Plant actin depolymerizing factor: actin microfilament disassembly and more

Regulation of heterochromatin organization in plants

Heterochromatin is a nuclear area that contains highly condensed and transcriptionally inactive chromatin. Alterations in the organization of heterochromatin are correlated with changes in gene expression and genome stability, which affect various aspects of plant life. Thus, studies of the molecular mechanisms that regulate heterochromatin organization are important for understanding the regulation of plant physiology. Microscopically, heterochromatin can be characterized as chromocenters that are intensely stained with DNA-binding fluorescent dyes. Arabidopsis thaliana exhibits distinctive chromocenters in interphase nuclei, and genetic studies combined with cytological analyses have identified a number of factors that are involved in heterochromatin assembly and organization. In this review, I will summarize the factors involved in the regulation of heterochromatin organization in plants.

AtHMGB15 regulates tapetal apoptosis in pollen development and actin dynamics during pollen germination in arabidopsis

KEY MESSAGE

ARID-HMG DNA binding protein, AtHMGB15, regulates pollen development and pollen germination in Arabidopsis. Previous studies have shown that ARID-HMG DNA binding protein, AtHMGB15 regulate pollen development and pollen germination in Arabidopsis. Here, we performed transcriptome and cytological studies to understand the role of AtHMGB15 in regulating pollen wall morphology and the pollen tube germination rate. Our result showed abnormal vacuolization in the tapetal cells during anther maturation and prolonged PCD in AtHMGB15 loss-of-function mutant. The tapetum has the ability to perform both secretory and biosynthetic activities critical for pollen maturation and pollen viability. Interestingly, expression of PCD executer genes CEP1, MC9 and RNS3 were significant down-regulation of in athmgb15-4. The growth of pollen tubes is regulated by the actin cytoskeleton dynamics. To address the defect in pollen tube growth of athmgb15, we monitored the actin network in growing pollen tubes of wildtype and athmgb15-4 using Rhodamine-phalloidin fluorescence. Our results indicate a highly fragmented actin distribution in athmgb15-4 pollen tubes with a lesser number of long actin fibers and significantly low f-actin concentration at the apex. q-RTPCR further indicates significant downy-regulation of actin regulatory proteins VLN2 and PRF4. Collectively, our results suggest that AtHMGB15 being a nuclear architectural protein orchestrates high-order chromatin organization to promote the transcription of genes responsible for pollen development and pollen germination.

Functional Characterization of the Soybean Glycine max Actin Depolymerization Factor GmADF13 for Plant Resistance to Drought Stress

Abiotic stress significantly affects plant growth and has devastating effects on crop production. Drought stress is one of the main abiotic stressors. Actin is a major component of the cytoskeleton, and actin-depolymerizing factors (ADFs) are conserved actin-binding proteins in eukaryotes that play critical roles in plant responses to various stresses. In this study, we found that GmADF13, an ADF gene from the soybean Glycine max, showed drastic upregulation under drought stress. Subcellular localization experiments in tobacco epidermal cells and tobacco protoplasts showed that GmADF13 was localized in the nucleus and cytoplasm. We characterized its biological function in transgenic Arabidopsis and hairy root composite soybean plants. Arabidopsis plants transformed with GmADF13 displayed a more robust drought tolerance than wild-type plants, including having a higher seed germination rate, longer roots, and healthy leaves under drought conditions. Similarly, GmADF13-overexpressing (OE) soybean plants generated via the Agrobacterium rhizogenes-mediated transformation of the hairy roots showed an improved drought tolerance. Leaves from OE plants showed higher relative water, chlorophyll, and proline contents, had a higher antioxidant enzyme activity, and had decreased malondialdehyde, hydrogen peroxide, and superoxide anion levels compared to those of control plants. Furthermore, under drought stress, GmADF13 OE activated the transcription of several drought-stress-related genes, such as GmbZIP1, GmDREB1A, GmDREB2, GmWRKY13, and GmANK114. Thus, GmADF13 is a positive regulator of the drought stress response, and it may play an essential role in plant growth under drought stress conditions. These results provide new insights into the functional elucidation of soybean ADFs. They may be helpful for breeding new soybean cultivars with a strong drought tolerance and further understanding how ADFs help plants adapt to abiotic stress.

Mechanisms of actin disassembly and turnover

Cellular actin networks exhibit a wide range of sizes, shapes, and architectures tailored to their biological roles. Once assembled, these filamentous networks are either maintained in a state of polarized turnover or induced to undergo net disassembly. Further, the rates at which the networks are turned over and/or dismantled can vary greatly, from seconds to minutes to hours or even days. Here, we review the molecular machinery and mechanisms employed in cells to drive the disassembly and turnover of actin networks. In particular, we highlight recent discoveries showing that specific combinations of conserved actin disassembly-promoting proteins (cofilin, GMF, twinfilin, Srv2/CAP, coronin, AIP1, capping protein, and profilin) work in concert to debranch, sever, cap, and depolymerize actin filaments, and to recharge actin monomers for new rounds of assembly.

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.

Arabidopsis thaliana Subclass I ACTIN DEPOLYMERIZING FACTORs Regulate Nuclear Organization and Gene Expression

ACTIN DEPOLYMERIZING FACTOR (ADF) is a conserved protein that regulates the organization and dynamics of actin microfilaments. Eleven ADFs in the Arabidopsis thaliana genome are grouped into four subclasses, and subclass I ADFs, ADF1-4, are all expressed throughout the plant. Previously, we showed that subclass I ADFs function in the regulation of the response against powdery mildew fungus as well as in the regulation of cell size and endoreplication. Here, we report a new role of subclass I ADFs in the regulation of nuclear organization and gene expression. Through microscopic observation of epidermal cells in mature leaves, we found that the size of chromocenters in both adf4 and transgenic lines where expression of subclass I ADFs is downregulated (ADF1-4Ri) was reduced compared with that of wild-type Col-0. Arabidopsis thaliana possesses eight ACTIN (ACT) genes, among which ACT2, -7 and -8 are expressed in vegetative organs. The chromocenter size in act7, but not in the act2/8 double mutant, was enlarged compared with that in Col-0. Microarray analysis revealed that 1,818 genes were differentially expressed in adf4 and ADF1-4Ri. In particular, expression of 22 nucleotide-binding leucine-rich repeat genes, which are involved in effector-triggered plant immunity, was reduced in adf4 and ADF1-4Ri. qRT-PCR confirmed the altered expressions shown with microarray analysis. Overall, these results suggest that ADF regulates various aspects of plant physiology through its role in regulation of nuclear organization and gene expression. The mechanism how ADF and ACT regulate nuclear organization and gene expression is discussed.

Effects of environmental stress factors on the actin cytoskeleton of fungi and plants: Ionizing radiation and ROS

Actin is an abundant and multifaceted protein in eukaryotic cells that has been detected in the cytoplasm as well as in the nucleus. In cooperation with numerous interacting accessory-proteins, monomeric actin (G-actin) polymerizes into microfilaments (F-actin) which constitute ubiquitous subcellular higher order structures. Considering the extensive spatial dimensions and multifunctionality of actin superarrays, the present study analyses the issue if and to what extent environmental stress factors, specifically ionizing radiation (IR) and reactive oxygen species (ROS), affect the cellular actin-entity. In that context, this review particularly surveys IR-response of fungi and plants. It examines in detail which actin-related cellular constituents and molecular pathways are influenced by IR and related ROS. This comprehensive survey concludes that the general integrity of the total cellular actin cytoskeleton is a requirement for IR-tolerance. Actin's functions in genome organization and nuclear events like chromatin remodeling, DNA-repair, and transcription play a key role. Beyond that, it is highly significant that the macromolecular cytoplasmic and cortical actin-frameworks are affected by IR as well. In response to IR, actin-filament bundling proteins (fimbrins) are required to stabilize cables or patches. In addition, the actin-associated factors mediating cellular polarity are essential for IR-survivability. Moreover, it is concluded that a cellular homeostasis system comprising ROS, ROS-scavengers, NADPH-oxidases, and the actin cytoskeleton plays an essential role here. Consequently, besides the actin-fraction which controls crucial genome-integrity, also the portion which facilitates orderly cellular transport and polarized growth has to be maintained in order to survive IR.

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.

VILLIN2 regulates cotton defense against Verticillium dahliae by modulating actin cytoskeleton remodeling

The active structural change of actin cytoskeleton is a general host response upon pathogen attack. This study characterized the function of the cotton (Gossypium hirsutum) actin-binding protein VILLIN2 (GhVLN2) in host defense against the soilborne fungus Verticillium dahliae. Biochemical analysis demonstrated that GhVLN2 possessed actin-binding, -bundling, and -severing activities. A low concentration of GhVLN2 could shift its activity from actin bundling to actin severing in the presence of Ca2+. Knockdown of GhVLN2 expression by virus-induced gene silencing reduced the extent of actin filament bundling and interfered with the growth of cotton plants, resulting in the formation of twisted organs and brittle stems with a decreased cellulose content of the cell wall. Upon V. dahliae infection, the expression of GhVLN2 was downregulated in root cells, and silencing of GhVLN2 enhanced the disease tolerance of cotton plants. The actin bundles were less abundant in root cells of GhVLN2-silenced plants than in control plants. However, upon infection by V. dahliae, the number of actin filaments and bundles in the cells of GhVLN2-silenced plants was raised to a comparable level as those in control plants, with the dynamic remodeling of the actin cytoskeleton appearing several hours in advance. GhVLN2-silenced plants exhibited a higher incidence of actin filament cleavage in the presence of Ca2+, suggesting that pathogen-responsive downregulation of GhVLN2 could activate its actin-severing activity. These data indicate that the regulated expression and functional shift of GhVLN2 contribute to modulating the dynamic remodeling of the actin cytoskeleton in host immune responses against V. dahliae.

Actin Depolymerization Factor ADF1 Regulated by MYB30 Plays an Important Role in Plant Thermal Adaptation

Actin filaments are essential for plant adaptation to high temperatures. However, the molecular mechanisms of actin filaments in plant thermal adaptation remain unclear. Here, we found that the expression of Arabidopsis actin depolymerization factor 1 (AtADF1) was repressed by high temperatures. Compared with wild-type seedlings (WT), the mutation of AtADF1 and the overexpression of AtADF1 led to promoted and inhibited plant growth under high temperature conditions, respectively. Further, high temperatures induced the stability of actin filaments in plants. Compared with WT, Atadf1-1 mutant seedlings showed more stability of actin filaments under normal and high temperature conditions, while the AtADF1 overexpression seedlings showed the opposite results. Additionally, AtMYB30 directly bound to the promoter of AtADF1 at a known AtMYB30 binding site, AACAAAC, and promoted the transcription of AtADF1 under high temperature treatments. Genetic analysis further indicated that AtMYB30 regulated AtADF1 under high temperature treatments. Chinese cabbage ADF1 (BrADF1) was highly homologous with AtADF1. The expression of BrADF1 was inhibited by high temperatures. BrADF1 overexpression inhibited plant growth and reduced the percentage of actin cable and the average length of actin filaments in Arabidopsis, which were similar to those of AtADF1 overexpression seedlings. AtADF1 and BrADF1 also affected the expression of some key heat response genes. In conclusion, our results indicate that ADF1 plays an important role in plant thermal adaptation by blocking the high-temperature-induced stability of actin filaments and is directly regulated by MYB30.

Seasonal Transcriptome Profiling of Susceptible and Tolerant Citrus Cultivars to Citrus Huanglongbing

Citrus huanglongbing (HLB) caused by 'Candidatus Liberibacter asiaticus' (CLas) is the most devastating citrus disease worldwide. Most commercial citrus cultivars are susceptible to HLB, with a few more tolerant exceptions such as 'LB8-9' Sugar Belle mandarin. Transcriptomic analyses have been widely used to investigate the potential mechanisms for disease susceptibility, resistance, or tolerance. Previous transcriptomic studies related to HLB mostly focused on single time point data collection. We hypothesize that changes in day length and temperature throughout the seasons have profound effects on citrus-CLas interactions. Here, we conducted RNA-seq analyses on HLB-susceptible Valencia sweet orange and HLB-tolerant mandarin 'LB8-9' in winter, spring, summer, and fall. Significant variations in differentially expressed genes (DEGs) related to HLB were observed among the four seasons. For both cultivars, the highest number of DEGs were found in the spring. CLas infection stimulates the expression of immune-related genes such as NBS-LRR, RLK, RLCK, CDPK, MAPK pathway, reactive oxygen species (ROS), and PR genes in both cultivars, consistent with the model that HLB is a pathogen-triggered immune disease. HLB-positive mandarin 'LB8-9' trees contained higher concentrations of maltose and sucrose, which are known to scavenge ROS. In addition, mandarin 'LB8-9' showed higher expression of genes involved in phloem regeneration, which might contribute to its HLB tolerance. This study shed light on the pathogenicity mechanism of the HLB pathosystem and the tolerance mechanism against HLB, providing valuable insights into HLB management.

Maize cytolines as models to study the impact of different cytoplasms on gene expression under heat stress conditions

BACKGROUND

Crops are under constant pressure due to global warming, which unfolds at a much faster pace than their ability to adapt through evolution. Agronomic traits are linked to cytoplasmic-nuclear genome interactions. It thus becomes important to understand the influence exerted by the organelles on gene expression under heat stress conditions and profit from the available genetic diversity. Maize (Zea mays) cytolines allow us to investigate how the gene expression changes under heat stress conditions in three different cytoplasmic environments, but each having the same nucleus. Analyzing retrograde signaling in such an experimental set-up has never been done before. Here, we quantified the response of three cytolines to heat stress as differentially expressed genes (DEGs), and studied gene expression patterns in the context of existing polymorphism in their organellar genomes.

RESULTS

Our study unveils a plethora of new genes and GO terms that are differentially expressed or enriched, respectively, in response to heat stress. We report 19,600 DEGs as responding to heat stress (out of 30,331 analyzed), which significantly enrich 164 GO biological processes, 30 GO molecular functions, and 83 GO cell components. Our approach allowed for the discovery of a significant number of DEGs and GO terms that are not common in the three cytolines and could therefore be linked to retrograde signaling. Filtering for DEGs with a fold regulation > 2 (absolute values) that are exclusive to just one of the cytolines, we find a total of 391 up- and down-DEGs. Similarly, there are 19 GO terms with a fold enrichment > 2 that are cytoline-specific. Using GBS data we report contrasting differences in the number of DEGs and GO terms in each cytoline, which correlate with the genetic distances between the mitochondrial genomes (but not chloroplast) and the original nuclei of the cytolines, respectively.

CONCLUSIONS

The experimental design used here adds a new facet to the paradigm used to explain how gene expression changes in response to heat stress, capturing the influence exerted by different organelles upon one nucleus rather than investigating the response of several nuclei in their innate cytoplasmic environments.

The Cytoskeleton in Plant Immunity: Dynamics, Regulation, and Function

The plant cytoskeleton, consisting of actin filaments and microtubules, is a highly dynamic filamentous framework involved in plant growth, development, and stress responses. Recently, research has demonstrated that the plant cytoskeleton undergoes rapid remodeling upon sensing pathogen attacks, coordinating the formation of microdomain immune complexes, the dynamic and turnover of pattern-recognizing receptors (PRRs), the movement and aggregation of organelles, and the transportation of defense compounds, thus serving as an important platform for responding to pathogen infections. Meanwhile, pathogens produce effectors targeting the cytoskeleton to achieve pathogenicity. Recent findings have uncovered several cytoskeleton-associated proteins mediating cytoskeletal remodeling and defense signaling. Furthermore, the reorganization of the actin cytoskeleton is revealed to further feedback-regulate reactive oxygen species (ROS) production and trigger salicylic acid (SA) signaling, suggesting an extremely complex role of the cytoskeleton in plant immunity. Here, we describe recent advances in understanding the host cytoskeleton dynamics upon sensing pathogens and summarize the effectors that target the cytoskeleton. We highlight advances in the regulation of cytoskeletal remodeling associated with the defense response and assess the important function of the rearrangement of the cytoskeleton in the immune response. Finally, we propose suggestions for future research in this area.

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.

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.

Arabidopsis thaliana subclass I ACTIN DEPOLYMERIZING FACTORs and vegetative ACTIN2/8 are novel regulators of endoreplication

Endoreplication is a type of cell cycle where genome replication occurs without mitosis. An increase of ploidy level by endoreplication is often associated with cell enlargement and an enhanced plant growth. Here we report Arabidopsis thaliana subclass I ACTIN DEPOLYMERIZING FACTORs (ADFs) and vegetative ACTIN2/8 as novel regulators of endoreplication. A. thaliana has 11 ADF members that are divided into 4 subclasses. Subclass I consists of four members, ADF1, -2, -3, and -4, all of which constitutively express in various tissues. We found that both adf4 knockout mutant and transgenic plants in which expressions of all of four subclass I ADFs are suppressed (ADF1-4Ri) showed an increased leaf area of mature first leaves, which was associated with a significant increase of epidermal pavement cell area. Ploidy analysis revealed that the ploidy level was significantly increased in mature leaves of ADF1-4Ri. The increased ploidy was also observed in roots of adf4 and ADF1-4Ri, as well as in dark-grown hypocotyls of adf4. Furthermore, double mutants of vegetative ACT2 and ACT8 (act2/8) exhibited an increase of leaf area and ploidy level in mature leaves. Therefore, actin-relating pathway could regulate endoreplication. The possible mechanisms that actin and ADFs regulate endoreplication are discussed.

Abiotic Stress-Induced Actin-Depolymerizing Factor 3 From Deschampsia antarctica Enhanced Cold Tolerance When Constitutively Expressed in Rice

The Antarctic flowering plant Deschampsia antarctica is highly sensitive to climate change and has shown rapid population increases during regional warming of the Antarctic Peninsula. Several studies have examined the physiological and biochemical changes related to environmental stress tolerance that allow D. antarctica to colonize harsh Antarctic environments; however, the molecular mechanisms of its responses to environmental changes remain poorly understood. To elucidate the survival strategies of D. antarctica in Antarctic environments, we investigated the functions of actin depolymerizing factor (ADF) in this species. We identified eight ADF genes in the transcriptome that were clustered into five subgroups by phylogenetic analysis. DaADF3, which belongs to a monocot-specific clade together with cold-responsive ADF in wheat, showed significant transcriptional induction in response to dehydration and cold, as well as under Antarctic field conditions. Multiple drought and low-temperature responsive elements were identified as possible binding sites of C-repeat-binding factors in the promoter region of DaADF3, indicating a close relationship between DaADF3 transcription control and abiotic stress responses. To investigate the functions of DaADF3 related to abiotic stresses in vivo, we generated transgenic rice plants overexpressing DaADF3. These transgenic plants showed greater tolerance to low-temperature stress than the wild-type in terms of survival rate, leaf chlorophyll content, and electrolyte leakage, accompanied by changes in actin filament organization in the root tips. Together, our results imply that DaADF3 played an important role in the enhancement of cold tolerance in transgenic rice plants and in the adaptation of D. antarctica to its extreme environment.

Hyperoside promotes pollen tube growth by regulating the depolymerization effect of actin-depolymerizing factor 1 on microfilaments in okra

Mature pollen germinates rapidly on the stigma, extending its pollen tube to deliver sperm cells to the ovule for fertilization. The success of this process is an important factor that limits output. The flavonoid content increased significantly during pollen germination and pollen tube growth, which suggests it may play an important role in these processes. However, the specific mechanism of this involvement has been little researched. Our previous research found that hyperoside can prolong the flowering period of Abelmoschus esculentus (okra), but its specific mechanism is still unclear. Therefore, in this study, we focused on the effect of hyperoside in regulating the actin-depolymerizing factor (ADF), which further affects the germination and growth of pollen. We found that hyperoside can prolong the effective pollination period of okra by 2-3-fold and promote the growth of pollen tubes in the style. Then, we used Nicotiana benthamiana cells as a research system and found that hyperoside accelerates the depolymerization of intercellular microfilaments. Hyperoside can promote pollen germination and pollen tube elongation in vitro. Moreover, AeADF1 was identified out of all AeADF genes as being highly expressed in pollen tubes in response to hyperoside. In addition, hyperoside promoted AeADF1-mediated microfilament dissipation according to microfilament severing experiments in vitro. In the pollen tube, the gene expression of AeADF1 was reduced to 1/5 by oligonucleotide transfection. The decrease in the expression level of AeADF1 partially reduced the promoting effect of hyperoside on pollen germination and pollen tube growth. This research provides new research directions for flavonoids in reproductive development.

Zucchini Plants Alter Gene Expression and Emission of (E)-β-Caryophyllene Following Aphis gossypii Infestation

Zucchini (Cucurbita pepo L.) is widely cultivated in temperate regions. One of the major production challenges is the damage caused by Aphis gossypii (Homoptera: Aphididae), a polyphagous aphid, which can negatively affect its host plant, both directly by feeding and indirectly by vectoring viruses. To gain insights into the transcriptome events that occur during the zucchini-aphid interaction and to understand the early-to-late defense response through gene expression profiles, we performed RNA-sequencing (RNA-Seq) on zucchini leaves challenged by A. gossypii (24, 48, and 96 h post-infestation; hpi). Data analysis indicated a complex and dynamic pattern of gene expression and a transient transcriptional reconfiguration that involved more than 700 differentially expressed genes (DEGs), including a large number of defense-related genes. The down-regulation of key genes of plant immunity, such as leucine-rich repeat (LRR) protein kinases, transcription factors, and genes associated with direct (i.e., protease inhibitors, cysteine peptidases, etc.) and indirect (i.e., terpene synthase) defense responses, suggests the aphid ability to manipulate plant immune responses. We also investigated the emission of volatile organic compounds (VOCs) from infested plants and observed a reduced emission of (E)-β-caryophyllene at 48 hpi, likely the result of aphid effectors, which reflects the down-regulation of two genes involved in the biosynthesis of terpenoids. We showed that (E)-β-caryophyllene emission was modified by the duration of plant infestation and by aphid density and that this molecule highly attracts Aphidius colemani, a parasitic wasp of A. gossypii. With our results we contributed to the identification of genes involved in cucurbit plant interactions with phloem feeders. Our findings may also help pave the way toward developing tolerant zucchini varieties and to identify molecules for sustainable management of harmful insect populations.

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.

Arabidopsis REM16 acts as a B3 domain transcription factor to promote flowering time via directly binding to the promoters of SOC1 and FT

Actin depolymerizing factor (ADF) is a key modulator for dynamic organization of actin cytoskeleton. Interestingly, it was found that the ADF1 gene silencing delays flowering, but its mechanism remains unclear. In this study, ADF1 was used as a bait to screen its interacting proteins by the yeast two-hybrid (Y2H) system. One of them, the REM16 transcription factor was identified. As one of the AP2/B3-like transcriptional factor family members, the REM16 contains two B3 domains and its transcript levels kept increasing during the floral transition stage. Overexpression of REM16 accelerates flowering while silencing of REM16 delays flowering. Gene expression analysis indicated that the key flowering activation genes such as CONSTANS (CO), FLOWERING LOCUS T (FT), LEAFY (LFY) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS (SOC1) were upregulated in the REM16 overexpression lines, while the transcription of the flowering suppression gene FLOWERING LOCUS C (FLC) was decreased. In contrast, the REM16 gene silencing lines contained lower transcript levels of the CO, FT, LFY and SOC1 but higher transcript levels of the FLC compared with the wild-type plants. It was proved that REM16 could directly bind to the promoter regions of SOC1 and FT by in vitro and in vivo assays. Genetic analysis supported that REM16 acts upstream of SOC1 and FT in flowering pathways. All these studies provided strong evidence demonstrating that REM16 promotes flowering by directly activating SOC1 and FT.

Genome-Wide Identification and Characterization of Actin-Depolymerizing Factor (ADF) Family Genes and Expression Analysis of Responses to Various Stresses in Zea Mays L

Actin-depolymerizing factor (ADF) is a small class of actin-binding proteins that regulates the dynamics of actin in cells. Moreover, it is well known that the plant ADF family plays key roles in growth, development and defense-related functions. Results: Thirteen maize (Zea mays L., ZmADFs) ADF genes were identified using Hidden Markov Model. Phylogenetic analysis indicated that the 36 identified ADF genes in Physcomitrella patens, Arabidopsis thaliana, Oryza sativa japonica, and Zea mays were clustered into five groups. Four pairs of segmental genes were found in the maize ADF gene family. The tissue-specific expression of ZmADFs and OsADFs was analyzed using microarray data obtained from the Maize and Rice eFP Browsers. Five ZmADFs (ZmADF1/2/7/12/13) from group V exhibited specifically high expression in tassel, pollen, and anther. The expression patterns of 13 ZmADFs in seedlings under five abiotic stresses were analyzed using qRT-PCR, and we found that the ADFs mainly responded to heat, salt, drought, and ABA. Conclusions: In our study, we identified ADF genes in maize and analyzed the gene structure and phylogenetic relationships. The results of expression analysis demonstrated that the expression level of ADF genes was diverse in various tissues and different stimuli, including abiotic and phytohormone stresses, indicating their different roles in plant growth, development, and response to external stimulus. This report extends our knowledge to understand the function of ADF genes in maize.

Understanding Cytoskeletal Dynamics During the Plant Immune Response

The plant cytoskeleton is a dynamic framework of cytoplasmic filaments that rearranges as the needs of the cell change during growth and development. Incessant turnover mechanisms allow these networks to be rapidly redeployed in defense of host cytoplasm against microbial invaders. Both chemical and mechanical stimuli are recognized as danger signals to the plant, and these are perceived and transduced into cytoskeletal dynamics and architecture changes through a collection of well-recognized, previously characterized players. Recent advances in quantitative cell biology approaches, along with the powerful molecular genetics techniques associated with Arabidopsis, have uncovered two actin-binding proteins as key intermediaries in the immune response to phytopathogens and defense signaling. Certain bacterial phytopathogens have adapted to the cytoskeletal-based defense mechanism during the basal immune response and have evolved effector proteins that target actin filaments and microtubules to subvert transcriptional reprogramming, secretion of defense-related proteins, and cell wall-based defenses. In this review, we describe current knowledge about host cytoskeletal dynamics operating at the crossroads of the molecular and cellular arms race between microbes and plants.

A Ca2+-dependent remodelled actin network directs vesicle trafficking to build wall ingrowth papillae in transfer cells

The transport function of transfer cells is conferred by an enlarged plasma membrane area, enriched in nutrient transporters, that is supported on a scaffold of wall ingrowth (WI) papillae. Polarized plumes of elevated cytosolic Ca2+ define loci at which WI papillae form in developing adaxial epidermal transfer cells of Vicia faba cotyledons that are induced to trans-differentiate when the cotyledons are placed on culture medium. We evaluated the hypothesis that vesicle trafficking along a Ca2+-regulated remodelled actin network is the mechanism that underpins this outcome. Polarized to the outer periclinal cytoplasm, a Ca2+-dependent remodelling of long actin bundles into short, thin bundles was found to be essential for assembling WI papillae but not the underlying uniform wall layer. The remodelled actin network directed polarized vesicle trafficking to sites of WI papillae construction, and a pharmacological study indicated that both exo- and endocytosis contributed to assembly of the papillae. Potential candidates responsible for the Ca2+-dependent actin remodelling, along with those underpinning polarized exo- and endocyotosis, were identified in a transcriptome RNAseq database generated from the trans-differentiating epidermal cells. Of most significance, endocytosis was controlled by up-regulated expression of a dynamin-like isoform. How a cycle of localized exo- and endocytosis, regulated by Ca2+-dependent actin remodelling, assembles WI papillae is discussed.

Textile Hemp vs. Salinity: Insights from a Targeted Gene Expression Analysis

Soil salinity is a serious threat to agriculture, because it compromises biomass production and plant productivity, by negatively affecting the vegetative growth and development of plants. Fiber crops like textile hemp (Cannabis sativa L.) are important natural resources that provide, sustainably, both cellulosic and woody fibers for industry. In this work, the response to salinity (200 mM NaCl) of a fiber variety of hemp (Santhica 27) was studied using quantitative real-time PCR. The responses of plantlets aged 15 days were analyzed by microscopy and by measuring the changes in expression of cell wall-related genes, as well as in the general response to exogenous constraints. The results presented here show that a different response is present in the hemp hypocotyls and leaves. In the leaves, genes coding for heat shock proteins were significantly upregulated, together with a phytohormone-related transcript (ethylene-responsive factor 1 ERF1) and genes involved in secondary cell wall biosynthesis (cellulose synthase CesA4, fasciclin-like arabinogalactan proteins FLA10 and FLA8). Moreover, a tendency towards upregulation was also observed in the leaves for genes involved in lignification (4CL, CAD, PAL); a finding that suggests growth arrest. In the hypocotyl, the genes involved in lignification did not show changes in expression, while a gene related to expansion (expansin EXPA8), as well as transcripts coding for calcium-dependent lipid-binding family proteins (CALB), were upregulated. Microscopic analyses on the hypocotyl cross sections revealed changes in the vascular tissues of salt-exposed plantlets, where the lumen of xylem vessels was reduced. The gene expression results show that a different response is present in the hemp hypocotyls and leaves. The data presented contribute to our understanding of the regulatory gene network in response to salinity in different tissues of an important fiber crop.