorylation cascade. However, the identity of the phosphatases and their regulatory mechanisms that would deactivate the SOS pathway remain unclear. In this study, we demonstrate that PP2C.D6 and PP2C.D7, which belong to clade D of the protein phosphatase 2C (PP2C) subfamily in Arabidopsis thaliana, directly interact with SOS1 and inhibit its Na+/H+ antiporter activity under non-salt-stress conditions. Upon salt stress, SOS3-LIKE CALCIUM-BINDING PROTEIN8 (SCaBP8), a member of the SOS pathway, interacts with the PP2Cs and suppresses their phosphatase activity; simultaneously, SCaBP8 regulates the subcellular localization of PP2C.D6 by releasing it from the plasma membrane. Thus, we identified two negative regulators of the SOS pathway that repress SOS1 activity under nonstress conditions. These processes set the stage for the activation of SOS1 by the kinase SOS2 to achieve plant salt tolerance. Our results suggest that reversible phosphorylation/dephosphorylation is crucial for the regulation of the SOS pathway, and that calcium sensors play dual roles in activating/deactivating SOS2 and PP2C phosphatases under salt stress.

The plant cytoskeleton takes center stage in abiotic stress responses and resilience

Stress resilience behaviours in plants are defensive mechanisms that develop under adverse environmental conditions to promote growth, development and yield. Over the past decades, improving stress resilience, especially in crop species, has been a focus of intense research for global food security and economic growth. Plants have evolved specific mechanisms to sense external stress and transmit information to the cell interior and generate appropriate responses. Plant cytoskeleton, comprising microtubules and actin filaments, takes a center stage in stress-induced signalling pathways, either as a direct target or as a signal transducer. In the past few years, it has become apparent that the function of the plant cytoskeleton and other associated proteins are not merely limited to elementary processes of cell growth and proliferation, but they also function in stress response and resilience. This review summarizes recent advances in the role of plant cytoskeleton and associated proteins in abiotic stress management. We provide a thorough overview of the mechanisms that plant cells employ to withstand different abiotic stimuli such as hypersalinity, dehydration, high temperature and cold, among others. We also discuss the crucial role of the plant cytoskeleton in organellar positioning under the influence of high light intensity.

CKL2 mediates the crosstalk between abscisic acid and brassinosteroid signaling to promote swift growth recovery after stress in Arabidopsis

Plants must adapt to the constantly changing environment. Adverse environmental conditions trigger various defensive responses, including growth inhibition mediated by phytohormone abscisic acid (ABA). When the stress recedes, plants must transit rapidly from stress defense to growth recovery, but the underlying mechanisms by which plants switch promptly and accurately between stress resistance and growth are poorly understood. Here, using quantitative phosphoproteomics strategy, we discovered that early ABA signaling activates upstream components of brassinosteroid (BR) signaling through CASEIN KINASE 1-LIKE PROTEIN 2 (CKL2). Further investigations showed that CKL2 interacts with and phosphorylates BRASSINOSTEROID INSENSITIVE1 (BRI1), the main BR receptor, to maintain the basal activity of the upstream of BR pathway in plants exposed to continuous stress conditions. When stress recedes, the elevated phosphorylation of BRI1 by CKL2 contributes to the swift reactivation of BR signaling, which results in quick growth recovery. These results suggest that CKL2 plays a critical regulatory role in the rapid switch between growth and stress resistance. Our evidence expands the understanding of how plants modulate stress defense and growth by integrating ABA and BR signaling cascades.

CARK3-mediated ADF4 regulates hypocotyl elongation and soil drought stress in Arabidopsis

Actin depolymerization factors (ADFs), as actin-binding proteins, act a crucial role in plant development and growth, as well as in response to abiotic and biotic stresses. Here, we found that CARK3 plays a role in regulating hypocotyl development and links a cross-talk between actin filament and drought stress through interaction with ADF4. By using bimolecular fluorescence complementation (BiFC) and GST pull-down, we confirmed that CARK3 interacts with ADF4 in vivo and in vitro. Next, we generated and characterized double mutant adf4cark3-4 and OE-ADF4:cark3-4. The hypocotyl elongation assay indicated that the cark3-4 mutant seedlings were slightly longer hypocotyls when compared with the wild type plants (WT), while CARK3 overexpressing seedlings had no difference with WT. In addition, overexpression of ADF4 significantly inhibited long hypocotyls of cark3-4 mutants. Surprisingly, we found that overexpression of ADF4 markedly enhance drought resistance in soil when compared with WT. On the other hand, drought tolerance analysis showed that overexpression of CARK3 could rescue adf4 drought susceptibility. Taken together, our results suggest that CARK3 acts as a regulator in hypocotyl elongation and drought tolerance likely via regulating ADF4 phosphorylation.

Binding of 14-3-3κ to ADF4 is involved in the regulation of hypocotyl growth and response to osmotic stress in Arabidopsis

14-3-3 proteins, a family of conserved molecules in eukaryotes, target a number of protein clients through their ability to recognize well-defined phosphorylated motifs. ADF4, as one of Actin-Depolymerizing Factor (ADF) family of proteins, is involved in plant development, and response to biotic and abiotic stresses. Here, we show that 14-3-3κ specially interacted with ADF4 in vitro and in vivo. The 14-3-3κ×adf4 double mutant displayed less F-actin bundle and shorter hypocotyl compared with adf4 mutant, indicating that 14-3-3κ acts upstream of ADF4 to mediate the hypocotyl growth in the dark-grown seedlings. Under the osmotic stress, 14-3-3κ mutants displayed less survival rate than wild-type plants. The adf4 mutants exhibited markedly enhanced survival rate under osmotic treatment, while ADF4-overexpressing plants displayed the opposite results, indicating that ADF4 plays a negative role in response to osmotic stress in Arabidopsis. The interaction between ADF4 and 14-3-3κ inhibited the association of ADF4 with actin filament. Moreover, the in vitro phosphorylation assay demonstrates that the phosphorylation of ADF4 by CASEIN KINASE1-LIKE PROTEIN2 (CKL2) was enhanced by binding 14-3-3κ. Collectively, our data infer a fundamental role for the interaction between 14-3-3κ and ADF4 in regulating hypocotyl growth and osmotic tolerance of plants.

Distinct Roles for KASH Proteins SINE1 and SINE2 in Guard Cell Actin Reorganization, Calcium Oscillations, and Vacuolar Remodeling

The linker of nucleoskeleton and cytoskeleton (LINC) complex is a protein complex spanning the inner and outer membranes of the nuclear envelope. Outer nuclear membrane KASH proteins interact in the nuclear envelope lumen with inner nuclear membrane SUN proteins. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and ABA. Previous studies have shown F-actin organization, cytoplasmic calcium (Ca²⁺) oscillations, and vacuolar morphology changes are involved in ABA-induced stomatal closure. Here, we show that SINE1 and SINE2 are both required for actin pattern changes during ABA-induced stomatal closure, but influence different, temporally distinguishable steps. External Ca²⁺ partially overrides the mutant defects. ABA-induced cytoplasmic Ca²⁺ oscillations are diminished in sine2-1 but not sine1-1, and this defect can be rescued by both exogenous Ca²⁺ and F-actin depolymerization. We show first evidence for nuclear Ca²⁺ oscillations during ABA-induced stomatal closure, which are disrupted in sine2-1. Vacuolar fragmentation is impaired in both mutants and is partially rescued by F-actin depolymerization. Together, these data indicate distinct roles for SINE1 and SINE2 upstream of this network of players involved in ABA-based stomatal closure, suggesting a role for the nuclear surface in guard cell ABA signaling.

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.

The PYR-PP2C-CKL2 module regulates ABA-mediated actin reorganization during stomatal closure

Limiting water loss by reducing transpiration helps plants survive when water is limited. Under drought stress, abscisic acid (ABA)-mediated gene expression and anion channel activation regulate stomatal closure and stress responses. ABA-induced actin reorganization also affects stomatal closure, but the underlying molecular mechanism remains unclear. In this study, we discovered that under nonstress conditions, the clade A PP2C phosphatases, such as ABI1 and ABI2, interact with CKL2 and inhibit its kinase activity in Arabidopsis. Under drought stress, CKL2 kinase activity was released through the formation of a complex containing ABA, PP2C and a PYR1/PYL/RCAR family (PYL) receptor. The activated CKL2 regulating actin reorganization is another important process to maintain stomatal closure besides ABA-activated SnRK2 signaling. Moreover, CKL2 phosphorylated PYR1-LIKE 1, ABI1 and ABI2 at amino acid residues conserved among PYLs and PP2Cs, and stabilized ABI1 protein. Our results reveal that ABA signaling regulates actin reorganization to maintain stomatal closure during drought stress, and the feedback regulation of PYL1, ABI1 and ABI2 by the CKL2 kinase might fine-tune ABA signaling and affect plant ABA responses.

Phosphatidylinositol 3-phosphate regulates SCAB1-mediated F-actin reorganization during stomatal closure in Arabidopsis

Stomatal movement is critical for plant responses to environmental changes and is regulated by the important signaling molecule phosphatidylinositol 3-phosphate (PI3P). However, the molecular mechanism underlying this process is not well understood. In this study, we show that PI3P binds to stomatal closure-related actin-binding protein1 (SCAB1), a plant-specific F-actin-binding and -bundling protein, and inhibits the oligomerization of SCAB1 to regulate its activity on F-actin in guard cells during stomatal closure in Arabidopsis thaliana. SCAB1 binds specifically to PI3P, but not to other phosphoinositides. Treatment with wortmannin, an inhibitor of phosphoinositide kinase that generates PI3P, leads to an increase of the intermolecular interaction and oligomerization of SCAB1, stabilization of F-actin, and retardation of F-actin reorganization during abscisic acid (ABA)-induced stomatal closure. When the binding activity of SCAB1 to PI3P is abolished, the mutated proteins do not rescue the stability and realignment of F-actin regulated by SCAB1 and the stomatal closure in the scab1 mutant. The expression of PI3P biosynthesis genes is consistently induced when the plants are exposed to drought and ABA treatments. Furthermore, the binding of PI3P to SCAB1 is also required for vacuolar remodeling during stomatal closure. Our results illustrate a PI3P-regulated pathway during ABA-induced stomatal closure, which involves the mediation of SCAB1 activity in F-actin reorganization.

Controlling the Gate: The Functions of the Cytoskeleton in Stomatal Movement

Stomata are specialized epidermal structures composed of two guard cells and are involved in gas and water exchange between plants and the environment and pathogen entry into the plant interior. Stomatal movement is a response to many internal and external stimuli to increase adaptability to environmental change. The cytoskeleton, including actin filaments and microtubules, is highly dynamic in guard cells during stomatal movement, and the destruction of the cytoskeleton interferes with stomatal movement. In this review, we discuss recent progress on the organization and dynamics of actin filaments and microtubule network in guard cells, and we pay special attention to cytoskeletal-associated protein-mediated cytoskeletal rearrangements during stomatal movement. We also discuss the potential mechanisms of stomatal movement in relation to the cytoskeleton and attempt to provide a foundation for further research in this field.

OsFH3 Encodes a Type II Formin Required for Rice Morphogenesis

The actin cytoskeleton is crucial for plant morphogenesis, and organization of actin filaments (AF) is dynamically regulated by actin-binding proteins. However, the roles of actin-binding proteins, particularly type II formins, in this process remain poorly understood in plants. Here, we report that a type II formin in rice, Oryza sativa formin homolog 3 (OsFH3), acts as a major player to modulate AF dynamics and contributes to rice morphogenesis. osfh3 mutants were semi-dwarf with reduced size of seeds and unchanged responses to light or gravity compared with mutants of osfh5, another type II formin in rice. osfh3 osfh5 mutants were dwarf with more severe developmental defectiveness. Recombinant OsFH3 could nucleate actin, promote AF bundling, and cap the barbed end of AF to prevent elongation and depolymerization, but in the absence of profilin, OsFH3 could inhibit AF elongation. Different from other reported type II formins, OsFH3 could bind, but not bundle, microtubules directly. Furthermore, its N-terminal phosphatase and tensin homolog domain played a key role in modulating OsFH3 localization at intersections of AF and punctate structures of microtubules, which differed from other reported plant formins. Our results, thus, provide insights into the biological function of type II formins in modulating plant morphology by acting on AF dynamics.

Arabidopsis ADF1 is Regulated by MYB73 and is Involved in Response to Salt Stress Affecting Actin Filament Organization

Actin cytoskeleton and transcription factors play key roles in plant response to salt stress; however, little is known about the link between the two regulators in response to salt stress. Actin-depolymerizing factors (ADFs) are conserved actin-binding proteins in eukaryotes. Here, we revealed that the expression level of ADF1 was induced by salt stress. The adf1 mutants showed significantly reduced survival rate, increased percentage of actin cable and reduced density of actin filaments, while ADF1 overexpression seedlings displayed the opposite results when compared with WT under the same condition. Furthermore, biochemical assays revealed that MYB73, a R2R3 MYB transcription factor, binds to the promoter of ADF1 and represses its expression via the MYB-binding site core motif ACCTAC. Taken together, our results indicate that ADF1 participates in salt stress by regulating actin organization and may also serve as a potential downstream target of MYB73, which is a negative regulator of salt stress.

MPK3- and MPK6-mediated VLN3 phosphorylation regulates actin dynamics during stomatal immunity in Arabidopsis

Upon perception of pathogens, plants can rapidly close their stomata to restrict pathogen entry into internal tissue, leading to stomatal immunity as one aspect of innate immune responses. The actin cytoskeleton is required for plant defense against microbial invaders. However, the precise functions of host actin during plant immunity remain largely unknown. Here, we report that Arabidopsis villin3 (VLN3) is critical for plant resistance to bacteria by regulating stomatal immunity. Our in vitro and in vivo phosphorylation assays show that VLN3 is a physiological substrate of two pathogen-responsive mitogen-activated protein kinases, MPK3/6. Quantitative analyses of actin dynamics and genetic studies reveal that VLN3 phosphorylation by MPK3/6 modulates actin remodeling to activate stomatal defense in Arabidopsis.

Plants can rapidly close stomata to restrict pathogen entry into leaves. Here the authors show that phosphorylation of villin3 by mitogen-activated protein kinases modulates actin remodeling to activate stomatal defense in Arabidopsis.

Stomatal morphology and physiology explain varied sensitivity to abscisic acid across vascular plant lineages

Abscisic acid (ABA) can induce rapid stomatal closure in seed plants, but the action of this hormone on the stomata of fern and lycophyte species remains equivocal. Here, ABA-induced stomatal closure, signaling components, guard cell K+ and Ca2+ fluxes, vacuolar and actin cytoskeleton dynamics, and the permeability coefficient of guard cell protoplasts (Pf) were analyzed in species spanning the diversity of vascular land plants including 11 seed plants, 6 ferns, and 1 lycophyte. We found that all 11 seed plants exhibited ABA-induced stomatal closure, but the fern and lycophyte species did not. ABA-induced hydrogen peroxide elevation was observed in all species, but the signaling pathway downstream of nitric oxide production, including ion channel activation, was only observed in seed plants. In the angiosperm faba bean (Vicia faba), ABA application caused large vacuolar compartments to disaggregate, actin filaments to disintegrate into short fragments and Pf to increase. None of these changes was observed in the guard cells of the fern Matteuccia struthiopteris and lycophyte Selaginella moellendorffii treated with ABA, but a hypertonic osmotic solution did induce stomatal closure in fern and the lycophyte. Our results suggest that there is a major difference in the regulation of stomata between the fern and lycophyte plants and the seed plants. Importantly, these findings have uncovered the physiological and biophysical mechanisms that may have been responsible for the evolution of a stomatal response to ABA in the earliest seed plants.

Arabidopsis ADF5 Acts as a Downstream Target Gene of CBFs in Response to Low-Temperature Stress

Low temperature is a major adverse environment that affects normal plant growth. Previous reports showed that the actin cytoskeleton plays an important role in the plant response to low-temperature stress, but the regulatory mechanism of the actin cytoskeleton in this process is not clear. C-repeat binding factors (CBFs) are the key molecular switches for plants to adapt to cold stress. However, whether CBFs are involved in the regulation of the actin cytoskeleton has not been reported. We found that Arabidopsis actin depolymerizing factor 5 (ADF5), an ADF that evolved F-actin bundling function, was up-regulated at low temperatures. We also demonstrated that CBFs bound to the ADF5 promoter directly in vivo and in vitro. The cold-induced expression of ADF5 was significantly inhibited in the cbfs triple mutant. The freezing resistance of the adf5 knockout mutant was weaker than that of wild type (WT) with or without cold acclimation. After low-temperature treatment, the actin cytoskeleton of WT was relatively stable, but the actin cytoskeletons of adf5, cbfs, and adf5 cbfs were disturbed to varying degrees. Compared to WT, the endocytosis rate of the amphiphilic styryl dye FM4-64 in adf5, cbfs, and adf5 cbfs at low temperature was significantly reduced. In conclusion, CBFs directly combine with the CRT/DRE DNA regulatory element of the ADF5 promoter after low-temperature stress to transcriptionally activate the expression of ADF5; ADF5 further regulates the actin cytoskeleton dynamics to participate in the regulation of plant adaptation to a low-temperature environment.

Multidimensional analysis of actin depolymerising factor family in pigeon pea under different environmental stress revealed specific response genes in each subgroup

Actin depolymerising factor (ADF) is an actin binding protein that is ubiquitous in animal and plant cells. It plays an important role in plant growth and development, as well as resistance to biotic and abiotic stress. The research of plant ADF family has been restricted to Arabidopsis thaliana (L.) Heynh. and some herb crops, but no woody cash crops have been reported to date. All members of the Cajanus cajan (L.) Millsp. ADF (CcADF) family were identified from the pigeon pea genome, and distributed among the four subfamilies by phylogenetic analysis. CcADFs were relatively conservative in gene structure evolution, protein structure and functional expression, and different CcADFs showed specific expression patterns under different treatments. The expression characteristics of several key CcADFs were revealed by analysing the stress response pattern of CcADFs and the time series RNA-seq of aluminium stress. Among them, CcADF9 in the first subgroup specifically responded to aluminium stress in the roots; CcADF3 in the second subgroup intensively responded to fungal infection in the leaves; and CcADF2 in the fourth subgroup positively responded to various stress treatments in different tissues. This study extended the relationship between plant ADF family and aluminium tolerance, as well as adding to the understanding of CcADF family in woody crops.

Regulation of cytoskeleton-associated protein activities: Linking cellular signals to plant cytoskeletal function

The plant cytoskeleton undergoes dynamic remodeling in response to diverse developmental and environmental cues. Remodeling of the cytoskeleton coordinates growth in plant cells, including trafficking and exocytosis of membrane and wall components during cell expansion, and regulation of hypocotyl elongation in response to light. Cytoskeletal remodeling also has key functions in disease resistance and abiotic stress responses. Many stimuli result in altered activity of cytoskeleton-associated proteins, microtubule-associated proteins (MAPs) and actin-binding proteins (ABPs). MAPs and ABPs are the main players determining the spatiotemporally dynamic nature of the cytoskeleton, functioning in a sensory hub that decodes signals to modulate plant cytoskeletal behavior. Moreover, MAP and ABP activities and levels are precisely regulated during development and environmental responses, but our understanding of this process remains limited. In this review, we summarize the evidence linking multiple signaling pathways, MAP and ABP activities and levels, and cytoskeletal rearrangements in plant cells. We highlight advances in elucidating the multiple mechanisms that regulate MAP and ABP activities and levels, including calcium and calmodulin signaling, ROP GTPase activity, phospholipid signaling, and post-translational modifications.

ABF3 enhances drought tolerance via promoting ABA-induced stomatal closure by directly regulating ADF5 in Populus euphratica

Water availability is a main limiting factor for plant growth, development, and distribution throughout the world. Stomatal movement mediated by abscisic acid (ABA) is particularly important for drought adaptation, but the molecular mechanisms in trees are largely unclear. Here, we isolated an ABA-responsive element binding factor, PeABF3, in Populus euphratica. PeABF3 was preferentially expressed in the xylem and young leaves, and was induced by dehydration and ABA treatments. PeABF3 showed transactivation activity and was located in the nucleus. To study its functional mechanism in poplar responsive to drought stress, transgenic triploid white poplars (Populus tomentosa 'YiXianCiZhu B385') overexpressing PeABF3 were generated. PeABF3 overexpression significantly enhanced stomatal sensitivity to exogenous ABA. When subjected to drought stress, PeABF3 overexpression maintained higher photosynthetic activity and promoted cell membrane integrity, resulting in increased water-use efficiency and enhanced drought tolerance compared with wild-type controls. Moreover, a yeast one-hybrid assay and an electrophoretic mobility shift assay revealed that PeABF3 activated the expression of Actin-Depolymerizing Factor-5 (PeADF5) by directly binding to its promoter, promoting actin cytoskeleton remodeling and stomatal closure in poplar under drought stress. Taken together, our results indicate that PeABF3 enhances drought tolerance via promoting ABA-induced stomatal closure by directly regulating PeADF5 expression.

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.

Review: F-Actin remodelling during plant signal transduction via biomolecular assembly

During signal transduction, multivalent interactions establish dynamic molecular connectivities that propagate molecular cascades throughout the entire signaling pathway. Such multivalent interactions include the initial activation, cascade signal transduction, and the amplification and assembly of structural machinery. For example, plants rapidly remodel the actin cytoskeleton during signal transduction by perceiving a wide range of mechanical and chemical cues from developmental and defense pathways. Actin treadmilling is stepwise-regulated by interactions between actin and actin-binding proteins (ABPs). Emerging evidence shows that intrinsically disordered regions (IDRs) enable flexible and promiscuous interactions that serve as the functional hub for generating cellular interactomes underlying various signaling events. Though IDRs are present in a majority of ABPs, few of the functional roles of IDR in the interaction and functions of ABPs have been defined. The distinct features of IDRs create diverse and dynamic molecular interactions that introduce a new paradigm to our understanding of the structure-function relationships for actin assembly. In this review, we will create a snapshot of recent advances in IDR-mediated plant actin remodeling and discuss future research directions in studying the complexity of actin assembly via multifaceted biomolecular assembly during signal transduction.

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.

Arabidopsis KASH Proteins SINE1 and SINE2 Are Involved in Microtubule Reorganization During ABA-Induced Stomatal Closure

Abscisic acid (ABA) induces stomatal closure by utilizing complex signaling mechanisms, allowing for sessile plants to respond rapidly to ever-changing environmental conditions. ABA regulates the activity of plasma membrane ion channels and calcium-dependent protein kinases, Ca2+ oscillations, and reactive oxygen species (ROS) concentrations. Throughout ABA-induced stomatal closure, the cytoskeleton undergoes dramatic changes that appear important for efficient closure. However, the precise role of this cytoskeletal reorganization in stomatal closure and the nature of its regulation are unknown. We have recently shown that the plant KASH proteins SINE1 and SINE2 are connected to actin organization during ABA-induced stomatal closure but their role in microtubule (MT) organization remains to be investigated. We show here that depolymerizing MTs using oryzalin can restore ABA-induced stomatal closure deficits in sine1-1 and sine2-1 mutants. GFP-MAP4-visualized MT organization is compromised in sine1-1 and sine2-1 mutants during ABA-induced stomatal closure. Loss of SINE1 or SINE2 results in loss of radially organized MT patterning in open guard cells, aberrant MT organization during stomatal closure, and an overall decrease in the number of MT filaments or bundles. Thus, SINE1 and SINE2 are necessary for establishing MT patterning and mediating changes in MT rearrangement, which is required for ABA-induced stomatal closure.

Systematic analysis of SmWD40s, and responding of SmWD40-170 to drought stress by regulation of ABA- and H2O2-induced stomal movement in Salvia miltiorrhiza bunge

WD40 proteins play crucial roles in response to abiotic stress. By screening the genome sequences of Salvia miltiorrhiza Bunge, 225 SmWD40 genes were identified and divided into 9 subfamilies (I-IX). Physiological, biochemical, gene structure, conserved protein motif and GO annotation analyses were performed on SmWD40 family members. The SmWD40-170 was found in 110 SmWD40 genes that contain drought response elements, SmWD40-170 was one of these genes whose response in terms of expression under drought was significant. The expression of SmWD40-170 was also up-regulated by ABA and H₂O₂. Through observed the stomatal phenotype of SmWD40-170 transgenic lines, the stomatal closure was abolished under dehydration, ABA and H₂O₂ treatment in SmWD40-170 knockdown lines. Abscisic acid (ABA), as the key phytohormone, elevates reactive oxygen species (ROS) levels under drought stress. The ABA-ROS interaction mediated the generation of H₂O₂ and the activation of anion channel in guard cells. The osmolality alteration of guard cells further accelerated the stomatal closure. As a second messenger, nitric oxide (NO) regulated ABA signaling, the NO stimulated protein kinase activity inhibited the K⁺ influx which result in stomatal closure. These NO-relevant events were essential for ABA-induced stomatal closure. The reduction of NO production was also observed in the guard cells of SmWD40-170 knockdown lines. The abolished of stomatal closure attributed to the SmWD40-170 deficiency induced the reduction of NO content. In general, the SmWD40-170 is a critical drought response gene in SmWD40 gene family and regulates ABA- and H₂O₂-induced stomatal movement by affecting the synthesis of NO.

Environmental pollutants damage airway epithelial cell cilia: Implications for the prevention of obstructive lung diseases

Mucociliary epithelium lining the upper and lower respiratory tract constitutes the first line of defense of the airway and lungs against inhaled pollutants and pathogens. The concerted beating of multiciliated cells drives mucociliary clearance. Abnormalities in both the structure and function of airway cilia have been implicated in obstructive lung diseases. Emerging evidence reveals a close correlation between lung diseases and environmental stimuli such as sulfur dioxide and tobacco particles. However, the underlying mechanism remains to be described. In this review, we emphasize the importance of airway cilia in mucociliary clearance and discuss how environmental pollutants affect the structure and function of airway cilia, thus shedding light on the function of airway cilia in preventing obstructive lung diseases and revealing the negative effects of environmental pollutants on human health.

Mut9p-LIKE KINASE Family Members: New Roles of the Plant-Specific Casein Kinase I in Plant Growth and Development

: Casein kinase I (CK1), a ubiquitous serine/threonine (Ser/Thr) protein kinase in eukaryotes, plays pivotal roles in a wide spectrum of cellular functions including metabolism, cell cycle progression, developmental control and stress responses. Plant CK1 evolves a lineage expansion, resulting in a unique branch of members exclusive to the kingdom. Among them, Arabidopsis Mut9p-LIKE KINASEs (MLKs) target diverse substrates including histones and the key regulatory proteins involving in physiological processes of light signaling, circadian rhythms, phytohormone and plant defense. Deregulation of the kinase activity by mutating the enzyme or the phosphorylation sites of substrates causes developmental disorders and susceptibility to adverse environmental conditions. MLKs have evolved as a general kinase that modifies transcription factors or primary regulatory proteins in a dynamic way. Here, we summarize the current knowledge of the roles of MLKs and MLK orthologs in several commercially important crops.

A Role for Plant KASH Proteins in Regulating Stomatal Dynamics

Stomatal movement, which regulates gas exchange in plants, is controlled by a variety of environmental factors, including biotic and abiotic stresses. The stress hormone abscisic acid (ABA) initiates a signaling cascade, which leads to increased H₂O₂ and Ca²⁺ levels and F-actin reorganization, but the mechanism of, and connection between, these events is unclear. SINE1, an outer nuclear envelope component of a plant Linker of Nucleoskeleton and Cytoskeleton complex, associates with F-actin and is, along with its putative paralog SINE2, expressed in guard cells. Here, we have determined that Arabidopsis (Arabidopsis thaliana) SINE1 and SINE2 play an important role in stomatal opening and closing. Loss of SINE1 or SINE2 results in ABA hyposensitivity and impaired stomatal dynamics but does not affect stomatal closure induced by the bacterial elicitor flg22. The ABA-induced stomatal closure phenotype is, in part, attributed to impairments in Ca²⁺ and F-actin regulation. Together, the data suggest that SINE1 and SINE2 act downstream of ABA but upstream of Ca²⁺ and F-actin. While there is a large degree of functional overlap between the two proteins, there are also critical differences. Our study makes an unanticipated connection between stomatal regulation and nuclear envelope-associated proteins, and adds two new players to the increasingly complex system of guard cell regulation.

Cell Cycle Regulation in the Plant Response to Stress

As sessile organisms, plants face a variety of environmental challenges. Their reproduction and survival depend on their ability to adapt to these stressors, which include water, heat stress, high salinity, and pathogen infection. Failure to adapt to these stressors results in programmed cell death and decreased viability, as well as reduced productivity in the case of crop plants. The growth and development of plants are maintained by meiosis and mitosis as well as endoreduplication, during which DNA replicates without cytokinesis, leading to polyploidy. As in other eukaryotes, the cell cycle in plants consists of four stages (G1, S, G2, and M) with two major check points, namely, the G1/S check point and G2/M check point, that ensure normal cell division. Progression through these checkpoints involves the activity of cyclin-dependent kinases and their regulatory subunits known as cyclins. In order for plants to survive, cell cycle control must be balanced with adaption to dynamic environmental conditions. In this review, we summarize recent advances in our understanding of cell cycle regulation in plants, with a focus on the molecular interactions of cell cycle machinery in the context of stress tolerance.

Mapping proteome-wide targets of protein kinases in plant stress responses

Protein kinases are major regulatory components in almost all cellular processes in eukaryotic cells. By adding phosphate groups, protein kinases regulate the activity, localization, protein-protein interactions, and other features of their target proteins. It is known that protein kinases are central components in plant responses to environmental stresses such as drought, high salinity, cold, and pathogen attack. However, only a few targets of these protein kinases have been identified. Moreover, how these protein kinases regulate downstream biological processes and mediate stress responses is still largely unknown. In this study, we introduce a strategy based on isotope-labeled in vitro phosphorylation reactions using in vivo phosphorylated peptides as substrate pools and apply this strategy to identify putative substrates of nine protein kinases that function in plant abiotic and biotic stress responses. As a result, we identified more than 5,000 putative target sites of osmotic stress-activated SnRK2.4 and SnRK2.6, abscisic acid-activated protein kinases SnRK2.6 and casein kinase 1-like 2 (CKL2), elicitor-activated protein kinase CDPK11 and MPK6, cold-activated protein kinase MPK6, H₂O₂-activated protein kinase OXI1 and MPK6, and salt-induced protein kinase SOS1 and MPK6, as well as the low-potassium-activated protein kinase CIPK23. These results provide comprehensive information on the role of these protein kinases in the control of cellular activities and could be a valuable resource for further studies on the mechanisms underlying plant responses to environmental stresses.

Environmental pollutants damage airway epithelial cell cilia: Implications for the prevention of obstructive lung diseases

Mucociliary epithelium lining the upper and lower respiratory tract constitutes the first line of defense of the airway and lungs against inhaled pollutants and pathogens. The concerted beating of multiciliated cells drives mucociliary clearance. Abnormalities in both the structure and function of airway cilia have been implicated in obstructive lung diseases. Emerging evidence reveals a close correlation between lung diseases and environmental stimuli such as sulfur dioxide and tobacco particles. However, the underlying mechanism remains to be described. In this review, we emphasize the importance of airway cilia in mucociliary clearance and discuss how environmental pollutants affect the structure and function of airway cilia, thus shedding light on the function of airway cilia in preventing obstructive lung diseases and revealing the negative effects of environmental pollutants on human health.

BAG6 is a novel microtubule-binding protein that regulates ciliogenesis by modulating the cell cycle and interacting with γ-tubulin

Microtubule-binding proteins provide an alternative and vital pathway to the functional diversity of microtubules. Considerable work is still required to understand the complexities of microtubule-associated cellular processes and to identify novel microtubule-binding proteins. In this study, we identify Bcl2-associated athanogene cochaperone 6 (BAG6) as a novel microtubule-binding protein and reveal that it is crucial for primary ciliogenesis. By immunofluorescence we show that BAG6 largely colocalizes with intracellular microtubules and by co-immunoprecipitation we demonstated that it can interact with α-tubulin. Additionally, both the UBL and BAG domains of BAG6 are indispensable for its interaction with α-tubulin. Moreover, the assembly of primary cilia in RPE-1 cells is significantly inhibited upon the depletion of BAG6. Notably, BAG6 inhibition leads to an abnormal G0/G1 transition during the cell cycle. In addition, BAG6 colocalizes and interactes with the centrosomal protein γ-tubulin, suggesting that BAG6 might regulate primary ciliogenesis through its action in centrosomal function. Collectively, our findings suggest that BAG6 is a novel microtubule-bindng protein crucial for primary ciliogenesis.

Understanding the functions and mechanisms of plant cytoskeleton in response to environmental signals

Plants perceive multiple physiological and environmental signals in order to fine-tune their growth and development. The highly dynamic plant cytoskeleton, including actin and microtubule networks, can rapidly alter their organization, stability and dynamics in response to internal and external stimuli, which is considered vital for plant growth and adaptation to the environment. The cytoskeleton-associated proteins have been shown to be key regulatory molecules in mediating cytoskeleton reorganization in response to multiple environmental signals, such as light, salt, drought and biotic stimuli. Recent findings, including our studies, have expanded knowledge about the functions and underlying mechanisms of the plant cytoskeleton in environmental adaptation.

Structure-function study of a novel inhibitor of the casein kinase 1 family in Arabidopsis thaliana

Casein kinase 1 (CK1) is an evolutionarily conserved protein kinase family among eukaryotes. Studies in non-plants have shown CK1-dependent divergent biological processes, but the collective knowledge regarding the biological roles of plant CK1 lags far behind other members of the Eukarya. One reason for this is that plants have many more genes encoding CK1 than do animals. To accelerate our understanding of the plant CK1 family, a strong CK1 inhibitor that efficiently inhibits multiple members of the CK1 protein family in vivo (i.e., in planta) is required. Here, we report a novel, specific, and effective CK1 inhibitor in Arabidopsis. Using circadian period-lengthening activity as an estimation of the CK1 inhibitor effect in vivo, we performed a structure-activity relationship study of analogues of the CK1 inhibitor PHA767491 (1,5,6,7-tetrahydro-2-(4-pyridinyl)-4H-pyrrolo[3,2-c]pyridin-4-one hydrochloride). A propargyl group at the pyrrole nitrogen atom (AMI-212) or a bromine atom at the pyrrole C3 position (AMI-23) had stronger CK1 inhibitory activity than PHA767491. A hybrid molecule of AMI-212 and AMI-23 (AMI-331) was about 100-fold more inhibitory than the parent molecule PHA767491. Affinity proteomics using an AMI-331 probe showed that the targets of AMI-331 inhibition are mostly CK1 kinases. As such, AMI-331 is a potent and selective CK1 inhibitor that shows promise in the research of CK1 in plants.

AP3M harbors actin filament binding activity that is crucial for vacuole morphology and stomatal closure in Arabidopsis

Stomatal movement is essential for plant growth. This process is precisely regulated by various cellular activities in guard cells. F-actin dynamics and vacuole morphology are both involved in stomatal movement. The sorting of cargoes by clathrin adaptor protein (AP) complexes from the Golgi to the vacuole is critical for establishing a normal vacuole morphology. In this study, we demonstrate that the medium subunit of the AP3 complex (AP3M) binds to and severs actin filaments in vitro and that it participates in the sorting of cargoes (such as the sucrose exporter SUC4) to the tonoplast, and thereby regulates stomatal closure in Arabidopsis thaliana Defects in AP3 or SUC4 led to more rapid water loss and delayed stomatal closure, as well as hypersensitivity to drought stress. In ap3m mutants, the F-actin status was altered compared to the wild type, and the sorted cargoes failed to localize to the tonoplast. AP3M contains a previously unidentified F-actin binding domain that is conserved in AP3M homologs in both plants and animals. Mutations in the F-actin binding domain of AP3M abolished its F-actin binding activity in vitro, leading to an aberrant vacuole morphology and reduced levels of SUC4 on the tonoplast in guard cells. Our findings indicate that the F-actin binding activity of AP3M is required for the precise localization of AP3-dependent cargoes to the tonoplast and for the regulation of vacuole morphology in guard cells during stomatal closure.

Altering microtubule stability affects microtubule clearance and nuclear extrusion during erythropoiesis

Mammalian erythrocytes are highly specialized cells that have adapted to lose their nuclei and cellular components during maturation to ensure oxygen delivery. Nuclear extrusion, the most critical event during erythropoiesis, represents an extreme case of asymmetric partitioning that requires a dramatic reorganization of the cytoskeleton. However, the precise role of the microtubule cytoskeleton in the enucleation process remains controversial. In this study, we show that microtubule reorganization is critical for microtubule clearance and nuclear extrusion during erythropoiesis. Using a rodent anemia model, we found that microtubules were present in erythroblasts and reticulocytes but were undetectable in erythrocytes. Further analysis demonstrated that microtubules became disordered in reticulocytes and revealed that microtubule stabilization was critical for tubulin degradation. Disruption of microtubule dynamics using the microtubule-stabilizing agent paclitaxel or the microtubule-destabilizing agent nocodazole did not affect the efficiency of erythroblast enucleation. However, paclitaxel treatment resulted in the retention of tubulin in mature erythrocytes, and nocodazole treatment led to a defect in pyrenocyte morphology. Taken together, our data reveals a critical role for microtubules in erythrocyte development. Our findings also implicate the disruption of microtubule dynamics in the pathogenesis of anemia-associated diseases, providing new insight into the pathogenesis of the microtubule-targeted agent-associated anemia frequently observed during cancer chemotherapy.

Guard Cell Microfilament Analyzer Facilitates the Analysis of the Organization and Dynamics of Actin Filaments in Arabidopsis Guard Cells

The actin cytoskeleton is involved in regulating stomatal movement, which forms distinct actin arrays within guard cells of stomata with different apertures. How those actin arrays are formed and maintained remains largely unexplored. Elucidation of the dynamic behavior of differently oriented actin filaments in guard cells will enhance our understanding in this regard. Here, we initially developed a program called ‘guard cell microfilament analyzer’ (GCMA) that enables the selection of individual actin filaments and analysis of their orientations semiautomatically in guard cells. We next traced the dynamics of individual actin filaments and performed careful quantification in open and closed stomata. We found that de novo nucleation of actin filaments occurs at both dorsal and ventral sides of guard cells from open and closed stomata. Interestingly, most of the nucleated actin filaments elongate radially and longitudinally in open and closed stomata, respectively. Strikingly, radial filaments tend to form bundles whereas longitudinal filaments tend to be removed by severing and depolymerization in open stomata. By contrast, longitudinal filaments tend to form bundles that are severed less frequently in closed stomata. These observations provide insights into the formation and maintenance of distinct actin arrays in guard cells in stomata of different apertures.