S-acylation of CBL10/SCaBP8 by PAT10 is crucial for its tonoplast association and function in salt tolerance.

Crop yield is sensitive to salt stresses, for which Calcineurin B-like proteins (CBLs) are major response factors. This study shows that Arabidopsis CBL10, through protein S-acylation by protein S-acyl transferase10, targets to the vacuolar membrane to confer salt tolerance.

Targeting and signaling of Rho of plants guanosine triphosphatases require synergistic interaction between guanine nucleotide inhibitor and vesicular trafficking.

Most eukaryotic cells are polarized. Common toolbox regulating cell polarization includes Rho guanosine triphosphatases (GTPases), in which spatiotemporal activation is regulated by a plethora of regulators. Rho of plants (ROPs) are the only Rho GTPases in plants. Although vesicular trafficking was hinted in the regulation of ROPs, it was unclear where vesicle-carried ROP starts, whether it is dynamically regulated, and which components participate in vesicle-mediated ROP targeting. In addition, although vesicle trafficking and guanine nucleotide inhibitor (GDI) pathways in Rho signaling have been extensively studied in yeast, it is unknown whether the two pathways interplay. Unclear are also cellular and developmental consequences of their interaction in multicellular organisms. Here, we show that the dynamic targeting of ROP through vesicles requires coat protein complex II and ADP-ribosylation factor 1-mediated post-Golgi trafficking. Trafficking of vesicle-carried ROPs between the plasma membrane and the trans-Golgi network is mediated through adaptor protein 1 and sterol-mediated endocytosis. Finally, we show that GDI and vesicle trafficking synergistically regulate cell polarization and ROP targeting, suggesting that the establishment and maintenance of cell polarity is regulated by an evolutionarily conserved mechanism.

HAPLESS13-Mediated Trafficking of STRUBBELIG Is Critical for Ovule Development in Arabidopsis.

Planar morphogenesis, a distinct feature of multicellular organisms, is crucial for the development of ovule, progenitor of seeds. Both receptor-like kinases (RLKs) such as STRUBBELIG (SUB) and auxin gradient mediated by PIN-FORMED1 (PIN1) play instructive roles in this process. Fine-tuned intercellular communications between different cell layers during ovule development demands dynamic membrane distribution of these cell-surface proteins, presumably through vesicle-mediated sorting. However, the way it's achieved and the trafficking routes involved are obscure. We report that HAPLESS13 (HAP13)-mediated trafficking of SUB is critical for ovule development. HAP13 encodes the µ subunit of adaptor protein 1 (AP1) that mediates protein sorting at the trans-Golgi network/early endosome (TGN/EE). The HAP13 mutant, hap13-1, is defective in outer integument growth, resulting in exposed nucellus accompanied with impaired pollen tube guidance and reception. SUB is mis-targeted in hap13-1. However, unlike that of PIN2, the distribution of PIN1 is independent of HAP13. Genetic interference of exocytic trafficking at the TGN/EE by specifically downregulating HAP13 phenocopied the defects of hap13-1 in SUB targeting and ovule development, supporting a key role of sporophytically expressed SUB in instructing female gametogenesis.

Arabidopsis PROTEIN S-ACYL TRANSFERASE4 mediates root hair growth.

Polar growth of root hairs is critical for plant survival and requires fine-tuned Rho of plants (ROP) signaling. Multiple ROP regulators participate in root hair growth. However, protein S-acyl transferases (PATs), mediating the S-acylation and membrane partitioning of ROPs, are yet to be found. Using a reverse genetic approach, combining fluorescence probes, pharmacological drugs, site-directed mutagenesis and genetic analysis with related root-hair mutants, we have identified and characterized an Arabidopsis PAT, which may be responsible for ROP2 S-acylation in root hairs. Specifically, functional loss of PAT4 resulted in reduced root hair elongation, which was rescued by a wild-type but not an enzyme-inactive PAT4. Membrane-associated ROP2 was significantly reduced in pat4, similar to S-acylation-deficient ROP2 in the wild type. We further showed that PAT4 and SCN1, a ROP regulator, additively mediate the stability and targeting of ROP2. The results presented here indicate that PAT4-mediated S-acylation mediates the membrane association of ROP2 at the root hair apex and provide novel insights into dynamic ROP signaling during plant tip growth.

PLURIPETALA mediates ROP2 localization and stability in parallel to SCN1 but synergistically with TIP1 in root hairs.

Prenylation, the post-translational attachment of prenyl groups to substrate proteins, can affect their distribution and interactomes. Arabidopsis PLURIPETALA (PLP) encodes the shared α subunit of two heterodimeric protein isoprenyltransferases, whose functional loss provides a unique opportunity to study developmental and cellular processes mediated by its prenylated substrates, such as ROP GTPases. As molecular switches, the distribution and activation of ROPs are mediated by various factors, including guanine nucleotide exchange factors, GTPase activating proteins, guanine nucleotide dissociation inhibitors (RhoGDIs), prenylation, and S-acylation. However, how these factors together ensure that dynamic ROP signalling is still obscure. We report here that a loss-of-function allele of PLP resulted in cytoplasmic accumulation of ROP2 in root hairs and reduced its stability. Consequently, two downstream events of ROP signalling, i.e. actin microfilament (MF) organization and the production of reactive oxygen species (ROS), were compromised. Genetic, cytological and biochemical evidence supports an additive interaction between prenylation and RhoGDI1/SCN1 in ROP2 distribution and stability whereas PLP acts synergistically with the protein S-acyl transferase TIP GROWTH DEFECTIVE1 during root hair growth. By using root hair growth as a model system, we uncovered complex interactions among prenylation, RhoGDIs, and S-acylation in dynamic ROP signalling.

Arabidopsis RhoGDIs Are Critical for Cellular Homeostasis of Pollen Tubes.

Rhos of plants (ROPs) play a key role in plant cell morphogenesis, especially in tip-growing pollen tubes and root hairs, by regulating an array of intracellular activities such as dynamic polymerization of actin microfilaments. ROPs are regulated by guanine nucleotide exchange factors (RopGEFs), GTPase activating proteins (RopGAPs), and guanine nucleotide dissociation inhibitors (RhoGDIs). RopGEFs and RopGAPs play evolutionarily conserved function in ROP signaling. By contrast, although plant RhoGDIs regulate the membrane extraction and cytoplasmic sequestration of ROPs, less clear are their positive roles in ROP signaling as do their yeast and metazoan counterparts. We report here that functional loss of all three Arabidopsis (Arabidopsis thaliana) GDIs (tri-gdi) significantly reduced male transmission due to impaired pollen tube growth in vitro and in vivo. We demonstrate that ROPs were ectopically activated at the lateral plasma membrane of the tri-gdi pollen tubes. However, total ROPs were reduced posttranslationally in the tri-gdi mutant, resulting in overall dampened ROP signaling. Indeed, a ROP5 mutant that was unable to interact with GDIs failed to induce growth, indicating the importance of the ROP-GDI interaction for ROP signaling. Functional loss of GDIs impaired cellular homeostasis, resulting in excess apical accumulation of wall components in pollen tubes, similar to that resulting from ectopic phosphatidylinositol 4,5-bisphosphate signaling. GDIs and phosphatidylinositol 4,5-bisphosphate may antagonistically coordinate to maintain cellular homeostasis during pollen tube growth. Our results thus demonstrate a more complex role of GDIs in ROP-mediated pollen tube growth.

Protein palmitoylation is critical for the polar growth of root hairs in Arabidopsis.

BACKGROUND: Protein palmitoylation, which is critical for membrane association and subcellular targeting of many signaling proteins, is catalyzed mainly by protein S-acyl transferases (PATs). Only a few plant proteins have been experimentally verified to be subject to palmitoylation, such as ROP GTPases, calcineurin B like proteins (CBLs), and subunits of heterotrimeric G proteins. However, emerging evidence from palmitoyl proteomics hinted that protein palmitoylation as a post-translational modification might be widespread. Nonetheless, due to the large number of genes encoding PATs and the lack of consensus motifs for palmitoylation, progress on the roles of protein palmitoylation in plants has been slow. RESULTS: We combined pharmacological and genetic approaches to examine the role of protein palmitoylation in root hair growth. Multiple PATs from different endomembrane compartments may participate in root hair growth, among which the Golgi-localized PAT24/TIP GROWTH DEFECTIVE1 (TIP1) plays a major role while the tonoplast-localized PAT10 plays a secondary role in root hair growth. A specific inhibitor for protein palmitoylation, 2-bromopalmitate (2-BP), compromised root hair elongation and polarity. Using various probes specific for cellular processes, we demonstrated that 2-BP impaired the dynamic polymerization of actin microfilaments (MF), the asymmetric plasma membrane (PM) localization of phosphatidylinositol (4,5)-bisphosphate (PIP2), the dynamic distribution of RabA4b-positive post-Golgi secretion, and endocytic trafficking in root hairs. CONCLUSIONS: By combining pharmacological and genetic approaches and using root hairs as a model, we show that protein palmitoylation, regulated by protein S-acyl transferases at different endomembrane compartments such as the Golgi and the vacuole, is critical for the polar growth of root hairs in Arabidopsis. Inhibition of protein palmitoylation by 2-BP disturbed key intracellular activities in root hairs. Although some of these effects are likely indirect, the cytological data reported here will contribute to a deep understanding of protein palmitoylation during tip growth in plants.

Arabidopsis COBRA-LIKE 10, a GPI-anchored protein, mediates directional growth of pollen tubes.

Successful reproduction of flowering plants requires constant communication between female tissues and growing pollen tubes. Female cells secrete molecules and peptides as nutrients or guidance cues for fast and directional tube growth, which is executed by dynamic changes of intracellular activities within pollen tubes. Compared with the extensive interest in female cues and intracellular activities of pollen tubes, how female cues are sensed and interpreted intracellularly in pollen is poorly understood. We show here that COBL10, a glycosylphosphatidylinositol (GPI)-anchored protein, is one component of this pollen tube internal machinery. Mutations in COBL10 caused gametophytic male sterility due to reduced pollen tube growth and compromised directional sensing in the female transmitting tract. Deposition of the apical pectin cap and cellulose microfibrils was disrupted in cobl10 pollen tubes. Pollen tube localization of COBL10 at the apical plasma membrane is critical for its function and relies on proper GPI processing and its C-terminal hydrophobic residues. GPI-anchored proteins are widespread cell sensors in mammals, especially during egg-sperm communication. Our results that COBL10 is critical for directional growth of pollen tubes suggest that they play critical roles in cell-cell communications in plants.

Arabidopsis COP1 suppresses root hair development by targeting type I ACS proteins for ubiquitination and degradation.

Root hairs (RHs) are an innovation of vascular plants whose development is coordinated by endogenous and environmental cues, such as ethylene and light conditions. However, the potential crosstalk between ethylene and light conditions in RH development is unclear. We report that Arabidopsis constitutive photomorphogenic 1 (COP1) integrates ethylene and light signaling to mediate RH development. Darkness suppresses RH development largely through COP1. COP1 inhibits both cell fate determination of trichoblast and tip growth of RHs based on pharmacological, genetic, and physiological analyses. Indeed, COP1 interacts with and catalyzes the ubiquitination of ACS2 and ACS6. COP1- or darkness-promoted proteasome-dependent degradation of ACS2/6 leads to a low ethylene level in underground tissues. The negative role of COP1 in RH development by downregulating ethylene signaling may be coordinated with the positive role of COP1 in hypocotyl elongation by upregulating ethylene signaling, providing an evolutionary advantage for seedling fitness.

Arabidopsis RopGEF4 and RopGEF10 are important for FERONIA-mediated developmental but not environmental regulation of root hair growth.

We investigated a genetic pathway in root hair development in Arabidopsis thaliana, involving the receptor-like kinase FERONIA (FER), two guanine nucleotide exchange factors for ROPs (RopGEF4 and RopGEF10), and the small GTPase Rho of plants (ROPs). Loss- and gain-of-function analyses demonstrated distinct roles of RopGEF4 and RopGEF10 such that RopGEF4 is only important for root hair elongation, while RopGEF10 mainly contributes to root hair initiation. Domain dissection by truncation and domain-swapping experiments indicated that their functional distinctions were mainly contributed by the noncatalytic domains. Using fluorescent ratio imaging, we showed that functional loss of RopGEF4 and RopGEF10 additively reduced reactive oxygen species (ROS) production. Bimolecular fluorescence complementation experiments demonstrated that RopGEF4 and RopGEF10 had the same interaction specificity as ROPs, suggesting common downstream components. We further showed that the promoting effects of environmental cues such as exogenous auxin and phosphate limitation on root hair development depended on FER. However, although functional loss of RopGEF4 and RopGEF10 largely abolished FER-induced ROS production, it did not compromise the responses to FER-mediated environmental cues on root hair development. Our results demonstrated that RopGEF4 and RopGEF10 are genetic components in FER-mediated, developmentally (but not environmentally) regulated, root hair growth.

The juxtamembrane and carboxy-terminal domains of Arabidopsis PRK2 are critical for ROP-induced growth in pollen tubes.

Polarized growth of pollen tubes is a critical step for successful reproduction in angiosperms and is controlled by ROP GTPases. Spatiotemporal activation of ROP (Rho GTPases of plants) necessitates a complex and sophisticated regulatory system, in which guanine nucleotide exchange factors (RopGEFs) are key components. It was previously shown that a leucine-rich repeat receptor-like kinase, Arabidopsis pollen receptor kinase 2 (AtPRK2), interacted with RopGEF12 for its membrane recruitment. However, the mechanisms underlying AtPRK2-mediated ROP activation in vivo are yet to be defined. It is reported here that over-expression of AtPRK2 induced tube bulging that was accompanied by the ectopic localization of ROP-GTP and the ectopic distribution of actin microfilaments. Tube depolarization was also induced by a potentially kinase-dead mutant, AtPRK2K366R, suggesting that the over-expression effect of AtPRK2 did not require its kinase activity. By contrast, deletions of non-catalytic domains in AtPRK2, i.e. the juxtamembrane (JM) and carboxy-terminal (CT) domains, abolished its ability to affect tube polarization. Notably, AtPRK2K366R retained the ability to interact with RopGEF12, whereas AtPRK2 truncations of these non-catalytic domains did not. Lastly, it has been shown that the JM and CT domains of AtPRK2 were not only critical for its interaction with RopGEF12 but also critical for its distribution at the plasma membrane. These results thus provide further insight into pollen receptor kinase-mediated ROP activation during pollen tube growth.

Croton tiglium essential oil compounds have anti-proliferative and pro-apoptotic effects in A549 lung cancer cell lines.

As a traditional Chinese medicine, Croton tiglium has the characteristics of laxative, analgesic, antibacterial and swelling. This study aimed to analyze the chemical composition of C. tiglium essential oil (CTEO) extracted from the seeds of C. tiglium and its cytotoxicity and antitumor effect in vitro. Supercritical CO2 fluid extraction technology was used to extract CTEO and the chemical constituents of the essential oil were identified by comparing the retention indices and mass spectra data taken from the NIST library with those calculated based on the C7-C40 n-alkanes standard. In vitro cytotoxicity of the CTEO was assessed against cancer cell lines (A549) and the human normal bronchial epithelial cells (HBE) using the CCK-8 assay. Proliferation was detected by colony formation experiments. Wound scratch and cell invasion assays were used to detect cell migration and invasion. Levels of apoptotic markers, signaling molecules, and cell cycle regulators expression were characterized by Western blot analysis. As the results, twenty-eight compounds representing 92.39% of the total oil were identified in CTEO. The CTEO has significant antitumor activity on A549 cancer cells (IC50 48.38 µg/mL). In vitro antitumor experiments showed that CTEO treatment significantly inhibited the proliferation and migration of A549 cells, disrupted the cell cycle process, and reduced the expression levels of cyclin A, cyclin B and CDK1. CTEO can also reduce mitochondrial membrane potential, activate caspase-dependent apoptosis pathway, and finally induce apoptosis. CTEO may become an effective anti-cancer drug and will be further developed for cancer treatment.

AGC1.5 Kinase Phosphorylates RopGEFs to Control Pollen Tube Growth.

Double fertilization in angiosperms requires the targeted delivery of immotile sperm to the eggs through pollen tubes. The polarity of tip-growing pollen tubes is maintained through dynamic association of active Rho GTPases of plants (ROP-GTP) with the apical plasma membrane. Guanine nucleotide exchange factors for ROPs (RopGEFs) catalyze the activation of ROPs and thereby affect spatiotemporal ROP signaling. Whereas RopGEFs have been found to be phosphorylated proteins, the kinases responsible for their phosphorylation in vivo and biological consequences of RopGEF phosphorylation in pollen tube growth remain unclear. We report here that the Arabidopsis AGC1.5 subfamily of cytoplasmic kinases is critical for the restricted localization of ROP-GTP during pollen tube growth. Loss of AGC1.5 and AGC1.7 functions resulted in the mistargeting of active ROPs and defective events downstream of ROP signaling in pollen tubes. AGC1.5 interacts with RopGEFs via their catalytic PRONE domain and phosphorylates RopGEFs at a conserved Ser residue of PRONE domain. Loss of AGC1.5 and AGC1.7 functions resulted in the mistargeting of RopGEFs in pollen tubes, similar to the phenotype caused by the mutation that renders RopGEFs non-phosphorylatable by AGC1.5. Collectively, our results provide mechanistic insights into the spatiotemporal activation of ROPs during the polar growth of pollen tubes.