The role of Arabidopsis SCAR genes in ARP2-ARP3-dependent cell morphogenesis

Functional divergence of plant SCAR/WAVE proteins is determined by intrinsically disordered regions

Dynamic actin cytoskeleton reorganization enables plant developmental processes requiring polarized transport such as root hair and leaf trichome formation. The SCAR/WAVE complex plays a crucial role in regulating these dynamics through ARP2/3-mediated actin branching. SCAR/WAVE genes occur as small families across a wide range of plant species, but whether and how they fulfill different functions remains unclear. We use a systematic chimera approach to define the differential functionality of two closely related Medicago truncatula SCAR proteins in plant development. We show that SCAR/WAVE contribution to M. truncatula root hair or Arabidopsis thaliana trichome formation is dependent on two central intrinsically disordered regions (IDRs). Differential functionalities of M. truncatula SCAR proteins were furthermore associated with the presence/absence of a 42-amino acid sequence within the IDR that affected protein stability. Through uncovering a molecular basis for functional differences, we advance our understanding of plant SCAR/WAVE complexes.

The ER-PM interaction is essential for cytokinesis and recruits the actin cytoskeleton through the SCAR/WAVE complex

Plant cytokinesis requires coordination between the actin cytoskeleton, microtubules, and membranes to guide division plane formation and cell plate expansion; how these regulatory factors are coordinated remains unknown. The actin cytoskeleton assembly is controlled by several actin nucleation factors, such as the SCAR/WAVE complex, which regulates actin nucleation and branching through the activation of the ARP2/3 complex. The activity of these actin regulatory proteins is likely influenced by interactions with specific membranes; however, the molecular basis and the biological relevance of SCAR-membrane interactions are also unclear. In this study, we demonstrate that the ER-PM tethering protein VAP27-1 directly interacts with SCAR2 at the ER membrane and that they colocalize to guide cell plate orientation during cell division. In the root meristem, both VAP27-1 and SCAR2 exhibit polarized localization at the cell plates, where the interaction between ER and PM is abundant. VAP27-1 recruits SCAR2 to the cell division plane, where there is a high concentration of actin filaments. In the vap27-1346 mutant, the densities of cortical ER, SCAR2, and consequently actin filaments are significantly reduced at the cell division plane, affecting cell plate orientation, cell division, and root development. A similar phenomenon is also observed in the scar1234 mutant, suggesting that VAP27 and SCAR proteins regulate cell division through a similar pathway. In conclusion, our data reveal a plant-specific function of VAP27-regulated ER-PM interaction and advance our understanding of plant ER-PM contact site and its role in cell division.

The Plastidial Protein Acetyltransferase GNAT1 Forms a Complex With GNAT2, yet Their Interaction Is Dispensable for State Transitions

Protein N-acetylation is one of the most abundant co- and post-translational modifications in eukaryotes, extending its occurrence to chloroplasts within vascular plants. Recently, a novel plastidial enzyme family comprising eight acetyltransferases that exhibit dual lysine and N-terminus acetylation activities was unveiled in Arabidopsis. Among these, GNAT1, GNAT2, and GNAT3 reveal notable phylogenetic proximity, forming a subgroup termed NAA90. Our study focused on characterizing GNAT1, closely related to the state transition acetyltransferase GNAT2. In contrast to GNAT2, GNAT1 did not prove essential for state transitions and displayed no discernible phenotypic difference compared to the wild type under high light conditions, while gnat2 mutants were severely affected. However, gnat1 mutants exhibited a tighter packing of the thylakoid membranes akin to gnat2 mutants. In vitro studies with recombinant GNAT1 demonstrated robust N-terminus acetylation activity on synthetic substrate peptides. This activity was confirmed in vivo through N-terminal acetylome profiling in two independent gnat1 knockout lines. This attributed several acetylation sites on plastidial proteins to GNAT1, reflecting a subset of GNAT2's substrate spectrum. Moreover, co-immunoprecipitation coupled with mass spectrometry revealed a robust interaction between GNAT1 and GNAT2, as well as a significant association of GNAT2 with GNAT3 - the third acetyltransferase within the NAA90 subfamily. This study unveils the existence of at least two acetyltransferase complexes within chloroplasts, whereby complex formation might have a critical effect on the fine-tuning of the overall acetyltransferase activities. These findings introduce a novel layer of regulation in acetylation-dependent adjustments in plastidial metabolism.

TRANSPARENT TESTA GLABRA2 defines trichome cell shape by modulating actin cytoskeleton in Arabidopsis thaliana

The Arabidopsis (Arabidopsis thaliana) TRANSPARENT TESTA GLABRA2 (TTG2) gene encodes a WRKY transcription factor that regulates a range of development events like trichome, seed coat, and atrichoblast formation. Loss-of-function of TTG2 was previously shown to reduce or eliminate trichome specification and branching. Here, we report the identification of an allele of TTG2, ttg2-6. In contrast to the ttg2 mutants described before, ttg2-6 displayed unique trichome phenotypes. Some ttg2-6 mutant trichomes were hyper-branched, whereas others were hypo-branched, distorted, or clustered. Further, we found that in addition to specifically activating R3 MYB transcription factor TRIPTYCHON (TRY) to modulate trichome specification, TTG2 also integrated cytoskeletal signaling to regulate trichome morphogenesis. The ttg2-6 trichomes displayed aberrant cortical microtubules (cMTs) and actin filaments (F-actin) configurations. Moreover, genetic and biochemical analyses showed that TTG2 could directly bind to the promoter and regulate the expression of BRICK1 (BRK1), which encodes a subunit of the actin nucleation promoting complex suppressor of cyclic AMP repressor (SCAR)/Wiskott-Aldrich syndrome protein family verprolin homologous protein (WAVE). Collectively, taking advantage of ttg2-6, we uncovered a function for TTG2 in facilitating cMTs and F-actin cytoskeleton-dependent trichome development, providing insight into cellular signaling events downstream of the core transcriptional regulation during trichome development in Arabidopsis.

The caleosin RD20/CLO3 regulates lateral root development in response to abscisic acid and regulates flowering time in conjunction with the caleosin CLO7

The caleosins are encoded by multi-gene families in Arabidopsis thaliana and other plant species. This work investigates the role of two family members, RD20/CLO3 and CLO7, in flowering transition and in root development in response to ABA treatment. Gene expression of the caleosin RD20/CLO3 is induced by ABA in the root tissues and RD20/CLO3 has a negative affect on the total number of lateral roots as well as the length of the lateral roots in response to ABA treatment. The rd20/clo3 mutant has more and longer lateral roots in response to ABA treatment compared to the wild-type, showing that RD20/CLO3 plays a role in the ABA signaling pathway affecting this trait. In contrast, the caleosin CLO7 is not expressed in the roots and does not affect root architecture in response to ABA treatment. The disruption of both RD20/CLO3 and CLO7 together causes a dramatic early-flowering phenotype under long-day conditions, whereas single mutations in these genes do not affect flowering time under these conditions. Both yeast two-hybrid and bimolecular fluorescence complementation showed that both RD20/CLO3 and CLO7 interact with each other and can form homodimers and heterodimers. Taken together, these findings suggest that members of the caleosin gene family play both different and redundant roles in plant development.

The caleosin CLO7 and its role in the heterotrimeric G-protein signalling network

The investigation of the caleosin CLO7 in relation to heterotrimeric G-protein signalling in Arabidopsis showed that the gene plays a role in seed germination and embryo viability. The caleosin CLO7 belongs to a multi-gene family of calcium-binding proteins which are characterized by single EF-hand motifs. Other members of the caleosin gene family have been shown to affect transpiration and seed germination as well as play a role in both abiotic and biotic stress responses. The proteins are associated with lipid droplets/oil bodies and some members of the gene family have been shown to have peroxygenase activity. Members of the gene family have also been shown to interact with the α subunit of the heterotrimeric G protein complex. In this study, we further expand on the diversity of physiological responses in which members of this gene family play regulatory roles. Utilizing BiFC and Y2H protein-protein interaction assays, CLO7 is identified as an interactor of the heterotrimeric G protein α subunit, GPA1. The full-length CLO7 is shown to interact with both the wild-type GPA1 and its constitutively active form, GPA1^QL, at the plasma membrane. Point mutations to critical amino acids for calcium binding in the EF-hand of CLO7 indicate that the interaction with GPA1 is calcium-dependent and that the interaction with GPA1^QL is enhanced by calcium. Protein-protein interaction assays also show that CLO7 interacts with Pirin1, a member of the cupin gene superfamily and a known downstream effector of GPA1, and this interaction is calcium-dependent. The N-terminal portion of CLO7 is responsible for these interactions. GFP-tagged CLO7 protein localizes to the endoplasmic reticulum (ER) and to lipid bodies. Characterization of the clo7 mutant line has shown that CLO7 is implicated in the abscisic acid (ABA) and mannitol-mediated inhibition of seed germination, with the clo7 mutant displaying higher germination rates in response to osmotic stress and ABA hormone treatment. These results provide insight into the role of CLO7 in seed germination in response to abiotic stress as well as its interaction with GPA1 and Pirin1. CLO7 also plays a role in embryo viability with the clo7gpa1 double mutant displaying embryo lethality, and therefore the double mutant cannot be recovered.

Genome-wide association study uncovers major genetic loci associated with flowering time in response to active accumulated temperature in wild soybean population

Flowering time and active accumulated temperature (AAT) are two key factors that limit the expanded production especially for soybean across different regions. Wild soybean provides an important germplasm for functional genomics study in cultivar soybean. However, the studies on genetic basis underlying flowering time in response to AAT especially in wild soybean were rarely reported. In this study, we used 294 wild soybean accessions derived from major soybean production region characterized by different AAT in Northeast of China. Based on genome-wide association study (GWAS), we identified 96 SNPs corresponded to 342 candidate genes that significantly associated with flowering time recorded in two-year experiments. Gene Ontology enrichment analysis suggests that the pathways of photosynthesis light reaction and actin filament binding were significantly enriched. We found three lead SNPs with -log10(p-value) > 32 across the two-year experiments, i.e., Chr02:9490318, Chr04:8545910 and Chr09:49553555. Linkage disequilibrium block analysis shows 28 candidate genes within the genomic region centered on the lead SNPs. Among them, expression levels of three genes (aspartic peptidase 1, serine/threonine-protein kinase and protein SCAR2-like) were significantly differed between two subgroups possessing contrasting flowering time distributed at chromosome 2, 4 and 9, respectively. There are 6, 7 and 3 haplotypes classified on the coding regions of the three genes, respectively. Collectively, accessions with late flowering time phenotype are typically derived from AAT zone 1, which is associated with the haplotypic distribution and expression levels of the three genes. This study provides an insight into a potential mechanism responsible for flowering time in response to AAT in wild soybean, which could promote the understanding of genetic basis for other major crops.

CRISPR-Cas9 Arabidopsis mutants of genes for ARPC1 and ARPC3 subunits of ARP2/3 complex reveal differential roles of complex subunits

Protein complex Arp2/3 has a conserved role in the nucleation of branched actin filaments. It is constituted of seven subunits, including actin-like subunits ARP2 and ARP3 plus five other subunits called Arp2/3 Complex Component 1 to 5, which are not related to actin. Knock-out plant mutants lacking individual plant ARP2/3 subunits have a typical phenotype of distorted trichomes, altered pavement cells shape and defects in cell adhesion. While knock-out mutant Arabidopsis plants for most ARP2/3 subunits have been characterized before, Arabidopsis plant mutants missing ARPC1 and ARPC3 subunits have not yet been described. Using CRISPR/Cas9, we generated knock-out mutants lacking ARPC1 and ARPC3 subunits. We confirmed that the loss of ARPC1 subunits results in the typical ARP2/3 mutant phenotype. However, the mutants lacking ARPC3 subunits resulted in plants with surprisingly different phenotypes. Our results suggest that plant ARP2/3 complex function in trichome shaping does not require ARPC3 subunit, while the fully assembled complex is necessary for the establishment of correct cell adhesion in the epidermis.

Tandem C2 domains mediate dynamic organelle targeting of a DOCK family guanine nucleotide exchange factor

Multicellular organisms use DOCK family guanine nucleotide exchange factors to activate Rac/Rho-of-Plants small GTPases and coordinate cell shape change. In developing tissues, DOCK signals integrate cell-cell interactions with cytoskeleton remodeling, and the GEFs cluster reversibly at specific organelle surfaces to orchestrate cytoskeletal reorganization. The domain organizations among DOCK orthologs are diverse, and the mechanisms of localization control are poorly understood. Here we use combinations of transgene complementation and live cell imaging assays to uncover an evolutionarily conserved and essential localization determinant in the DOCK-GEF named SPIKE1. The SPIKE1-DHR3 domain is sufficient for organelle association in vivo, and displays a complicated lipid binding selectivity for both phospholipid head groups and fatty acid chain saturation. SPIKE1-DHR3 is predicted to adopt a C2-domain structure and functions as part of tandem C2 array that enables reversible clustering at the cell apex. This work provides mechanistic insight into how DOCK GEFs sense compositional and biophysical membrane properties at the interface of two organelle systems.

Formation of self-organizing functionally distinct Rho of plants domains involves a reduced mobile population

Rho family proteins are central to the regulation of cell polarity in eukaryotes. Rho of Plants-Guanyl nucleotide Exchange Factor (ROPGEF) can form self-organizing polar domains following co-expression with an Rho of Plants (ROP) and an ROP GTPase-Activating Protein (ROPGAP). Localization of ROPs in these domains has not been demonstrated, and the mechanisms underlying domain formation and function are not well understood. Here we show that six different ROPs form self-organizing domains when co-expressed with ROPGEF3 and GAP1 in Nicotiana benthamiana or Arabidopsis (Arabidopsis thaliana). Domain formation was associated with ROP-ROPGEF3 association, reduced ROP mobility, as revealed by time-lapse imaging and Fluorescence Recovery After Photobleaching beam size analysis, and was independent of Rho GTP Dissociation Inhibitor mediated recycling. The domain formation depended on the ROPs' activation/inactivation cycles and interaction with anionic lipids via a C-terminal polybasic domain. Coexpression with the microtubule-associated protein ROP effector INTERACTOR OF CONSTITUTIVELY ACTIVE ROP 1 (ICR1) revealed differential function of the ROP domains in the ability to recruit ICR1. Taken together, the results reveal mechanisms underlying self-organizing ROP domain formation and function.

Insights into the molecular evolution of fertilization mechanism in land plants

Comparative genetics and genomics among green plants, including algae, provide deep insights into the evolution of land plant sexual reproduction. Land plants have evolved successive changes during their conquest of the land and innovations in sexual reproduction have played a major role in their terrestrialization. Recent years have seen many revealing dissections of the molecular mechanisms of sexual reproduction and much new genomics data from the land plant lineage, including early diverging land plants, as well as algae. This new knowledge is being integrated to further understand how sexual reproduction in land plants evolved, identifying highly conserved factors and pathways, but also molecular changes that underpinned the emergence of new modes of sexual reproduction. Here, we review recent advances in the knowledge of land plant sexual reproduction from an evolutionary perspective and also revisit the evolution of angiosperm double fertilization.

The stress induced caleosin, RD20/CLO3, acts as a negative regulator of GPA1 in Arabidopsis

KEY MESSAGE

A stress induced calcium-binding protein, RD20/CLO3 interacts with the alpha subunit of the heterotrimeric G-protein complex in Arabidopsis and affects etiolation and leaf morphology. Heterotrimeric G proteins and calcium signaling have both been shown to play a role in the response to environmental abiotic stress in plants; however, the interaction between calcium-binding proteins and G-protein signaling molecules remains elusive. We investigated the interaction between the alpha subunit of the heterotrimeric G-protein complex, GPA1, of Arabidopsis thaliana with the calcium-binding protein, the caleosin RD20/CLO3, a gene strongly induced by drought, salt and abscisic acid. The proteins were found to interact in vivo by bimolecular fluorescent complementation (BiFC); the interaction was localized to the endoplasmic reticulum and to oil bodies within the cell. The constitutively GTP-bound GPA1 (GPA1QL) also interacts with RD20/CLO3 as well as its EF-hand mutant variations and these interactions are localized to the plasma membrane. The N-terminal portion of RD20/CLO3 was found to be responsible for the interaction with GPA1 and GPA1QL using both BiFC and yeast two-hybrid assays. RD20/CLO3 contains a single calcium-binding EF-hand in the N-terminal portion of the protein; disruption of the calcium-binding capacity of the protein obliterates interaction with GPA1 in in vivo assays and decreases the interaction between the caleosin and the constitutively active GPA1QL. Analysis of rd20/clo3 mutants shows that RD20/CLO3 plays a key role in the signaling pathway controlling hypocotyl length in dark grown seedlings and in leaf morphology. Our findings indicate a novel role for RD20/CLO3 as a negative regulator of GPA1.

The ARP2/3 complex, acting cooperatively with Class I formins, modulates penetration resistance in Arabidopsis against powdery mildew invasion

The actin cytoskeleton regulates an array of diverse cellular activities that support the establishment of plant-microbe interactions and plays a critical role in the execution of plant immunity. However, molecular and cellular mechanisms regulating the assembly and rearrangement of actin filaments (AFs) at plant-pathogen interaction sites remain largely elusive. Here, using live-cell imaging, we show that one of the earliest cellular responses in Arabidopsis thaliana upon powdery mildew attack is the formation of patch-like AF structures beneath fungal invasion sites. The AFs constituting actin patches undergo rapid turnover, which is regulated by the actin-related protein (ARP)2/3 complex and its activator, the WAVE/SCAR regulatory complex (W/SRC). The focal accumulation of phosphatidylinositol-4,5-bisphosphate at fungal penetration sites appears to be a crucial upstream modulator of the W/SRC-ARP2/3 pathway-mediated actin patch formation. Knockout of W/SRC-ARP2/3 pathway subunits partially compromised penetration resistance with impaired endocytic recycling of the defense-associated t-SNARE protein PEN1 and its deposition into apoplastic papillae. Simultaneously knocking out ARP3 and knocking down the Class I formin (AtFH1) abolished actin patch formation, severely impaired the deposition of cell wall appositions, and promoted powdery mildew entry into host cells. Our results demonstrate that the ARP2/3 complex and formins, two actin-nucleating systems, act cooperatively and contribute to Arabidopsis penetration resistance to fungal invasion.

Plant cytoskeletons and the endoplasmic reticulum network organization

Plant endoplasmic reticulum (ER) remodelling is likely to be important for its function in targeted protein secretion, organelle interaction and signal exchange. It has been known for decades that the structure and movement of the ER network is mainly regulated by the actin cytoskeleton through actin motor proteins and membrane-cytoskeleton adaptors. Recent discoveries also revealed alternative pathways that influence ER movement, through a microtubule-based machinery. Therefore, plants utilize both cytoskeletal components to drive ER dynamics, a process that is likely to be dependent on the cell type and the developmental stages. On the other hand, the ER membrane also has a direct effect towards the organization of the cytoskeletal network and disrupting the tethering factors at the ER-PM interface also rearranges the cytoskeletal structure. However, the influence of the ER network on the cytoskeleton organization has not been studied. In this review, we will provide an overview of the ER-cytoskeleton network in plants, and discuss the most recent discoveries in the field.

Formins control dynamics of F-actin in the central cell of Arabidopsis thaliana

In the female gamete of flowering plants, sperm nuclear migration is controlled by a constant inward movement of actin filaments (F-actin) for successful fertilization. This dynamic F-actin movement is ARP2/3-independent, raising the question of how actin nucleation and polymerization is controlled in the female gamete. Using confocal microscopy live-cell imaging in combination with a pharmacological approach, we assessed the involvement of another group of actin nucleators, formins, in F-actin inward movement in the central cell of Arabidopsis thaliana. We identify that the inhibition of the formin function, by formin inhibitor SMIFH2, significantly reduced the dynamic inward movement of F-actin in the central cell, indicating that formins play a major role in actin nucleation required for F-actin inward movement in the central cell.

The HD-Zip IV transcription factor SlHDZIV8 controls multicellular trichome morphology by regulating the expression of Hairless-2

Trichomes are specialized epidermal appendages that serve as excellent models to study cell morphogenesis. Although the molecular mechanism underlying trichome morphogenesis in Arabidopsis has been well characterized, most of the regulators essential for multicellular trichome morphology remain unknown in tomato. In this study, we determined that the recessive hairless-2 (hl-2) mutation in tomato causes severe distortion of all trichome types, along with increased stem fragility. Using map-based cloning, we found that the hl-2 phenotype was associated with a 100 bp insertion in the coding region of Nck-associated protein 1, a component of the SCAR/WAVE complex. Direct protein-protein interaction was detected between Hl-2 and Hl (SRA1, specifically Rac1-associated protein) using yeast two-hybrid and co-immunoprecipitation assays, suggesting that these proteins may work together during trichome formation. In addition, knock-down of a HD-Zip IV transcription factor, HDZIPIV8, distorted trichomes similar to the hl-2 mutant. HDZIPIV8 regulates the expression of Hl-2 by binding to the L1-box in the Hl-2 promoter region, and is involved in organizing actin filaments. The brittleness of hl-2 stems was found to result from decreased cellulose content. Taken together, these findings suggest that the Hl-2 gene plays an important role in controlling multicellular trichome morphogenesis and mechanical properties of stems in tomato plants.

ARP2/3-independent WAVE/SCAR pathway and class XI myosin control sperm nuclear migration in flowering plants

After eukaryotic fertilization, gamete nuclei migrate to fuse parental genomes in order to initiate development of the next generation. In most animals, microtubules control female and male pronuclear migration in the zygote. Flowering plants, on the other hand, have evolved actin filament (F-actin)-based sperm nuclear migration systems for karyogamy. Flowering plants have also evolved a unique double-fertilization process: two female gametophytic cells, the egg and central cells, are each fertilized by a sperm cell. The molecular and cellular mechanisms of how flowering plants utilize and control F-actin for double-fertilization events are largely unknown. Using confocal microscopy live-cell imaging with a combination of pharmacological and genetic approaches, we identified factors involved in F-actin dynamics and sperm nuclear migration in Arabidopsis thaliana (Arabidopsis) and Nicotiana tabacum (tobacco). We demonstrate that the F-actin regulator, SCAR2, but not the ARP2/3 protein complex, controls the coordinated active F-actin movement. These results imply that an ARP2/3-independent WAVE/SCAR-signaling pathway regulates F-actin dynamics in female gametophytic cells for fertilization. We also identify that the class XI myosin XI-G controls active F-actin movement in the Arabidopsis central cell. XI-G is not a simple transporter, moving cargos along F-actin, but can generate forces that control the dynamic movement of F-actin for fertilization. Our results provide insights into the mechanisms that control gamete nuclear migration and reveal regulatory pathways for dynamic F-actin movement in flowering plants.

The domesticated transposase ALP2 mediates formation of a novel Polycomb protein complex by direct interaction with MSI1, a core subunit of Polycomb Repressive Complex 2 (PRC2)

A large fraction of plant genomes is composed of transposable elements (TE), which provide a potential source of novel genes through “domestication”–the process whereby the proteins encoded by TE diverge in sequence, lose their ability to catalyse transposition and instead acquire novel functions for their hosts. In Arabidopsis, ANTAGONIST OF LIKE HETEROCHROMATIN PROTEIN 1 (ALP1) arose by domestication of the nuclease component of Harbinger class TE and acquired a new function as a component of POLYCOMB REPRESSIVE COMPLEX 2 (PRC2), a histone H3K27me3 methyltransferase involved in regulation of host genes and in some cases TE. It was not clear how ALP1 associated with PRC2, nor what the functional consequence was. Here, we identify ALP2 genetically as a suppressor of Polycomb-group (PcG) mutant phenotypes and show that it arose from the second, DNA binding component of Harbinger transposases. Molecular analysis of PcG compromised backgrounds reveals that ALP genes oppose silencing and H3K27me3 deposition at key PcG target genes. Proteomic analysis reveals that ALP1 and ALP2 are components of a variant PRC2 complex that contains the four core components but lacks plant-specific accessory components such as the H3K27me3 reader LIKE HETEROCHROMATION PROTEIN 1 (LHP1). We show that the N-terminus of ALP2 interacts directly with ALP1, whereas the C-terminus of ALP2 interacts with MULTICOPY SUPPRESSOR OF IRA1 (MSI1), a core component of PRC2. Proteomic analysis reveals that in alp2 mutant backgrounds ALP1 protein no longer associates with PRC2, consistent with a role for ALP2 in recruitment of ALP1. We suggest that the propensity of Harbinger TE to insert in gene-rich regions of the genome, together with the modular two component nature of their transposases, has predisposed them for domestication and incorporation into chromatin modifying complexes.

A large part of the genomes of plants and animals consists of transposable elements (TE), which are usually considered as selfish or parasitic as they encode proteins (transposases) which promote TE proliferation but not functions useful for their hosts. As a result, hosts have evolved ways of reducing TE proliferation, usually by modifying the DNA or chromatin of TE so that their transposases are no longer produced. Once the TE are inactivated they can no longer proliferate and over time they accumulate mutations and can evolve new functions, often beneficial for their hosts. This process is known as domestication and is increasingly recognised as a potent source of evolutionary novelty. For example, the CRISPR/Cas system that has provided the basis for a revolution in genetic engineering (“genome editing”) has evolved via domestication of transposons in bacteria. We have identified the ALP proteins, two domesticated transposases which function as components of an enzyme complex (PRC2) involved in modifying chromatin and regulating host gene activity in plants. Here we show how ALPs contact PRC2 and direct formation of a novel complex that lacks several of the usual components. The ALPs and related proteins will provide valuable tools for manipulating plant chromatin.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

Resistance protein Pit interacts with the GEF OsSPK1 to activate OsRac1 and trigger rice immunity

Resistance (R) genes encode intracellular nucleotide-binding/leucine-rich repeat-containing (NLR) family proteins that serve as critical plant immune receptors to induce effector-triggered immunity (ETI). NLR proteins possess a tripartite domain architecture consisting of an N-terminal variable region, a central nucleotide-binding domain, and a C-terminal leucine-rich repeat. N-terminal coiled-coil (CC) or Toll-interleukin 1 receptor (TIR) domains of R proteins appear to serve as platforms to trigger immune responses, because overexpression of the CC or TIR domain of some R proteins is sufficient to induce an immune response. Because direct downstream signaling molecules of R proteins remain obscure, the molecular mechanisms by which R proteins regulate downstream signaling are largely unknown. We reported previously that a rice R protein named Pit triggers ETI through a small GTPase, OsRac1, although how Pit activates OsRac1 is unclear. Here, we identified OsSPK1, a DOCK family guanine nucleotide exchange factor, as an interactor of Pit and activator for OsRac1. OsSPK1 contributes to signaling by two disease-resistance genes, Pit and Pia, against the rice blast fungus Magnaporthe oryzae and facilitates OsRac1 activation in vitro and in vivo. The CC domain of Pit is required for its binding to OsSPK1, OsRac1 activation, and the induction of cell death. Overall, we conclude that OsSPK1 is a direct and key signaling target of Pit-mediated immunity. Our results shed light on how R proteins trigger ETI through direct downstream molecules.

The truncated NLR protein TIR-NBS13 is a MOS6/IMPORTIN-α3 interaction partner required for plant immunity

Importin-α proteins mediate the translocation of nuclear localization signal (NLS)-containing proteins from the cytoplasm into the nucleus through nuclear pore complexes (NPCs). Genetically, Arabidopsis IMPORTIN-α3/MOS6 (MODIFIER OF SNC1, 6) is required for basal plant immunity and constitutive disease resistance activated in autoimmune mutant snc1 (suppressor of npr1-1, constitutive 1), suggesting that MOS6 plays a role in the nuclear import of proteins involved in plant defense signaling. Here, we sought to identify and characterize defense-regulatory cargo proteins and interaction partners of MOS6. We conducted both in silico database analyses and affinity purification of functional epitope-tagged MOS6 from pathogen-challenged stable transgenic plants coupled with mass spectrometry. We show that among the 13 candidate MOS6 interactors we selected for further functional characterization, the TIR-NBS-type protein TN13 is required for resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 lacking the type-III effector proteins AvrPto and AvrPtoB. When expressed transiently in N. benthamiana leaves, TN13 co-immunoprecipitates with MOS6, but not with its closest homolog IMPORTIN-α6, and localizes to the endoplasmic reticulum (ER), consistent with a predicted N-terminal transmembrane domain in TN13. Our work uncovered the truncated NLR protein TN13 as a component of plant innate immunity that selectively binds to MOS6/IMPORTIN-α3 in planta. We speculate that the release of TN13 from the ER membrane in response to pathogen stimulus, and its subsequent nuclear translocation, is important for plant defense signal transduction.

New advances in autophagy in plants: Regulation, selectivity and function

Autophagy is a major and conserved pathway for delivering unwanted proteins or damaged organelles to the vacuole for degradation and recycling. In plants, it functions as a housekeeping process to maintain cellular homeostasis under normal conditions and is induced by stress and senescence; it thus plays important roles in development, stress tolerance and metabolism. Autophagy can both execute bulk degradation and be highly selective in targeting cargos under specific environmental conditions or during certain developmental processes. Here, we review recent research on autophagy in plants, and discuss new insights into its core mechanism, regulation, selectivity and physiological roles. Potential future directions are also highlighted.

Homologs of SCAR/WAVE complex components are required for epidermal cell morphogenesis in rice

Filamentous actins (F-actins) play a vital role in epidermal cell morphogenesis. However, a limited number of studies have examined actin-dependent leaf epidermal cell morphogenesis events in rice. In this study, two recessive mutants were isolated: less pronounced lobe epidermal cell2-1 (lpl2-1) and lpl3-1, whose leaf and stem epidermis developed a smooth surface, with fewer serrated pavement cell (PC) lobes, and decreased papillae. The lpl2-1 also exhibited irregular stomata patterns, reduced plant height, and short panicles and roots. Molecular genetic studies demonstrated that LPL2 and LPL3 encode the PIROGI/Specifically Rac1-associated protein 1 (PIR/SRA1)-like and NCK-associated protein 1 (NAP1)-like proteins, respectively, two components of the suppressor of cAMP receptor/Wiskott-Aldrich syndrome protein-family verprolin-homologous protein (SCAR/WAVE) regulatory complex involved in actin nucleation and function. Epidermal cells exhibited abnormal arrangement of F-actins in both lpl2 and lpl3 expanding leaves. Moreover, the distorted trichomes of Arabidopsis pir could be partially restored by an overexpression of LPL2 A yeast two-hybrid assay revealed that LPL2 can directly interact with LPL3 in vitro Collectively, the results indicate that LPL2 and LPL3 are two functionally conserved homologs of the SCAR/WAVE complex components, and that they play an important role in controlling epidermal cell morphogenesis in rice by organising F-actin.

Seed Dormancy in Arabidopsis Requires Self-Binding Ability of DOG1 Protein and the Presence of Multiple Isoforms Generated by Alternative Splicing

The Arabidopsis protein DELAY OF GERMINATION 1 (DOG1) is a key regulator of seed dormancy, which is a life history trait that determines the timing of seedling emergence. The amount of DOG1 protein in freshly harvested seeds determines their dormancy level. DOG1 has been identified as a major dormancy QTL and variation in DOG1 transcript levels between accessions contributes to natural variation for seed dormancy. The DOG1 gene is alternatively spliced. Alternative splicing increases the transcriptome and proteome diversity in higher eukaryotes by producing transcripts that encode for proteins with altered or lost function. It can also generate tissue specific transcripts or affect mRNA stability. Here we suggest a different role for alternative splicing of the DOG1 gene. DOG1 produces five transcript variants encoding three protein isoforms. Transgenic dog1 mutant seeds expressing single DOG1 transcript variants from the endogenous DOG1 promoter did not complement because they were non-dormant and lacked DOG1 protein. However, transgenic plants overexpressing single DOG1 variants from the 35S promoter could accumulate protein and showed complementation. Simultaneous expression of two or more DOG1 transcript variants from the endogenous DOG1 promoter also led to increased dormancy levels and accumulation of DOG1 protein. This suggests that single isoforms are functional, but require the presence of additional isoforms to prevent protein degradation. Subsequently, we found that the DOG1 protein can bind to itself and that this binding is required for DOG1 function but not for protein accumulation. Natural variation for DOG1 binding efficiency was observed among Arabidopsis accessions and contributes to variation in seed dormancy.

SCARN a Novel Class of SCAR Protein That Is Required for Root-Hair Infection during Legume Nodulation

Rhizobial infection of legume root hairs requires a rearrangement of the actin cytoskeleton to enable the establishment of plant-made infection structures called infection threads. In the SCAR/WAVE (Suppressor of cAMP receptor defect/WASP family verpolin homologous protein) actin regulatory complex, the conserved N-terminal domains of SCAR proteins interact with other components of the SCAR/WAVE complex. The conserved C-terminal domains of SCAR proteins bind to and activate the actin-related protein 2/3 (ARP2/3) complex, which can bind to actin filaments catalyzing new actin filament formation by nucleating actin branching. We have identified, SCARN (SCAR-Nodulation), a gene required for root hair infection of Lotus japonicus by Mesorhizobium loti. Although the SCARN protein is related to Arabidopsis thaliana SCAR2 and SCAR4, it belongs to a distinct legume-sub clade. We identified other SCARN-like proteins in legumes and phylogeny analyses suggested that SCARN may have arisen from a gene duplication and acquired specialized functions in root nodule symbiosis. Mutation of SCARN reduced formation of infection-threads and their extension into the root cortex and slightly reduced root-hair length. Surprisingly two of the scarn mutants showed constitutive branching of root hairs in uninoculated plants. However we observed no effect of scarn mutations on trichome development or on the early actin cytoskeletal accumulation that is normally seen in root hair tips shortly after M. loti inoculation, distinguishing them from other symbiosis mutations affecting actin nucleation. The C-terminal domain of SCARN binds to ARPC3 and ectopic expression of the N-terminal SCAR-homology domain (but not the full length protein) inhibited nodulation. In addition, we found that SCARN expression is enhanced by M. loti in epidermal cells and that this is directly regulated by the NODULE INCEPTION (NIN) transcription factor.

Characterization of Lotus japonicus mutants defective for nodule infection by rhizobia led to the identification of a gene we named SCARN. Two of the five alleles caused formation of branched root-hairs in uninoculated seedlings, suggesting SCARN plays a role in the microtubule and actin-regulated polar growth of root hairs. SCARN is one of three L. japonicus proteins containing the conserved N and C terminal domains predicted to be required for rearrangement of the actin cytoskeleton. SCARN expression is induced in response to rhizobial nodulation factors by the NIN (NODULE INCEPTION) transcription factor and appears to be adapted to promoting rhizobial infection, possibly arising from a gene duplication event. SCARN binds to ARPC3, one of the predicted components in the actin-related protein complex involved in the activation of actin nucleation.

The SCAR/WAVE complex polarizes PAN receptors and promotes division asymmetry in maize

Pre-mitotic establishment of polarity is a key event in the preparation of mother cells for asymmetric cell divisions that produce daughters of distinct fates, and ensures correct cellular patterning of tissues and eventual organ function. Previous work has shown that two receptor-like kinases, PANGLOSS2 (PAN2) and PAN1, and the small GTPase RHO GTPASE OF PLANTS (ROP) promote mother cell polarity and subsequent division asymmetry in developing maize stomata. PAN proteins become polarized prior to asymmetric cell division, however, the mechanism of this polarization is unknown. Here we show that the SCAR/WAVE regulatory complex, which activates the actin-nucleating ARP2/3 complex, is the first known marker of polarity in this asymmetric division model and is required for PAN polarization. These findings implicate actin, and specifically branched actin networks, in PAN polarization and asymmetric cell division.

Interactions between plant endomembrane systems and the actin cytoskeleton

Membrane trafficking, organelle movement, and morphogenesis in plant cells are mainly controlled by the actin cytoskeleton. Not all proteins that regulate the cytoskeleton and membrane dynamics in animal systems have functional homologs in plants, especially for those proteins that form the bridge between the cytoskeleton and membrane; the membrane-actin adaptors. Their nature and function is only just beginning to be elucidated and this field has been greatly enhanced by the recent identification of the NETWORKED (NET) proteins, which act as membrane-actin adaptors. In this review, we will summarize the role of the actin cytoskeleton and its regulatory proteins in their interaction with endomembrane compartments and where they potentially act as platforms for cell signaling and the coordination of other subcellular events.

RHO GTPase in plants: Conservation and invention of regulators and effectors

Plants possess a single subfamily of Rho GTPases, ROP, which does usual things as do Rho-family GTPases in animal and fungal systems, namely participating in the spatial control of cellular processes by signaling to the cytoskeleton and vesicular trafficking. As one would expect, ROPs are modulated by conserved regulators such as DHR2-type GEFs, RhoGAPs and Rho GDIs. What is surprising is that plants have invented new regulators such as PRONE-type GEFs (known as RopGEFs) and effectors such as RICs and ICRs/RIPs in the regulation of the cytoskeleton and vesicular trafficking. This review will discuss recent work on characterizing ROP regulators and effectors as well as addressing why and how a mixture of conserved and novel Rho signaling mechanisms is utilized to modulate fundamental cellular processes such as cytoskeletal dynamics/reorganization and vesicular trafficking.

The Arabidopsis CURVY1 (CVY1) gene encoding a novel receptor-like protein kinase regulates cell morphogenesis, flowering time and seed production

BACKGROUND

A molecular-level understanding of the loss of CURVY1 (CVY1) gene expression (which encodes a member of the receptor-like protein kinase family) was investigated to gain insights into the mechanisms controlling cell morphogenesis and development in Arabidopsis thaliana.

RESULTS

Using a reverse genetic and cell biology approaches, we demonstrate that CVY1 is a new DISTORTED gene with similar phenotypic characterization to previously characterized ARP2/3 distorted mutants. Compared to the wild type, cvy1 mutant displayed a strong distorted trichome and altered pavement cell phenotypes. In addition, cvy1 null-mutant flowers earlier, grows faster and produces more siliques than WT and the arp2/3 mutants. The CVY1 gene is ubiquitously expressed in all tissues and seems to negatively regulate growth and yield in higher plants.

CONCLUSIONS

Our results suggest that CURVY1 gene participates in several biochemical pathways in Arabidopsis thaliana including (i) cell morphogenesis regulation through actin cytoskeleton functional networks, (ii) the transition of vegetative to the reproductive stage and (iii) the production of seeds.

The PpCMT chromomethylase affects cell growth and interacts with the homolog of LIKE HETEROCHROMATIN PROTEIN 1 in the moss Physcomitrella patens

Chromomethylases (CMTs) are plant-specific cytosine DNA methyltransferases that are involved in maintenance of CpNpG methylation. In seed plants, histone methylation and interaction of CMT with LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) is essential for recruitment of CMT to target sites. LHP1 has been characterized as a putative component of the POLYCOMB REPRESSIVE COMPLEX1 (PRC1) in plants, and functions downstream of PRC2 to maintain genes in repressed state for orchestrated development. In the present study, we show that targeted disruption of PpCMT results in an approximately 50% reduction in global cytosine methylation levels. This affects growth of apical cells, predominantly growth of side branch initials emerging from chloronema cells. In some places, these cells develop thick walls with plasmolyzed cellular contents. Transcript accumulation patterns of genes involved in apical cell extension and metabolism of hemicelluloses, such as xyloglucans, in the primary cell walls decreased many fold in ppcmt mutant lines, as determined by real-time PCR. Using yeast two-hybrid method and bimolecular fluorescence complementation assay, we show that PpCMT and PpLHP1 interact through their chromo domains, while PpLHP1 homodimerizes through its chromo shadow domain. The results presented in this study provide insight into the role of the single chromomethylase, PpCMT, in proliferation of protonema filaments, and shed light on the evolutionary conservation of proteins interacting with these methylases in the early land plant, Physcomitrella patens.

In vivo protein-protein interaction studies with BiFC: conditions, cautions, and caveats

Bimolecular fluorescence complementation (BiFC), performed with suitable controls and the right conditions, can be a straightforward and simple method to assess protein-protein interactions accessible to anyone with basic confocal microscopy skills. It is of course not without its own potential pitfalls and requires specific controls. Here we describe its use to study the interactions between pathogen effector proteins and host proteins inside plant cells.

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

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

The endoplasmic reticulum is a reservoir for WAVE/SCAR regulatory complex signaling in the Arabidopsis leaf

During plant cell morphogenesis, signal transduction and cytoskeletal dynamics interact to locally organize the cytoplasm and define the geometry of cell expansion. The WAVE/SCAR (for WASP family verprolin homologous/suppressor of cyclic AMP receptor) regulatory complex (W/SRC) is an evolutionarily conserved heteromeric protein complex. Within the plant kingdom W/SRC is a broadly used effector that converts Rho-of-Plants (ROP)/Rac small GTPase signals into Actin-Related Protein2/3 and actin-dependent growth responses. Although the components and biochemistry of the W/SRC pathway are well understood, a basic understanding of how cells partition W/SRC into active and inactive pools is lacking. In this paper, we report that the endoplasmic reticulum (ER) is an important organelle for W/SRC regulation. We determined that a large intracellular pool of the core W/SRC subunit NAP1, like the known positive regulator of W/SRC, the DOCK family guanine nucleotide-exchange factor SPIKE1 (SPK1), localizes to the surface of the ER. The ER-associated NAP1 is inactive because it displays little colocalization with the actin network, and ER localization requires neither activating signals from SPK1 nor a physical association with its W/SRC-binding partner, SRA1. Our results indicate that in Arabidopsis (Arabidopsis thaliana) leaf pavement cells and trichomes, the ER is a reservoir for W/SRC signaling and may have a key role in the early steps of W/SRC assembly and/or activation.

The cell morphogenesis ANGUSTIFOLIA (AN) gene, a plant homolog of CtBP/BARS, is involved in abiotic and biotic stress response in higher plants

BACKGROUND

ANGUSTIFOLIA (AN), one of the CtBP family proteins, plays a major role in microtubule-dependent cell morphogenesis. Microarray analysis of mammalian AN homologs suggests that AN might function as a transcriptional activator and regulator of a wide range of genes. Genetic characterization of AN mutants suggests that AN might be involved in multiple biological processes beyond cell morphology regulation.

RESULTS

Using a reverse genetic approach, we provide in this paper the genetic, biochemical, and physiological evidence for ANGUSTIFOLIA's role in other new biological functions such as abiotic and biotic stress response in higher plants. The T-DNA knockout an-t1 mutant exhibits not only all the phenotypes of previously described angustifolia null mutants, but also copes better than wild type under dehydration and pathogen attack. The stress tolerance is accompanied by a steady-state modulation of cellular H(2)O(2) content, malondialdehyde (MDA) derived from cellular lipid peroxidation, and over-expression of stress responsive genes. Our results indicate that ANGUSTIFOLIA functions beyond cell morphology control through direct or indirect functional protein interaction networks mediating other biological processes such as drought and pathogen attacks.

CONCLUSIONS

Our results indicate that the ANGUSTIFOLIA gene participates in several biochemical pathways controlling cell morphogenesis, abiotic, and biotic stress responses in higher plants. Our results suggest that the in vivo function of plant ANGUSTIFOLIA has been overlooked and it needs to be further studied beyond microtubule-dependent cell morphogenesis.

Identification of SHRUBBY, a SHORT-ROOT and SCARECROW interacting protein that controls root growth and radial patterning

The timing and extent of cell division is particularly important for the growth and development of multicellular organisms. Roots of the model organism Arabidopsis thaliana have been widely studied as a paradigm for organ development in plants. In the Arabidopsis root, the plant-specific GRAS family transcription factors SHORT-ROOT (SHR) and SCARECROW (SCR) are key regulators of root growth and of the asymmetric cell divisions that separate the ground tissue into two separate layers: the endodermis and cortex. To elucidate the role of SHR in root development, we identified 17 SHR-interacting proteins. Among those isolated was At5g24740, which we named SHRUBBY (SHBY). SHBY is a vacuolar sorting protein with similarity to the gene responsible for Cohen syndrome in humans. Hypomorphic alleles of shby caused poor root growth, decreased meristematic activity and defects in radial patterning that are characterized by an increase in the number of cell divisions in the ground tissue that lead to extra cells in the cortex and endodermis, as well as additional cell layers. Analysis of genetic and molecular markers indicates that SHBY acts in a pathway that partially overlaps with SHR, SCR, PLETHORA1 and PLETHORA2 (PLT1 and PLT2). The shby-1 root phenotype was partially phenocopied by treatment of wild-type roots with the proteosome inhibitor MG132 or the gibberellic acid (GA) synthesis inhibitor paclobutrazol (PAC). Our results indicate that SHBY controls root growth downstream of GA in part through the regulation of SHR and SCR.

ARP2/3-dependent growth in the plant kingdom: SCARs for life

In the human experience SCARs (suppressor of cAMP receptors) are permanent reminders of past events, not always based on bad decisions, but always those in which an interplay of opposing forces leaves behind a clear record in the form of some permanent watery mark. During plant morphogenesis, SCARs are important proteins that reflect an unusual evolutionary outcome, in which the plant kingdom relies heavily on this single class of actin-related protein (ARP) 2/3 complex activator to dictate the time and place of actin filament nucleation. This unusually simple arrangement may serve as a permanent reminder that cell shape control in plants is fundamentally different from that of crawling cells in mammals that use the power of actin polymerization to define and maintain cell shape. In plant cells, actin filaments indirectly affect cell shape by determining the transport properties of organelles and cargo molecules that modulate the mechanical properties of the wall. It is becoming increasingly clear that polarized bundles of actin filaments operate at whole cell spatial scales to organize the cytoplasm and dictate the patterns of long-distance intracellular transport and secretion. The number of actin-binding proteins and actin filament nucleators that are known to participate in the process of actin network formation are rapidly increasing. In plants, formins and ARP2/3 are two important actin filament nucleators. This review will focus on ARP2/3, and the apparent reliance of most plant species on the SCAR/WAVE (WASP family verprolin homologous) regulatory complex as the sole pathway for ARP2/3 activation.

Usefulness of Physcomitrella patens for studying plant organogenesis

In this chapter, we review the main organogenesis features and associated regulation processes of the moss Physcomitrella patens (P. patens), the model plant for the Bryophytes. We highlight how the study of this descendant of the earliest plant species that colonized earth, brings useful keys to understand the mechanisms that determine and control both vascular and non vascular plants organogenesis. Despite its simple morphogenesis pattern, P. patens still requires the fine tuning of organogenesis regulators, including hormone signalling, common to the whole plant kingdom, and which study is facilitated by a high number of molecular tools, among which the powerful possibility of gene targeting/replacement. The recent discovery of moss cells reprogramming capacity completes the picture of an excellent model for studying plant organogenesis.

Proteins from the FLOWERING LOCUS T-like subclade of the PEBP family act antagonistically to regulate floral initiation in tobacco

Flowering is an important agronomic trait that often depends on the integration of photoperiod, vernalization, gibberellin and/or autonomous signaling pathways by regulatory proteins such as FLOWERING LOCUS T (FT), a member of the phosphatidylethanolamine-binding protein (PEBP) family. Six PEBP family proteins control flowering in the model plant Arabidopsis thaliana, and their regulatory functions are well established, but variation in the number and structural diversity of PEBPs in different species means their precise functions must be determined on a case-by-case basis. We isolated four novel FT-like genes from Nicotiana tabacum (tobacco), and determined their expression profiles in wild-type plants and their overexpression phenotypes in transgenic plants. We found that all four genes were expressed in leaves under short-day conditions, and at least NtFT3 expression was restricted to phloem companion cells. We also found that the NtFT1, NtFT2 and NtFT3 proteins are floral inhibitors (atypical for FT-like proteins), whereas only NtFT4 is a floral inducer. We were unable to detect the expression of these genes under long-day conditions, suggesting that all four tobacco FT-like proteins may control flowering in response to short days. Phylogenetic analysis of PEBP family proteins and their functions in different solanaceous species confirmed that gene duplication and divergence within the FT-like clade has led to the evolution of antagonistic regulators that may help to fine-tune floral initiation in response to environmental cues.

Division polarity in developing stomata

Stomata are generated via asymmetric cell division in both dicots and monocots. Intrinsic or extrinsic polarity cues are perceived and acted upon to generate mother cell polarity and determine asymmetric division planes. Arabidopsis employs both intrinsic and extrinsic cues to orient a variable series of asymmetric stomatal divisions, using novel proteins such as BASL and POLAR to generate polarity. In contrast, maize appears to employ only extrinsic cues to orient the polarities of divisions occurring in an invariant sequence to generate stomatal complexes. Although both plants use receptor-like kinases to generate or orient division polarity in developing stomata, there are few similarities in the proteins and pathway identified to date as regulators of these processes.

Inhibition of actin polymerisation by low concentration Latrunculin B affects endocytosis and alters exocytosis in shank and tip of tobacco pollen tubes

Pollen tube growth depends on the integrity of the actin cytoskeleton that regulates cytoplasmic streaming and secretion. To clarify whether actin also plays a role in pollen tube endocytosis, Latrunculin B (LatB) was employed in internalisation experiments with tobacco pollen tubes, using the lipophilic dye FM4-64 and charged nanogold. Time-lapse analysis and dissection of endocytosis allowed us to identify internalisation pathways with different sensitivity to LatB. Co-localisation experiments and ultrastructural observations using positively charged nanogold revealed that LatB significantly inhibited endocytosis in the pollen tube shank, affecting internalisation of the plasma membrane (PM) recycled for secretion, as well as that conveyed to vacuoles. In contrast, endocytosis of negatively charged nanogold in the tip, which is also conveyed to vacuoles, was not influenced. Experiments of fluorescence recovery after photobleaching (FRAP) of the apical and subapical PM revealed domains with different rates of fluorescence recovery and showed that these differences depend on the actin cytoskeleton integrity. These results show the presence of distinct degradation pathways by demonstrating that actin-dependent and actin-indepedent endocytosis both operate in pollen tubes, internalising tracts of PM to be recycled and broken down. Intriguingly, although most studies concentrate on exocytosis and distension in the apex, the present paper shows that uncharacterised, actin-dependent secretory activity occurs in the shank of pollen tubes.

SCAR mediates light-induced root elongation in Arabidopsis through photoreceptors and proteasomes

The ARP2/3 complex, a highly conserved nucleator of F-actin, and its activator, the SCAR complex, are essential for growth in plants and animals. In this article, we present a pathway through which roots of Arabidopsis thaliana directly perceive light to promote their elongation. The ARP2/3-SCAR complex and the maintenance of longitudinally aligned F-actin arrays are crucial components of this pathway. The involvement of the ARP2/3-SCAR complex in light-regulated root growth is supported by our finding that mutants of the SCAR complex subunit BRK1/HSPC300, or other individual subunits of the ARP2/3-SCAR complex, showed a dramatic inhibition of root elongation in the light, which mirrored reduced growth of wild-type roots in the dark. SCAR1 degradation in dark-grown wild-type roots by constitutive photomorphogenic 1 (COP1) E3 ligase and 26S proteasome accompanied the loss of longitudinal F-actin and reduced root growth. Light perceived by the root photoreceptors, cryptochrome and phytochrome, suppressed COP1-mediated SCAR1 degradation. Taken together, our data provide a biochemical explanation for light-induced promotion of root elongation by the ARP2/3-SCAR complex.

BRANCHLESS TRICHOMES links cell shape and cell cycle control in Arabidopsis trichomes

Endoreplication, also called endoreduplication, is a modified cell cycle in which DNA is repeatedly replicated without subsequent cell division. Endoreplication is often associated with increased cell size and specialized cell shapes, but the mechanism coordinating DNA content with shape and size remains obscure. Here we identify the product of the BRANCHLESS TRICHOMES (BLT) gene, a protein of hitherto unknown function that has been conserved throughout angiosperm evolution, as a link in coordinating cell shape and nuclear DNA content in endoreplicated Arabidopsis trichomes. Loss-of-function mutations in BLT were found to enhance the multicellular trichome phenotype of mutants in the SIAMESE (SIM) gene, which encodes a repressor of endoreplication. Epistasis and overexpression experiments revealed that BLT encodes a key regulator of trichome branching. Additional experiments showed that BLT interacts both genetically and physically with STICHEL, another key regulator of trichome branching. Although blt mutants have normal trichome DNA content, overexpression of BLT results in an additional round of endoreplication, and blt mutants uncouple DNA content from morphogenesis in mutants with increased trichome branching, further emphasizing its role in linking cell shape and endoreplication.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

RTM3, which controls long-distance movement of potyviruses, is a member of a new plant gene family encoding a meprin and TRAF homology domain-containing protein

Restriction of long-distance movement of several potyviruses in Arabidopsis (Arabidopsis thaliana) is controlled by at least three dominant restricted TEV movement (RTM) genes, named RTM1, RTM2, and RTM3. RTM1 encodes a protein belonging to the jacalin family, and RTM2 encodes a protein that has similarities to small heat shock proteins. In this article, we describe the positional cloning of RTM3, which encodes a protein belonging to an undescribed protein family of 29 members that has a meprin and TRAF homology (MATH) domain in its amino-terminal region and a coiled-coil domain at its carboxy-terminal end. Involvement in the RTM resistance system is the first biological function experimentally identified for a member of this new gene family in plants. Our analyses showed that the coiled-coil domain is not only highly conserved between RTM3-homologous MATH-containing proteins but also in proteins lacking a MATH domain. The cluster organization of the RTM3 homologs in the Arabidopsis genome suggests the role of duplication events in shaping the evolutionary history of this gene family, including the possibility of deletion or duplication of one or the other domain. Protein-protein interaction experiments revealed RTM3 self-interaction as well as an RTM1-RTM3 interaction. However, no interaction has been detected involving RTM2 or the potyviral coat protein previously shown to be the determinant necessary to overcome the RTM resistance. Taken together, these observations strongly suggest the RTM proteins might form a multiprotein complex in the resistance mechanism to block the long-distance movement of potyviruses.

A role for ABIL3 in plant cell morphogenesis

Actin nucleation facilitated by the ARP2/3 complex plays a central role in plant cell shape development. The molecular characterization of the distorted class of trichome mutants has recently revealed the SCAR/WAVE complex as an essential upstream activator of ARP2/3 function in plants. The SCAR/WAVE complex is conserved from animals to plants and, generally, is composed of the five subunits SCAR/WAVE, PIR121, NAP125, BRICK and ABI. In plants, four of the five subunits have been shown to participate in trichome and pavement morphogenesis. Plant ABI-like proteins (ABIL), however, which constitute a small four-member protein family in Arabidopsis thaliana, have not been characterized functionally, so far. Here we demonstrate that microRNA knock-down of the ABIL3 gene leads to a distorted trichome phenotype reminiscent of ARP2/3 mutant phenotypes and consistent with a crucial role of the ABIL3 protein in an ARP2/3-activating SCAR/WAVE complex. In contrast to ARP2/3 mutants, however, the ABIL3 knock-down stimulated cell elongation in the root, indicating distinct functions of the ABIL3 protein in different tissues. Furthermore, we provide evidence that ABIL3 associates with microtubules in vivo, opening up the intriguing possibility that ABI-like proteins have a function in linking SCAR/WAVE-dependent actin nucleation with organization of the microtubule cytoskeleton.