Arabidopsis Histone Variant H2A.X Functions in the DNA Damage-Coupling Abscisic Acid Signaling Pathway.

Environmental variations initiate chromatin modifications, leading to the exchange of histone subunits or the repositioning of nucleosomes. The phosphorylated histone variant H2A.X (γH2A.X) is recognized for the formation of foci that serve as established markers of DNA double-strand breaks (DSBs). Nevertheless, the precise roles of H2A.X in the cellular response to genotoxic stress and the impact of the plant hormone abscisic acid (ABA) remain incompletely understood. In this investigation, we implemented CRISPR/Cas9 technology to produce loss-of-function mutants of AtHTA3 and AtHTA5 in Arabidopsis. The phenotypes of the athta3 and athta5 single mutants were nearly identical to those of the wild-type Col-0. Nevertheless, the athta3 athta5 double mutants exhibited aberrant embryonic development, increased sensitivity to DNA damage, and higher sensitivity to ABA. The RT-qPCR analysis indicates that AtHTA3 and AtHTA5 negatively regulate the expression of AtABI3, a fundamental regulator in the ABA signaling pathway. Subsequent investigation demonstrated that AtABI3 participates in the genotoxic stress response by influencing the expression of DNA damage response genes, such as AtBRCA1, AtRAD51, and AtWEE1. Our research offers new insights into the role of H2A.X in the genotoxic and ABA responses of Arabidopsis.

Jasmonic acid-inducible TSA1 facilitates ER body formation.

Members of the Brassicales contain an organelle, the endoplasmic reticulum (ER) body, which is derived from the ER. Recent studies have shed light on the biogenesis of the ER body and its physiological role in plants. However, formation of the ER body and its physiological role are not fully understood. Here, we investigated the physiological role of TSK-associating protein 1 (TSA1), a close homolog of NAI2 that is involved in ER body formation, and provide evidence that it is involved in ER body biogenesis under wound-related stress conditions. TSA1 is N-glycosylated and localizes to the ER body as a luminal protein. TSA1 was highly induced by the plant hormone, methyl jasmonate (MeJA). Ectopic expression of TSA1:GFP induced ER body formation in root tissues of transgenic Arabidopsis thaliana and in leaf tissues of Nicotiana benthamiana. TSA1 and NAI2 formed a heterocomplex and showed an additive effect on ER body formation in N. benthamiana. MeJA treatment induced ER body formation in leaf tissues of nai2 and tsa1 plants, but not nai2/tsa1 double-mutant plants. However, constitutive ER body formation was altered in young seedlings of nai2 plants but not tsa1 plants. Based on these results, we propose that TSA1 plays a critical role in MeJA-induced ER body formation in plants.

SH3 Domain-Containing Protein 2 Plays a Crucial Role at the Step of Membrane Tubulation during Cell Plate Formation.

During cytokinesis in plants, trans-Golgi network-derived vesicles accumulate at the center of dividing cells and undergo various structural changes to give rise to the planar cell plate. However, how this conversion occurs at the molecular level remains elusive. In this study, we report that SH3 Domain-Containing Protein 2 (SH3P2) in Arabidopsis thaliana plays a crucial role in converting vesicles to the planar cell plate. SH3P2 RNAi plants showed cytokinesis-defective phenotypes and produced aggregations of vesicles at the leading edge of the cell plate. SH3P2 localized to the leading edge of the cell plate, particularly the constricted or curved regions of the cell plate. The BAR domain of SH3P2 induced tubulation of vesicles. SH3P2 formed a complex with dynamin-related protein 1A (DRP1A) and affected DRP1A accumulation to the cell plate. Based on these results, we propose that SH3P2 functions together with DRP1A to convert the fused vesicles to tubular structures during cytokinesis.

A Plant-Derived Antigen-Antibody Complex Induces Anti-Cancer Immune Responses by Forming a Large Quaternary Structure.

The antigen-antibody complex (AAC) has novel functions for immunomodulation, encouraging the application of diverse quaternary protein structures for vaccination. In this study, GA733 antigen and anti-GA733 antibody proteins were both co-expressed to obtain the AAC protein structures in a F1 plant obtained by crossing the plants expressing each protein. In F1 plant, the antigen and antibody assembled to form a large quaternary circular ACC structure (~30 nm). The large quaternary protein structures induced immune response to produce anticancer immunoglobulins G (IgGs) that are specific to the corresponding antigens in mouse. The serum containing the anticancer IgGs inhibited the human colorectal cancer cell growth in the xenograft nude mouse. Taken together, antigens and antibodies can be assembled to form AAC protein structures in plants. Plant crossing represents an alternative strategy for the formation of AAC vaccines that efficiently increases anticancer antibody production.

Biogenesis of chloroplast outer envelope membrane proteins.

Most organisms on Earth use glucose, a photosynthetic product, as energy source. The chloroplast, the home of photosynthesis, is the most representative and characteristic organelle in plants and is enclosed by the outer envelope and inner envelope membranes. The chloroplast biogenesis and unique functions are very closely associated with proteins in the two envelope membranes of the chloroplast. Especially, the chloroplast outer envelope membrane proteins have important roles in signal transduction, protein import, lipid biosynthesis and remodeling, exchange of ions and numerous metabolites, plastid division, movement, and host defense. Therefore, biogenesis of these membrane proteins of chloroplast outer envelope membrane is very important for biogenesis of the entire chloroplast proteome as well as plant development. Most proteins among the outer envelope membrane proteins are encoded by the nuclear genome and are post-translationally targeted to the chloroplast outer envelope membrane. In this process, cytoplasmic receptor and import machineries are required for efficient and correct targeting of these membrane proteins. In this review, we have summarized recent advances on the sorting, targeting, and insertion mechanisms of the outer envelope membrane proteins of chloroplasts and also provide future direction of the study on these topics.

An Arabidopsis SUMO E3 Ligase, SIZ1, Negatively Regulates Photomorphogenesis by Promoting COP1 Activity.

COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1), a ubiquitin E3 ligase, is a central negative regulator of photomorphogenesis. However, how COP1 activity is regulated by post-translational modifications remains largely unknown. Here we show that SUMO (small ubiquitin-like modifier) modification enhances COP1 activity. Loss-of-function siz1 mutant seedlings exhibit a weak constitutive photomorphogenic phenotype. SIZ1 physically interacts with COP1 and mediates the sumoylation of COP1. A K193R substitution in COP1 blocks its SUMO modification and reduces COP1 activity in vitro and in planta. Consistently, COP1 activity is reduced in siz1 and the level of HY5, a COP1 target protein, is increased in siz1. Sumoylated COP1 may exhibits higher transubiquitination activity than does non-sumoylated COP1, but SIZ1-mediated SUMO modification does not affect COP1 dimerization, COP1-HY5 interaction, and nuclear accumulation of COP1. Interestingly, prolonged light exposure reduces the sumoylation level of COP1, and COP1 mediates the ubiquitination and degradation of SIZ1. These regulatory mechanisms may maintain the homeostasis of COP1 activity, ensuing proper photomorphogenic development in changing light environment. Our genetic and biochemical studies identify a function for SIZ1 in photomorphogenesis and reveal a novel SUMO-regulated ubiquitin ligase, COP1, in plants.

The Prenylated Rab GTPase Receptor PRA1.F4 Contributes to Protein Exit from the Golgi Apparatus.

Prenylated Rab acceptor1 (PRA1) functions in the recruitment of prenylated Rab proteins to their cognate organelles. Arabidopsis (Arabidopsis thaliana) contains a large number of proteins belonging to the AtPRA1 family. However, their physiological roles remain largely unknown. Here, we investigated the physiological role of AtPRA1.F4, a member of the AtPRA1 family. A T-DNA insertion knockdown mutant of AtPRA1.F4, atpra1.f4, was smaller in stature than parent plants and possessed shorter roots, whereas transgenic plants overexpressing HA:AtPRA1.F4 showed enhanced development of secondary roots and root hairs. However, both overexpression and knockdown plants exhibited increased sensitivity to high-salt stress, lower vacuolar Na+/K+-ATPase and plasma membrane ATPase activities, lower and higher pH in the vacuole and apoplast, respectively, and highly vesiculated Golgi apparatus. HA:AtPRA1.F4 localized to the Golgi apparatus and assembled into high-molecular-weight complexes. atpra1.f4 plants displayed a defect in vacuolar trafficking, which was complemented by low but not high levels of HA:AtPRA1.F4 Overexpression of HA:AtPRA1.F4 also inhibited protein trafficking at the Golgi apparatus, albeit differentially depending on the final destination or type of protein: trafficking of vacuolar proteins, plasma membrane proteins, and trans-Golgi network (TGN)-localized SYP61 was strongly inhibited; trafficking of TGN-localized SYP51 was slightly inhibited; and trafficking of secretory proteins and TGN-localized SYP41 was negligibly or not significantly inhibited. Based on these results, we propose that Golgi-localized AtPRA1.F4 is involved in the exit of many but not all types of post-Golgi proteins from the Golgi apparatus. Additionally, an appropriate level of AtPRA1.F4 is crucial for its function at the Golgi apparatus.

Miktoarm Amphiphilic Block Copolymer with Singlet Oxygen-Labile Stereospecific ß-Aminoacrylate Junction: Synthesis, Self-Assembly, and Photodynamically Triggered Drug Release.

Incorporation of a desired stimuli-responsive unit in a stereospecific manner at the specific location within a nonlinear block copolymer architecture is a challenging task in synthetic polymer chemistry. Herein, we report a facile and versatile method to synthesize AB2 miktoarm block copolymers bearing a singlet oxygen (1O2)-labile regio and stereospecific ß-aminoacrylate linkage with 100% E-configuration at the junction via a combination of amino-yne click chemistry and ring opening polymerization. Using this strategy, a series of 1O2-responsive AB2 amphiphilic miktoarm (MA) copolymers composed of hydrophilic polyethylene glycol (PEG) as the A constituent and hydrophobic polycaprolactone (PCL) as the B constituent (MA-PEG- b-PCL2) was synthesized by varying the block length of PCL. The self-assembly characteristics of these well-defined MA-PEG- b-PCL2 copolymers in an aqueous condition were studied by solvent displacement and thin-film hydration method, and their morphologies were investigated using transmission electron microscopy. The copolymers formed spherical, cylindrical, or lamella morphologies, depending on the chain length and preparation conditions. A hydrophobic photosensitizer chlorin e6 (Ce6) and anticancer drug doxorubicin (DOX) were efficiently encapsulated into the hydrophobic core of MA-PEG- b-PCL2 copolymer micelles. These coloaded micelles were taken up by human breast cancer (MDA-MB-231) cells. Upon red laser light irradiation, the 1O2-generated by the Ce6 induced photocleavage of the ß-aminoacrylate moiety, leading to the dissociation of the micellar structure and triggered intracellular drug release for effective therapy. Overall, rapid disassembly upon 1O2 generation and subsequent controlled intracellular drug release suggested that these micelles bearing ß-aminoacrylate linkage have a huge potential for on-demand drug delivery.

Adaptor protein complex 2-mediated endocytosis is crucial for male reproductive organ development in Arabidopsis.

Fertilization in flowering plants requires the temporal and spatial coordination of many developmental processes, including pollen production, anther dehiscence, ovule production, and pollen tube elongation. However, it remains elusive as to how this coordination occurs during reproduction. Here, we present evidence that endocytosis, involving heterotetrameric adaptor protein complex 2 (AP-2), plays a crucial role in fertilization. An Arabidopsis thaliana mutant ap2m displays multiple defects in pollen production and viability, as well as elongation of staminal filaments and pollen tubes, all of which are pivotal processes needed for fertilization. Of these abnormalities, the defects in elongation of staminal filaments and pollen tubes were partially rescued by exogenous auxin. Moreover, DR5rev:GFP (for green fluorescent protein) expression was greatly reduced in filaments and anthers in ap2m mutant plants. At the cellular level, ap2m mutants displayed defects in both endocytosis of N-(3-triethylammonium-propyl)-4-(4-diethylaminophenylhexatrienyl) pyridinium dibromide, a lypophilic dye used as an endocytosis marker, and polar localization of auxin-efflux carrier PIN FORMED2 (PIN2) in the stamen filaments. Moreover, these defects were phenocopied by treatment with Tyrphostin A23, an inhibitor of endocytosis. Based on these results, we propose that AP-2-dependent endocytosis plays a crucial role in coordinating the multiple developmental aspects of male reproductive organs by modulating cellular auxin level through the regulation of the amount and polarity of PINs.

The Arabidopsis NAC transcription factor ANAC096 cooperates with bZIP-type transcription factors in dehydration and osmotic stress responses.

Multiple transcription factors (TFs) play essential roles in plants under abiotic stress, but how these multiple TFs cooperate in abiotic stress responses remains largely unknown. In this study, we provide evidence that the NAC (for NAM, ATAF1/2, and CUC2) TF ANAC096 cooperates with the bZIP-type TFs ABRE binding factor and ABRE binding protein (ABF/AREB) to help plants survive under dehydration and osmotic stress conditions. ANAC096 directly interacts with ABF2 and ABF4, but not with ABF3, both in vitro and in vivo. ANAC096 and ABF2 synergistically activate RD29A transcription. Our genome-wide gene expression analysis revealed that a major proportion of abscisic acid (ABA)-responsive genes are under the transcriptional regulation of ANAC096. We found that the Arabidopsis thaliana anac096 mutant is hyposensitive to exogenous ABA and shows impaired ABA-induced stomatal closure and increased water loss under dehydration stress conditions. Furthermore, we found the anac096 abf2 abf4 triple mutant is much more sensitive to dehydration and osmotic stresses than the anac096 single mutant or the abf2 abf4 double mutant. Based on these results, we propose that ANAC096 is involved in a synergistic relationship with a subset of ABFs for the transcriptional activation of ABA-inducible genes in response to dehydration and osmotic stresses.

The immediate upstream region of the 5'-UTR from the AUG start codon has a pronounced effect on the translational efficiency in Arabidopsis thaliana.

The nucleotide sequence around the translational initiation site is an important cis-acting element for post-transcriptional regulation. However, it has not been fully understood how the sequence context at the 5'-untranslated region (5'-UTR) affects the translational efficiency of individual mRNAs. In this study, we provide evidence that the 5'-UTRs of Arabidopsis genes showing a great difference in the nucleotide sequence vary greatly in translational efficiency with more than a 200-fold difference. Of the four types of nucleotides, the A residue was the most favourable nucleotide from positions -1 to -21 of the 5'-UTRs in Arabidopsis genes. In particular, the A residue in the 5'-UTR from positions -1 to -5 was required for a high-level translational efficiency. In contrast, the T residue in the 5'-UTR from positions -1 to -5 was the least favourable nucleotide in translational efficiency. Furthermore, the effect of the sequence context in the -1 to -21 region of the 5'-UTR was conserved in different plant species. Based on these observations, we propose that the sequence context immediately upstream of the AUG initiation codon plays a crucial role in determining the translational efficiency of plant genes.

Direct targeting of proteins from the cytosol to organelles: the ER versus endosymbiotic organelles.

In eukaryotic cells consisting of many different types of organelles, targeting of organellar proteins is one of the most fundamental cellular processes. Proteins belonging to the endoplasmic reticulum (ER), chloroplasts and mitochondria are targeted individually from the cytosol to their cognate organelles. As the targeting to these organelles occurs in the cytosol during or after translation, the most crucial aspect is how specific targeting to these three organelles can be achieved without interfering with other targeting pathways. For these organelles, multiple mechanisms are used for targeting proteins, but the exact mechanism used depends on the type of protein and organelle, the location of targeting signals in the protein and the location of the protein in the organelle. In this review, we discuss the various mechanisms involved in protein targeting to the ER, chloroplasts and mitochondria, and how the targeting specificity is determined for these organelles in plant cells.

Oral immunization of haemaggulutinin H5 expressed in plant endoplasmic reticulum with adjuvant saponin protects mice against highly pathogenic avian influenza A virus infection.

Pandemics in poultry caused by the highly pathogenic avian influenza (HPAI) A virus occur too frequently globally, and there is growing concern about the HPAI A virus due to the possibility of a pandemic among humans. Thus, it is important to develop a vaccine against HPAI suitable for both humans and animals. Various approaches are underway to develop such vaccines. In particular, an edible vaccine would be a convenient way to vaccinate poultry because of the behaviour of the animals. However, an edible vaccine is still not available. In this study, we developed a strategy of effective vaccination of mice by the oral administration of transgenic Arabidopsis plants (HA-TG) expressing haemagglutinin (HA) in the endoplasmic reticulum (ER). Expression of HA in the ER resulted in its high-level accumulation, N-glycosylation, protection from proteolytic degradation and long-term stability. Oral administration of HA-TG with saponin elicited high levels of HA-specific systemic IgG and mucosal IgA responses in mice, which resulted in protection against a lethal influenza virus infection with attenuated inflammatory symptoms. Based on these results, we propose that oral administration of freeze-dried leaf powders from transgenic plants expressing HA in the ER together with saponin is an attractive strategy for vaccination against influenza A virus.

Abscisic acid uridine diphosphate glucosyltransferases play a crucial role in abscisic acid homeostasis in Arabidopsis.

The phytohormone abscisic acid (ABA) is crucial for plant growth and adaptive responses to various stress conditions. Plants continuously adjust the ABA level to meet physiological needs, but how ABA homeostasis occurs is not fully understood. This study provides evidence that UGT71B6, an ABA uridine diphosphate glucosyltransferase (UGT), and its two closely related homologs, UGT71B7 and UGT71B8, play crucial roles in ABA homeostasis and in adaptation to dehydration, osmotic stress, and high-salinity stresses in Arabidopsis (Arabidopsis thaliana). UGT RNA interference plants that had low levels of these three UGT transcripts displayed hypersensitivity to exogenous ABA and high-salt conditions during germination and exhibited a defect in plant growth. However, the ectopic expression of UGT71B6 in the atbg1 (for ß-glucosidase) mutant background aggravated the ABA-deficient phenotype of atbg1 mutant plants. In addition, modulation of the expression of the three UGTs affects the expression of CYP707A1 to CYP707A4, which encode ABA 8'-hydroxylases; four CYP707As were expressed at higher levels in the UGT RNA interference plants but at lower levels in the UGT71B6:GFP-overexpressing plants. Based on these data, this study proposes that UGT71B6 and its two homologs play a critical role in ABA homeostasis by converting active ABA to an inactive form (abscisic acid-glucose ester) depending on intrinsic cellular and environmental conditions in plants.

A vacuolar ß-glucosidase homolog that possesses glucose-conjugated abscisic acid hydrolyzing activity plays an important role in osmotic stress responses in Arabidopsis.

The phytohormone abscisic acid (ABA) plays a critical role in various physiological processes, including adaptation to abiotic stresses. In Arabidopsis thaliana, ABA levels are increased both through de novo biosynthesis and via ß-glucosidase homolog1 (BG1)-mediated hydrolysis of Glc-conjugated ABA (ABA-GE). However, it is not known how many different ß-glucosidase proteins produce ABA from ABA-GE and how the multiple ABA production pathways are coordinated to increase ABA levels. Here, we report that a previously undiscovered ß-glucosidase homolog, BG2, produced ABA by hydrolyzing ABA-GE and plays a role in osmotic stress response. BG2 localized to the vacuole as a high molecular weight complex and accumulated to high levels under dehydration stress. BG2 hydrolyzed ABA-GE to ABA in vitro. In addition, BG2 increased ABA levels in protoplasts upon application of exogenous ABA-GE. Overexpression of BG2 rescued the bg1 mutant phenotype, as observed for the overexpression of NCED3 in bg1 mutants. Multiple Arabidopsis bg2 alleles with a T-DNA insertion in BG2 were more sensitive to dehydration and NaCl stress, whereas BG2 overexpression resulted in enhanced resistance to dehydration and NaCl stress. Based on these observations, we propose that, in addition to the de novo biosynthesis, ABA is produced in multiple organelles by organelle-specific ß-glucosidases in response to abiotic stresses.

Both the hydrophobicity and a positively charged region flanking the C-terminal region of the transmembrane domain of signal-anchored proteins play critical roles in determining their targeting specificity to the endoplasmic reticulum or endosymbiotic organelles in Arabidopsis cells.

Proteins localized to various cellular and subcellular membranes play pivotal roles in numerous cellular activities. Accordingly, in eukaryotic cells, the biogenesis of organellar proteins is an essential process requiring their correct localization among various cellular and subcellular membranes. Localization of these proteins is determined by either cotranslational or posttranslational mechanisms, depending on the final destination. However, it is not fully understood how the targeting specificity of membrane proteins is determined in plant cells. Here, we investigate the mechanism by which signal-anchored (SA) proteins are differentially targeted to the endoplasmic reticulum (ER) or endosymbiotic organelles using in vivo targeting, subcellular fractionation, and bioinformatics approaches. For targeting SA proteins to endosymbiotic organelles, the C-terminal positively charged region (CPR) flanking the transmembrane domain (TMD) is necessary but not sufficient. The hydrophobicity of the TMD in CPR-containing proteins also plays a critical role in determining targeting specificity; TMDs with a hydrophobicity value >0.4 on the Wimley and White scale are targeted primarily to the ER, whereas TMDs with lower values are targeted to endosymbiotic organelles. Based on these data, we propose that the CPR and the hydrophobicity of the TMD play a critical role in determining the targeting specificity between the ER and endosymbiotic organelles.

Bioactivity-Guided Fraction from Viscera of Abalone, Haliotis discus hannai Suppresses Cellular Basophils Activation and Anaphylaxis in Mice.

Basophils and mast cells are specialized effector cells in allergic reactions. Haliotis discus hannai (abalone), is valuable seafood. Abalone male viscera, which has a brownish color and has not been previously reported to show anti-allergic activities, was extracted with acetone. Six different acetone/hexane fractions (0, 10, 20, 30, 40, and 100%) were obtained using a silica column via ß-hexosaminidase release inhibitory activity-guided selection in phorbol myristate acetate and a calcium ionophore, A23187 (PMACI)-induced human basophils, KU812F cells. The 40% acetone/hexane fraction (A40) exhibited the strongest inhibition of PMACI-induced-ß-hexosaminidase release. This fraction dose-dependently inhibited reactive oxygen species (ROS) production and calcium mobilization without cytotoxicity. Western blot analysis revealed that A40 down-regulated PMACI-induced MAPK (ERK 1/2, p-38, and JNK) phosphorylation, and the NF-κB translocation from the cytosol to membrane. Moreover, A40 inhibited PMACI-induced interleukin (IL)-1ß, IL-6, and IL-8 production. Anti-allergic activities of A40 were confirmed based on inhibitory effects on IL-4 and tumor necrosis factor alpha (TNF-α) production in compound (com) 48/80-induced rat basophilic leukemia (RBL)-2H3 cells. A40 inhibited ß-hexosaminidase release and cytokine production such as IL-4 and TNF-α produced by com 48/80-stimulated RBL-2H3 cells. Furthermore, it's fraction attenuated the IgE/DNP-induced passive cutaneous anaphylaxis (PCA) reaction in the ears of BALB/c mice. Our results suggest that abalone contains the active fraction, A40 is a potent therapeutic and functional material to treat allergic diseases.

Anti-obesity and immunomodulatory effects of oil and fermented extract dried from Tenebrio molitor larvae on aged obese mice.

Preventing disease and maintaining the health of the elderly are crucial goals for an aging population, with obesity and immune function restoration being of paramount importance. Obesity, particularly visceral obesity characterized by excessive fat accumulation around the abdominal organs, is linked to chronic conditions such as diabetes, hypertension, cardiovascular diseases, and immune dysfunction. Globally, obesity is considered a disease, prompting significant research interest in its treatment. Therefore, it is essential to explore potential therapeutic and preventive strategies to address obesity and the decline in immune function brought about by aging. Tenebrio molitor larvae (TML), commonly known as 'mealworms,' are rich in unsaturated fatty acids, including oleic and linoleic acids, and essential amino acids, such as isoleucine and tyrosine. In this study, we aimed to investigate the effects of the consumption of TML oil and mealworm fermented extract (MWF-1) on obesity and immunological changes in aged obese mice. Our data showed reduced body fat in 23-week-old C57BL/6 mice fed processed TML products for 6 weeks. Additionally, the characteristically high levels of serum triglycerides decreased by treating with TML oil. The immune responsiveness results confirmed an increase in B cells by treating with MWF-1, while cytokine levels (interferon-gamma, tumor necrosis factor-alpha, interleukin-2, and -6) were restored to levels similar to young mice. These results suggest that TML oil and MWF-1 are promising dietary supplements for addressing obesity and restoring immune function.

An A/ENTH domain-containing protein functions as an adaptor for clathrin-coated vesicles on the growing cell plate in Arabidopsis root cells.

Cytokinesis is the process of partitioning the cytoplasm of a dividing cell, thereby completing mitosis. Cytokinesis in the plant cell is achieved by the formation of a new cell wall between daughter nuclei using components carried in Golgi-derived vesicles that accumulate at the midplane of the phragmoplast and fuse to form the cell plate. Proteins that play major roles in the development of the cell plate in plant cells are not well defined. Here, we report that an AP180 amino-terminal homology/epsin amino-terminal homology domain-containing protein from Arabidopsis (Arabidopsis thaliana) is involved in clathrin-coated vesicle formation from the cell plate. Arabidopsis Epsin-like Clathrin Adaptor1 (AtECA1; At2g01600) and its homologous proteins AtECA2 and AtECA4 localize to the growing cell plate in cells undergoing cytokinesis and also to the plasma membrane and endosomes in nondividing cells. AtECA1 (At2g01600) does not localize to nascent cell plates but localizes at higher levels to expanding cell plates even after the cell plate fuses with the parental plasma membrane. The temporal and spatial localization patterns of AtECA1 overlap most closely with those of the clathrin light chain. In vitro protein interaction assays revealed that AtECA1 binds to the clathrin H chain via its carboxyl-terminal domain. These results suggest that these AP180 amino-terminal homology/epsin amino-terminal homology domain-containing proteins, AtECA1, AtECA2, and AtECA4, may function as adaptors of clathrin-coated vesicles budding from the cell plate.

ABA homeostasis and signaling involving multiple subcellular compartments and multiple receptors.

The plant hormone abscisic acid (ABA) plays pivotal roles in many important physiological processes including stomatal closure, seed dormancy, growth and various environmental stresses. In these responses, ABA action is under the control of complex regulatory mechanisms involving homeostasis, perception and signaling. Recent studies provide new insights into these processes, which are of great importance in understanding the mechanisms underlying the evolutionary principle of how plants can survive as a sessile organism under ever-changing environmental conditions. They also form the basis for designing plants that have an enhanced resistance to various stresses in particular abiotic stress.