Arabidopsis formin AtFH6 is a plasma membrane-associated protein upregulated in giant cells induced by parasitic nematodes

BLA1 Affects Leaf Angles by Altering Brassinosteroid Biosynthesis in Rice (Oryza sativa L.)

Brassinosteroids (BRs) are crucial plant hormones influencing diverse developmental processes in rice. While several enzymes in BR biosynthesis have been identified, their regulatory mechanisms remain largely unknown. This study highlights a novel regulatory pathway wherein the CHD3 chromatin remodeler, BLA1, epigenetically modulates the expression of key BR biosynthesis genes, BRD1 and D2. Phenotypic analysis of bla1 mutants revealed significant alterations, such as increased leaf angles and longer mesocotyls, which were alleviated by BR synthesis inhibitors. Moreover, the bla1 mutants showed elevated BR levels that correlated with the significant upregulation of the expression levels of BRD1 and D2, particularly at the lamina joint sites. Mechanistically, the yeast one-hybrid and chromatin immunoprecipitation assays revealed specific binding of BLA1 to the promoter regions of BRD1 and D2, accompanied by a marked enrichment of the transcriptionally active histone modification, H3K4me3, on these loci in the bla1 mutant. Functional assessments of the brd1 and d2 mutants confirmed their reduced sensitivity to BR, further underscoring their critical regulatory roles in BR-mediated developmental processes. Our findings uncovered an epigenetic mechanism that governs BR biosynthesis and orchestrates the expression of BRD1 and D2 to modulate BR levels and influence rice growth and development.

VND Genes Redundantly Regulate Cell Wall Thickening during Parasitic Nematode Infection

Plant parasitic root-knot nematodes are major agricultural pests worldwide, as they infect plant roots and cause substantial damages to crop plants. Root-knot nematodes induce specialized feeding cells known as giant cells (GCs) in the root vasculature, which serve as nutrient reservoirs for the infecting nematodes. Here, we show that the cell walls of GCs thicken to form pitted patterns that superficially resemble metaxylem cells. Interestingly, VASCULAR-RELATED NAC-DOMAIN1 (VND1) was found to be upregulated, while the xylem-type programmed cell death marker XYLEM CYSTEINE PEPTIDASE 1 was downregulated upon nematode infection. The vnd2 and vnd3 mutants showed reduced secondary cell wall pore size, while the vnd1 vnd2 vnd3 triple mutant produced significantly fewer nematode egg masses when compared with the wild type. These results suggest that the GC development pathway likely shares common signaling modules with the metaxylem differentiation pathway and VND1, VND2, and VND3 redundantly regulate plant-nematode interaction through secondary cell wall formation.

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

The status of the CRISPR/Cas9 research in plant-nematode interactions

MAIN CONCLUSION

As an important biotic stressor, plant-parasitic nematodes afflict global crop productivity. Deployment of CRISPR/Cas9 system that selectively knock out host susceptibility genes conferred improved nematode tolerance in crop plants. As an important biotic stressor, plant-parasitic nematodes cause a considerable yield decline in crop plants that eventually contributes to a negative impact on global food security. Being obligate plant parasites, the root-knot and cyst nematodes maintain an intricate and sophisticated relationship with their host plants by hijacking the host's physiological and metabolic pathways for their own benefit. Significant progress has been made toward developing RNAi-based transgenic crops that confer nematode resistance. However, the strategy of host-induced gene silencing that targets nematode effectors is likely to fail because the induced silencing of effectors (which interact with plant R genes) may lead to the development of nematode phenotypes that break resistance. Lately, the CRISPR/Cas9-based genome editing system has been deployed to achieve host resistance against bacteria, fungi, and viruses. In these studies, host susceptibility (S) genes were knocked out to achieve resistance via loss of susceptibility. As the S genes are recessively inherited in plants, induced mutations of the S genes are likely to be long-lasting and confer broad-spectrum resistance. A number of S genes contributing to plant susceptibility to nematodes have been identified in Arabidopsis thaliana, rice, tomato, cucumber, and soybean. A few of these S genes were targeted for CRISPR/Cas9-based knockout experiments to improve nematode tolerance in crop plants. Nevertheless, the CRISPR/Cas9 system was mostly utilized to interrogate the molecular basis of plant-nematode interactions rather than direct research toward achieving tolerance in crop plants. The current standalone article summarizes the progress made so far on CRISPR/Cas9 research in plant-nematode interactions.

Exploring the Role of the Plant Actin Cytoskeleton: From Signaling to Cellular Functions

The plant actin cytoskeleton is characterized by the basic properties of dynamic array, which plays a central role in numerous conserved processes that are required for diverse cellular functions. Here, we focus on how actins and actin-related proteins (ARPs), which represent two classical branches of a greatly diverse superfamily of ATPases, are involved in fundamental functions underlying signal regulation of plant growth and development. Moreover, we review the structure, assembly dynamics, and biological functions of filamentous actin (F-actin) from a molecular perspective. The various accessory proteins known as actin-binding proteins (ABPs) partner with F-actin to finely tune actin dynamics, often in response to various cell signaling pathways. Our understanding of the significance of the actin cytoskeleton in vital cellular activities has been furthered by comparison of conserved functions of actin filaments across different species combined with advanced microscopic techniques and experimental methods. We discuss the current model of the plant actin cytoskeleton, followed by examples of the signaling mechanisms under the supervision of F-actin related to cell morphogenesis, polar growth, and cytoplasmic streaming. Determination of the theoretical basis of how the cytoskeleton works is important in itself and is beneficial to future applications aimed at improving crop biomass and production efficiency.

Adaptor protein complex interaction map in Arabidopsis identifies P34 as a common stability regulator

Adaptor protein (AP) complexes are evolutionarily conserved vesicle transport regulators that recruit coat proteins, membrane cargoes and coated vesicle accessory proteins. As in plants endocytic and post-Golgi trafficking intersect at the trans-Golgi network, unique mechanisms for sorting cargoes of overlapping vesicular routes are anticipated. The plant AP complexes are part of the sorting machinery, but despite some functional information, their cargoes, accessory proteins and regulation remain largely unknown. Here, by means of various proteomics approaches, we generated the overall interactome of the five AP and the TPLATE complexes in Arabidopsis thaliana. The interactome converged on a number of hub proteins, including the thus far unknown adaptin binding-like protein, designated P34. P34 interacted with the clathrin-associated AP complexes, controlled their stability and, subsequently, influenced clathrin-mediated endocytosis and various post-Golgi trafficking routes. Altogether, the AP interactome network offers substantial resources for further discoveries of unknown endomembrane trafficking regulators in plant cells.

Formin protein DRT1 affects gross morphology and chloroplast relocation in rice

Plant height and tiller number are two major factors determining plant architecture and yield. However, in rice (Oryza sativa), the regulatory mechanism of plant architecture remains to be elucidated. Here, we reported a recessive rice mutant presenting dwarf and reduced tillering phenotypes (drt1). Map-based cloning revealed that the phenotypes are caused by a single point mutation in DRT1, which encodes the Class I formin protein O. sativa formin homolog 13 (OsFH13), binds with F-actin, and promotes actin polymerization for microfilament organization. DRT1 protein localized on the plasma membrane (PM) and chloroplast (CP) outer envelope. DRT1 interacted with rice phototropin 2 (OsPHOT2), and the interaction was interrupted in drt1. Upon blue light stimulus, PM localized DRT1 and OsPHOT2 were translocated onto the CP membrane. Moreover, deficiency of DRT1 reduced OsPHOT2 internalization and OsPHOT2-mediated CP relocation. Our study suggests that rice formin protein DRT1/OsFH13 is necessary for plant morphology and CP relocation by modulating the actin-associated cytoskeleton network.

The kinesin-13 protein BR HYPERSENSITIVE 1 is a negative brassinosteroid signaling component regulating rice growth and development

Phytohormones performed critical roles in regulating plant architecture and thus determine grain yield in rice. However, the roles of brassinosteroids (BRs) compared to other phytohormones in shaping rice architecture are less studied. In this study, we report that BR hypersensitive1 (BHS1) plays a negative role in BR signaling and regulate rice architecture. BHS1 encodes the kinesin-13a protein and regulates grain length. We found that bhs1 was hypersensitive to BR, while BHS1-overexpression was less sensitive to BR compare to WT. BHS1 was down-regulated at RNA and protein level upon exogenous BR treatment, and proteasome inhibitor MG132 delayed the BHS1 degradation, indicating that both the transcriptional and posttranscriptional regulation machineries are involved in BHS1-mediated regulation of plant growth and development. Furthermore, we found that the BR-induced degradation of BHS1 was attenuated in Osbri1 and Osbak1 mutants, but not in Osbzr1 and Oslic mutants. Together, these results suggest that BHS1 is a novel component which is involved in negative regulation of the BR signaling downstream player of BRI1.

Membrane nanodomains modulate formin condensation for actin remodeling in Arabidopsis innate immune responses

The assembly of macromolecules on the plasma membrane concentrates cell surface biomolecules into nanometer- to micrometer-scale clusters (nano- or microdomains) that help the cell initiate or respond to signals. In plant-microbe interactions, the actin cytoskeleton undergoes rapid remodeling during pathogen-associated molecular pattern-triggered immunity (PTI). The nanoclustering of formin-actin nucleator proteins at the cell surface has been identified as underlying actin nucleation during plant innate immune responses. Here, we show that the condensation of nanodomain constituents and the self-assembly of remorin proteins enables this mechanism of controlling formin condensation and activity during innate immunity in Arabidopsis thaliana. Through intrinsically disordered region-mediated remorin oligomerization and formin interaction, remorin gradually recruits and condenses formins upon PTI activation in lipid bilayers, consequently increasing actin nucleation in a time-dependent manner postinfection. Such nanodomain- and remorin-mediated regulation of plant surface biomolecules is expected to be a general feature of plant innate immune responses that creates spatially separated biochemical compartments and fine tunes membrane physicochemical properties for transduction of immune signals in the host.

Comprehensive analysis of formin gene family highlights candidate genes related to pollen cytoskeleton and male fertility in wheat (Triticum aestivum L.)

Background

Formin, a highly conserved multi-domain protein, interacts with microfilaments and microtubules. Although specifically expressed formin genes in anthers are potentially significant in research on male sterility and hybrid wheat breeding, similar reports in wheat, especially in thermo-sensitive genic male sterile (TGMS) wheat, remain elusive.

Results

Herein, we systematically characterized the formin genes in TGMS wheat line BS366 named TaFormins (TaFHs) and predicted their functions in inducing stress response. In total, 25 TaFH genes were uncovered, majorly localized in 2A, 2B, and 2D chromosomes. According to the neighbor-joining (NJ) method, all TaFH proteins from wheat and other plants clustered in 6 sub-groups (A-F). The modeled 3D structures of TaFH1-A/B, TaFH2-A/B, TaFH3-A/B and TaFH3-B/D were validated. And different numbers of stress and hormone-responsive regulatory elements in their 1500 base pair promoter regions were contained in the TaFH genes copies. TaFHs had specific temporal and spatial expression characteristics, whereby TaFH1, TaFH4, and TaFH5 were expressed highly in the stamen of BS366. Besides, the accumulation of TaFHs was remarkably lower in a low-temperature sterile condition (Nanyang) than fertile condition (Beijing), particularly at the early stamen development stage. The pollen cytoskeleton of BS366 was abnormal in the three stages under sterile and fertile environments. Furthermore, under different stress levels, TaFHs expression could be induced by drought, salt, abscisic acid (ABA), salicylic acid (SA), methyl jasmonate (MeJA), indole-3-acetic acid (IAA), polyethylene glycol (PEG), and low temperature. Some miRNAs, including miR167, miR1120, and miR172, interacts with TaFH genes; thus, we constructed an interaction network between microRNAs, TaFHs, phytohormone responses, and distribution of cytoskeleton to reveal the regulatory association between upstream genes of TaFH family members and sterile.

Conclusions

Collectively, this comprehensive analysis provides novel insights into TaFHs and miRNA resources for wheat breeding. These findings are, therefore, valuable in understanding the mechanism of TGMS fertility conversion in wheat.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07878-7.

Xanthomonas effector XopR hijacks host actin cytoskeleton via complex coacervation

The intrinsically disordered region (IDR) is a preserved signature of phytobacterial type III effectors (T3Es). The T3E IDR is thought to mediate unfolding during translocation into the host cell and to avoid host defense by sequence diversification. Here, we demonstrate a mechanism of host subversion via the T3E IDR. We report that the Xanthomonas campestris T3E XopR undergoes liquid-liquid phase separation (LLPS) via multivalent IDR-mediated interactions that hijack the Arabidopsis actin cytoskeleton. XopR is gradually translocated into host cells during infection and forms a macromolecular complex with actin-binding proteins at the cell cortex. By tuning the physical-chemical properties of XopR-complex coacervates, XopR progressively manipulates multiple steps of actin assembly, including formin-mediated nucleation, crosslinking of F-actin, and actin depolymerization, which occurs through competition for actin-depolymerizing factor and depends on constituent stoichiometry. Our findings unravel a sophisticated strategy in which bacterial T3E subverts the host actin cytoskeleton via protein complex coacervation.

Analysis of formin functions during cytokinesis using specific inhibitor SMIFH2

The phragmoplast separates daughter cells during cytokinesis by constructing the cell plate, which depends on interaction between cytoskeleton and membrane compartments. Proteins responsible for these interactions remain unknown, but formins can link cytoskeleton with membranes and several members of formin protein family localize to the cell plate. Progress in functional characterization of formins in cytokinesis is hindered by functional redundancies within the large formin gene family. We addressed this limitation by employing Small Molecular Inhibitor of Formin Homology 2 (SMIFH2), a small-molecule inhibitor of formins. Treatment of tobacco (Nicotiana tabacum) tissue culture cells with SMIFH2 perturbed localization of actin at the cell plate; slowed down both microtubule polymerization and phragmoplast expansion; diminished association of dynamin-related proteins with the cell plate independently of actin and microtubules; and caused cell plate swelling. Another impact of SMIFH2 was shortening of the END BINDING1b (EB1b) and EB1c comets on the growing microtubule plus ends in N. tabacum tissue culture cells and Arabidopsis thaliana cotyledon epidermis cells. The shape of the EB1 comets in the SMIFH2-treated cells resembled that of the knockdown mutant of plant Xenopus Microtubule-Associated protein of 215 kDa (XMAP215) homolog MICROTUBULE ORGANIZATION 1/GEMINI 1 (MOR1/GEM1). This outcome suggests that formins promote elongation of tubulin flares on the growing plus ends. Formins AtFH1 (A. thaliana Formin Homology 1) and AtFH8 can also interact with EB1. Besides cytokinesis, formins function in the mitotic spindle assembly and metaphase to anaphase transition. Our data suggest that during cytokinesis formins function in: (1) promoting microtubule polymerization; (2) nucleating F-actin at the cell plate; (3) retaining dynamin-related proteins at the cell plate; and (4) remodeling of the cell plate membrane.

Using natural variation to understand plant responses to iron availability

Iron bioavailability varies dramatically between soil types across the globe. This has given rise to high levels of natural variation in plant iron responses, allowing members of even a single species to thrive across a wide range of soil types. In recent years we have seen the use of genome-wide association analysis to identify natural variants underlying plant responses to changes in iron availability in both Arabidopsis and important crop species. These studies have provided insights into which genes have been important in shaping local adaptation to iron availability in different plant species and have allowed the discovery of novel regulators and mechanisms, not previously identified using mutagenesis approaches. Furthermore, these studies have allowed the identification of markers that can be used to accelerate breeding of future elite varieties with increased resilience to iron stress and improved nutritional quality. The studies highlighted here show that, in addition to studying plant responses to iron alone, it is important to consider these responses within the context of plant nutrition more broadly and to also consider iron regulation in relation to additional traits of agronomic importance such as yield and disease resistance.

Formin nanoclustering-mediated actin assembly during plant flagellin and DSF signaling

Plants respond to bacterial infection acutely with actin remodeling during plant immune responses. The mechanisms by which bacterial virulence factors (VFs) modulate plant actin polymerization remain enigmatic. Here, we show that plant-type-I formin serves as the molecular sensor for actin remodeling in response to two bacterial VFs: Xanthomonas campestris pv. campestris (Xcc) diffusible signal factor (DSF), and pathogen-associated molecular pattern (PAMP) flagellin in pattern-triggered immunity (PTI). Both VFs regulate actin assembly by tuning the clustering and nucleation activity of formin on the plasma membrane (PM) at the nano-sized scale. By being integrated within the cell-wall-PM-actin cytoskeleton (CW-PM-AC) continuum, the dynamic behavior and function of formins are highly dependent on each scaffold layer's composition within the CW-PM-AC continuum during both DSF and PTI signaling. Our results reveal a central mechanism for rapid actin remodeling during plant-bacteria interactions, in which bacterial signaling molecules fine tune plant formin nanoclustering in a host mechanical-structure-dependent manner.

An Update on the Role of the Actin Cytoskeleton in Plasmodesmata: A Focus on Formins

Cell-to-cell communication in plants is mediated by plasmodesmata (PD) whose permeability is tightly regulated during plant growth and development. The actin cytoskeleton has been implicated in regulating the permeability of PD, but the underlying mechanism remains largely unknown. Recent characterization of PD-localized formin proteins has shed light on the role and mechanism of action of actin in regulating PD-mediated intercellular trafficking. In this mini-review article, we will describe the progress in this area.

A wheat cysteine-rich receptor-like kinase confers broad-spectrum resistance against Septoria tritici blotch

The poverty of disease resistance gene reservoirs limits the breeding of crops for durable resistance against evolutionary dynamic pathogens. Zymoseptoria tritici which causes Septoria tritici blotch (STB), represents one of the most genetically diverse and devastating wheat pathogens worldwide. No fully virulent Z. tritici isolates against synthetic wheats carrying the major resistant gene Stb16q have been identified. Here, we use comparative genomics, mutagenesis and complementation to identify Stb16q, which confers broad-spectrum resistance against Z. tritici. The Stb16q gene encodes a plasma membrane cysteine-rich receptor-like kinase that was recently introduced into cultivated wheat and which considerably slows penetration and intercellular growth of the pathogen.

β-Hexosaminidases Along the Secretory Pathway of Nicotiana benthamiana Have Distinct Specificities Toward Engineered Helminth N-Glycans on Recombinant Glycoproteins

Secretions of parasitic worms (helminths) contain a wide collection of immunomodulatory glycoproteins with the potential to treat inflammatory disorders, like autoimmune diseases. Yet, the identification of single molecules that can be developed into novel biopharmaceuticals is hampered by the limited availability of native parasite-derived proteins. Recently, pioneering work has shown that helminth glycoproteins can be produced transiently in Nicotiana benthamiana plants while simultaneously mimicking their native helminth N-glycan composition by co-expression of desired glycosyltransferases. However, efficient "helminthization" of N-glycans in plants by glyco-engineering seems to be hampered by the undesired truncation of complex N-glycans by β-N-acetyl-hexosaminidases, in particular when aiming for the synthesis of N-glycans with antennary GalNAcβ1-4GlcNAc (LacdiNAc or LDN). In this study, we cloned novel β-hexosaminidase open reading frames from N. benthamiana and characterized the biochemical activity of these enzymes. We identified HEXO2 and HEXO3 as enzymes responsible for the cleavage of antennary GalNAc residues of N-glycans on the model helminth glycoprotein kappa-5. Furthermore, we reveal that each member of the HEXO family has a distinct specificity for N-glycan substrates, where HEXO2 has strict β-galactosaminidase activity, whereas HEXO3 cleaves both GlcNAc and GalNAc. The identification of HEXO2 and HEXO3 as major targets for LDN cleavage will enable a targeted genome editing approach to reduce undesired processing of these N-glycans. Effective knockout of these enzymes could allow the production of therapeutically relevant glycoproteins with tailor-made helminth N-glycans in plants.

Multi-omics analysis revealed that MAPK signaling and flavonoid metabolic pathway contributed to resistance against Meloidogyne incognita in the introgression line cucumber

Inhibiting giant cells (GCs) development is an important characteristic of introgression line cucumber IL10-1 against Meloidogyne incognita proved by our previous study, but the systematic regulatory pathways were unknown. To reveal the regulation pathways more comprehensively, in the current study, we performed a joint analysis of RNA-Seq and lable-free quantitative proteomics between the IL10-1 (resistant) and the CC3 (susceptible cucumber) after inoculation with M. incognita. The results indicated that flavonoid biosynthesis pathway was specifically enriched in IL10-1. And protein species of Csa5P590220 and Csa5P589940 associated with flavonoid biosynthesis were highly translated in IL10-1 compared with these in CC3 at 3 days post inoculation (dpi), which would resulted in the excess of flavonoids in IL10-1 roots. In addition, phosphoproteomic analysis found that phosphorylated protein species involved in MAPK signaling cascade were enhanced in IL10-1, while they were inhibited in CC3 at 3 dpi. Accordingly, we speculate that the enhanced MAPK cascade signaling plays an important role in signal transduction for IL10-1 regulating the flavonoid biosynthesis. Knowledge from the study provide important regulatory pathways and protein species of introgression line cucumber against M. incognita, which will help in efforts to improve the recognition of the resistance mechanism of plants against nematode. SIGNIFICANCE: The current approach of joint analysis in transcription level, protein level and protein phosphorylation level more comprehensively revealed the different expression patterns at the molecular level of resistant and susceptible cucumber after inoculation with M. incognita. Based on the different expression patterns, we explore the pathway of resistance regulation of resistant cucumber IL10-1. Moreover, our results are helpful for the discovery of key genes and then apply them to M. incognita-resistance breeding.

Arabidopsis Class II Formins AtFH13 and AtFH14 Can Form Heterodimers but Exhibit Distinct Patterns of Cellular Localization

Formins are evolutionarily conserved multi-domain proteins participating in the control of both actin and microtubule dynamics. Angiosperm formins form two evolutionarily distinct families, Class I and Class II, with class-specific domain layouts. The model plant Arabidopsis thaliana has 21 formin-encoding loci, including 10 Class II members. In this study, we analyze the subcellular localization of two A. thaliana Class II formins exhibiting typical domain organization, the so far uncharacterized formin AtFH13 (At5g58160) and its distant homolog AtFH14 (At1g31810), previously reported to bind microtubules. Fluorescent protein-tagged full length formins and their individual domains were transiently expressed in Nicotiana benthamiana leaves under the control of a constitutive promoter and their subcellular localization (including co-localization with cytoskeletal structures and the endoplasmic reticulum) was examined using confocal microscopy. While the two formins exhibit distinct and only partially overlapping localization patterns, they both associate with microtubules via the conserved formin homology 2 (FH2) domain and with the periphery of the endoplasmic reticulum, at least in part via the N-terminal PTEN (Phosphatase and Tensin)-like domain. Surprisingly, FH2 domains of AtFH13 and AtFH14 can form heterodimers in the yeast two-hybrid assay-a first case of potentially biologically relevant formin heterodimerization mediated solely by the FH2 domain.

Systems genomics approaches provide new insights into Arabidopsis thaliana root growth regulation under combinatorial mineral nutrient limitation

The molecular mechanisms by which plants modulate their root growth rate (RGR) in response to nutrient deficiency are largely unknown. Using Arabidopsis thaliana accessions, we analyzed RGR variation under combinatorial mineral nutrient deficiencies involving phosphorus (P), iron (Fe), and zinc (Zn). While -P stimulated early RGR of most accessions, -Fe or -Zn reduced it. The combination of either -P-Fe or -P-Zn led to suppression of the growth inhibition exerted by -Fe or -Zn alone. Surprisingly, root growth responses of the reference accession Columbia (Col-0) were not representative of the species under -P nor -Zn. Using a systems approach that combines GWAS, network-based candidate identification, and reverse genetic screen, we identified new genes that regulate root growth in -P-Fe: VIM1, FH6, and VDAC3. Our findings provide a framework to systematically identifying favorable allelic variations to improve root growth, and to better understand how plants sense and respond to multiple environmental cues.

Plants thrive in highly heterogenous soils. How they compute a multitude of contrasting stimuli and mount an adaptive response without a centralized information processing unit is an intriguing question. For instance, below ground, roots can sense and respond to the single or multiple nutrient stresses, and adjust its growth rate accordingly. Nevertheless, the genetic architecture of root growth responses under single and combined stress remains poorly understood. To fill this gap in our understanding about such crucial phenomenon for plant survival, we explored the natural variation of root growth rate (RGR) in Arabidopsis grown under single and combined nutritional stress, including deficiencies of iron (-Fe), zinc (-Zn), phosphate and iron (-P-Fe) and phosphate and zinc (-P-Zn). Our GWAS revealed distinct genetic architectures underlying root growth responses to single or combined nutrient stresses. By integrating GWAS and coexpression networks, we identified and validated genes controlling the variation of root growth to combined nutrient-deficiency, namely VARIANT IN METHYLATION 1, FORMIN-LIKE-PROTEIN-6 and VOLTAGE-DEPENDENT ANION-SELECTIVE CHANNEL PROTEIN 3. Our findings provide a framework to accelerate future research aiming at better understanding how plants sense and respond to multiple environmental inputs, and promise to help designing new agronomical and biotechnological strategies to improve root growth.

Arabidopsis Class I Formin FH1 Relocates between Membrane Compartments during Root Cell Ontogeny and Associates with Plasmodesmata

Formins are evolutionarily conserved eukaryotic proteins engaged in actin nucleation and other aspects of cytoskeletal organization. Angiosperms have two formin clades with multiple paralogs; typical plant Class I formins are integral membrane proteins that can anchor cytoskeletal structures to membranes. For the main Arabidopsis housekeeping Class I formin, FH1 (At3g25500), plasmalemma localization was documented in heterologous expression and overexpression studies. We previously showed that loss of FH1 function increases cotyledon epidermal pavement cell shape complexity via modification of actin and microtubule organization and dynamics. Here, we employ transgenic Arabidopsis expressing green fluorescent protein-tagged FH1 (FH1-GFP) from its native promoter to investigate in vivo behavior of this formin using advanced microscopy techniques. The fusion protein is functional, since its expression complements the fh1 loss-of-function mutant phenotype. Accidental overexpression of FH1-GFP results in a decrease in trichome branch number, while fh1 mutation has the opposite effect, indicating a general role of this formin in controlling cell shape complexity. Consistent with previous reports, FH1-GFP associates with membranes. However, the protein exhibits surprising actin- and secretory pathway-dependent dynamic localization and relocates between cellular endomembranes and the plasmalemma during cell division and differentiation in root tissues, with transient tonoplast localization at the transition/elongation zones border. FH1-GFP also accumulates in actin-rich regions of cortical cytoplasm and associates with plasmodesmata in both the cotyledon epidermis and root tissues. Together with previous reports from metazoan systems, this suggests that formins might have a shared (ancestral or convergent) role at cell-cell junctions.

Arabidopsis class I formins control membrane-originated actin polymerization at pollen tube tips

A population of dynamic apical actin filaments is required for rapid polarized pollen tube growth. However, the cellular mechanisms driving their assembly remain incompletely understood. It was postulated that formin is a major player in nucleating apical actin assembly, but direct genetic and cytological evidence remains to be firmly established. Here we found that both Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5) are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes, with AtFH3 playing a more dominant role. We found that both formins have plasma membrane (PM) localization signals but exhibit distinct PM localization patterns in the pollen tube, and loss of their function reduces the amount of apical actin filaments. Live-cell imaging revealed that the reduction in filamentous actin is very likely due to the decrease in filament elongation. Furthermore, we found that the rate of tip-directed vesicle transport is reduced and the pattern of apical vesicle accumulation is altered in formin loss-of-function mutant pollen tubes, which explains to some extent the reduction in pollen tube elongation. Thus, we provide direct genetic and cytological evidence showing that formin is an important player in nucleating actin assembly from the PM at pollen tube tips.

Actin polymerization has been implicated in the regulation of rapid polarized pollen tube growth. The important role of actin polymerization is well appreciated, but the mechanisms that regulate rapid actin polymerization in pollen tubes remain incompletely understood. It was postulated that one of the major actin polymerization pathways in pollen tubes involves formin/profilin modules. However, direct genetic and cytological evidence is still required to support the role of formin in this framework. Using state-of-the-art live-cell imaging in combination with reverse genetic approaches, we demonstrate here that two class I formins, Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5), are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes. In support of the role of AtFH3 and AtFH5 in regulating membrane-originated apical actin polymerization, we found that both of them are localized to the plasma membrane (PM) at pollen tube tips. Live-cell imaging revealed that the reduction in filamentous actin is very likely due to the decrease in elongation of actin filaments originating from the apical membrane. We also found that AtFH3 and AtFH5 exhibit distinct PM localization patterns in the pollen tube, suggesting that they might have distinct roles in regulating actin polymerization in pollen tubes. Our study provides direct genetic and cytological evidence that formins act as important players in regulating apical actin assembly in pollen tubes.

Genetic Architecture of Soybean Yield and Agronomic Traits

Soybean is the world's leading source of vegetable protein and demand for its seed continues to grow. Breeders have successfully increased soybean yield, but the genetic architecture of yield and key agronomic traits is poorly understood. We developed a 40-mating soybean nested association mapping (NAM) population of 5,600 inbred lines that were characterized by single nucleotide polymorphism (SNP) markers and six agronomic traits in field trials in 22 environments. Analysis of the yield, agronomic, and SNP data revealed 23 significant marker-trait associations for yield, 19 for maturity, 15 for plant height, 17 for plant lodging, and 29 for seed mass. A higher frequency of estimated positive yield alleles was evident from elite founder parents than from exotic founders, although unique desirable alleles from the exotic group were identified, demonstrating the value of expanding the genetic base of US soybean breeding.

Arabidopsis formin 2 regulates cell-to-cell trafficking by capping and stabilizing actin filaments at plasmodesmata

Here, we demonstrate that Arabidopsis thaliana Formin 2 (AtFH2) localizes to plasmodesmata (PD) through its transmembrane domain and is required for normal intercellular trafficking. Although loss-of-function atfh2 mutants have no overt developmental defect, PD's permeability and sensitivity to virus infection are increased in atfh2 plants. Interestingly, AtFH2 functions in a partially redundant manner with its closest homolog AtFH1, which also contains a PD localization signal. Strikingly, targeting of Class I formins to PD was also confirmed in rice, suggesting that the involvement of Class I formins in regulating actin dynamics at PD may be evolutionarily conserved in plants. In vitro biochemical analysis showed that AtFH2 fails to nucleate actin assembly but caps and stabilizes actin filaments. We also demonstrate that the interaction between AtFH2 and actin filaments is crucial for its function in vivo. These data allow us to propose that AtFH2 regulates PD's permeability by anchoring actin filaments to PD.

The root-knot nematode effector MiPFN3 disrupts plant actin filaments and promotes parasitism

Root-knot nematodes secrete effectors that manipulate their host plant cells so that the nematode can successfully establish feeding sites and complete its lifecycle. The root-knot nematode feeding structures, their “giant cells,” undergo extensive cytoskeletal remodeling. Previous cytological studies have shown the cytoplasmic actin within the feeding sites looks diffuse. In an effort to study root-knot nematode effectors that are involved in giant cell organogenesis, we have identified a nematode effector called MiPFN3 (Meloidogyne incognita Profilin 3). MiPFN3 is transcriptionally up-regulated in the juvenile stage of the nematode. In situ hybridization experiments showed that MiPFN3 transcribed in the nematode subventral glands, where it can be secreted by the nematode stylet into the plant. Moreover, Arabidopsis plants that heterologously expressed MiPFN3 were more susceptible to root-knot nematodes, indicating that MiPFN3 promotes nematode parasitism. Since profilin proteins can bind and sequester actin monomers, we investigated the function of MiPFN3 in relation to actin. Our results show that MiPFN3 suppressed the aberrant plant growth phenotype caused by the misexpression of reproductive actin (AtACT1) in transgenic plants. In addition, it disrupted actin polymerization in an in vitro assay, and it reduced the filamentous actin network when expressed in Arabidopsis protoplasts. Over a decade ago, cytological studies showed that the cytoplasmic actin within nematode giant cells looked fragmented. Here we provide the first evidence that the nematode is secreting an effector that has significant, direct effects on the plant’s actin cytoskeleton.

Root-knot nematodes are microscopic plant pests that infect plant roots and significantly reduce yields of many crop plants. The nematodes enter the plant roots and modify plant cells into complex, multinuclear feeding sites called giant cells. The formation and maintenance of giant cells is critical to nematode survival. During giant cell organogenesis, the progenitor plant cells undergo many morphological changes, including a re-organization of the cytoplasmic actin cytoskeleton. As a result, the giant cell cytoplasmic actin appears fragmented and disorganized. Plant cells can regulate their actin filament assembly, in part, through the expression of actin binding proteins such as profilins. Here we show that infectious nematode juveniles express a profilin called MiPFN3. Expression of MiPFN3 in Arabidopsis plants made the plants more susceptible to root-knot nematodes, indicating that MiPFN3 acts as an effector that aids parasitism. We show evidence that the expression MiPFN3 in plant cells causes the fragmentation of plant actin filaments. The work here demonstrates that nematode effector MiPFN3 can directly affect plant actin filaments, whose reorganization is necessary for giant cell formation.

An ancient Sec10-formin fusion provides insights into actin-mediated regulation of exocytosis

Interactions between actin nucleators and the exocyst in yeast and mammals control membrane remodeling. van Gisbergen et al. now describe For1F, a fusion of an exocyst subunit (Sec10) and an actin nucleation factor (formin), retained in the moss lineage for more than 170 million years, which provides unique insight into the regulation of exocytosis by actin.

Exocytosis, facilitated by the exocyst, is fundamentally important for remodeling cell walls and membranes. Here, we analyzed For1F, a novel gene that encodes a fusion of an exocyst subunit (Sec10) and an actin nucleation factor (formin). We showed that the fusion occurred early in moss evolution and has been retained for more than 170 million years. In Physcomitrella patens, For1F is essential, and the expressed protein is a fusion of Sec10 and formin. Reduction of For1F or actin filaments inhibits exocytosis, and For1F dynamically associates with Sec6, another exocyst subunit, in an actin-dependent manner. Complementation experiments demonstrate that constitutive expression of either half of the gene or the paralogous Sec10b rescues loss of For1F, suggesting that fusion of the two domains is not essential, consistent with findings in yeast, where formin and the exocyst are linked noncovalently. Although not essential, the fusion may have had selective advantages and provides a unique opportunity to probe actin regulation of exocytosis.

Identification, Validation and Utilization of Novel Nematode-Responsive Root-Specific Promoters in Arabidopsis for Inducing Host-Delivered RNAi Mediated Root-Knot Nematode Resistance

The root-knot nematode (RKN), Meloidogyne incognita, is an obligate, sedentary endoparasite that infects a large number of crops and severely affects productivity. The commonly used nematode control strategies have their own limitations. Of late, RNA interference (RNAi) has become a popular approach for the development of nematode resistance in plants. Transgenic crops capable of expressing dsRNAs, specifically in roots for disrupting the parasitic process, offer an effective and efficient means of producing resistant crops. We identified nematode-responsive and root-specific (NRRS) promoters by using microarray data from the public domain and known conserved cis-elements. A set of 51 NRRS genes was identified which was narrowed down further on the basis of presence of cis-elements combined with minimal expression in the absence of nematode infection. The comparative analysis of promoters from the enriched NRRS set, along with earlier reported nematode-responsive genes, led to the identification of specific cis-elements. The promoters of two candidate genes were used to generate transgenic plants harboring promoter GUS constructs and tested in planta against nematodes. Both promoters showed preferential expression upon nematode infection, exclusively in the root in one and galls in the other. One of these NRRS promoters was used to drive the expression of splicing factor, a nematode-specific gene, for generating host-delivered RNAi-mediated nematode-resistant plants. Transgenic lines expressing dsRNA of splicing factor under the NRRS promoter exhibited upto a 32% reduction in number of galls compared to control plants.

LlFH1-mediated interaction between actin fringe and exocytic vesicles is involved in pollen tube tip growth

Pollen tube tip growth is an extreme form of polarized cell growth, which requires polarized exocytosis based on a dynamic actin cytoskeleton. However, the molecular basis for the connection between actin filaments and exocytic vesicles is unclear. Here, we identified a Lilium longiflorum pollen-specific formin (LlFH1) and found that it localized at the apical vesicles and plasma membrane (PM). Overexpression of LlFH1 induced excessive actin cables in the tube tip region, and downregulation of LlFH1 eliminated the actin fringe. Fluorescence recovery after photobleaching (FRAP) analysis revealed that LlFH1-labeled exocytic vesicles exhibited an initial accumulation at the shoulder of the apex and coincided with the leading edge of the actin fringe. Time-lapse analysis suggested that nascent actin filaments followed the emergence of the apical vesicles, implying that LlFH1 at apical vesicles could initiate actin polymerization. Biochemical assays showed that LlFH1 FH1FH2 could nucleate actin polymerization, but then capped the actin filament at the barbed end and inhibited its elongation. However, in the presence of lily profilins, LlFH1 FH1FH2 could accelerate barbed-end actin elongation. In addition, LlFH1 FH1FH2 was able to bundle actin filaments. Thus, we propose that LlFH1 and profilin coordinate the interaction between the actin fringe and exocytic vesicle trafficking during pollen tube growth of lily.

The Infection of Cucumber (Cucumis sativus L.) Roots by Meloidogyne incognita Alters the Expression of Actin-Depolymerizing Factor (ADF) Genes, Particularly in Association with Giant Cell Formation

Cucumber (Cucumis sativus L.) is threatened by substantial yield losses due to the south root-knot nematode (Meloidogyne incognita). However, understanding of the molecular mechanisms underlying the process of nematode infection is still limited. In this study, we found that M. incognita infection affected the structure of cells in cucumber roots and treatment of the cytoskeleton inhibitor (cytochalasin D) reduced root-knot nematode (RKN) parasitism. It is known that Actin-Depolymerizing Factor (ADF) affects cell structure, as well as the organization of the cytoskeleton. To address the hypothesis that nematode-induced abnormal cell structures and cytoskeletal rearrangements might be mediated by the ADF genes, we identified and characterized eight cucumber ADF (CsADF) genes. Phylogenetic analysis showed that the cucumber ADF gene family is grouped into four ancient subclasses. Expression analysis revealed that CsADF1, CsADF2-1, CsADF2-2, CsADF2-3 (Subclass I), and CsADF6 (Subclass III) have higher transcript levels than CsADF7-1, CsADF7-2 (Subclass II genes), and CsADF5 (Subclass IV) in roots. Members of subclass I genes (CsADF1, CsADF2-1, CsADF2-2, and CsADF2-3), with the exception of CsADF2-1, exhibited a induction of expression in roots 14 days after their inoculation (DAI) with nematodes. However, the expression of subclass II genes (CsADF7-1 and CsADF7-2) showed no significant change after inoculation. The transcript levels of CsADF6 (Subclass III) showed a specific induction at 21 DAI, while CsADF5 (Subclass IV) was weakly expressed in roots, but was strongly up-regulated as early as 7 DAI. In addition, treatment of roots with cytochalasin D caused an approximately 2-fold down-regulation of the CsADF genes in the treated plants. These results suggest that CsADF gene mediated actin dynamics are associated with structural changes in roots as a consequence of M. incognita infection.

Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease

Secreted peptides and their specific receptors frequently orchestrate cell-to-cell communication in plants. Phytosulfokines (PSKs) are secreted tyrosine-sulphated peptide hormones, which trigger cellular dedifferentiation and redifferentiation upon binding to their membrane receptor. Biotrophic plant pathogens frequently trigger the differentiation of host cells into specialized feeding structures, which are essential for successful infection. We found that oomycete and nematode infections were characterized by the tissue-specific transcriptional regulation of genes encoding Arabidopsis PSKs and the PSK receptor 1 (PSKR1). Subcellular analysis of PSKR1 distribution showed that the plasma membrane-bound receptor internalizes after binding of PSK-α. Arabidopsis pskr1 knockout mutants were impaired in their susceptibility to downy mildew infection. Impaired disease susceptibility depends on functional salicylic acid (SA) signalling, but not on the massive up-regulation of SA-associated defence-related genes. Knockout pskr1 mutants also displayed a major impairment of root-knot nematode reproduction. In the absence of functional PSKR1, giant cells arrested their development and failed to fully differentiate. Our findings indicate that the observed restriction of PSK signalling to cells surrounding giant cells contributes to the isotropic growth and maturation of nematode feeding sites. Taken together, our data suggest that PSK signalling in Arabidopsis promotes the differentiation of host cells into specialized feeding cells.

An update on cell surface proteins containing extensin-motifs

In recent years it has become clear that there are several molecular links that interconnect the plant cell surface continuum, which is highly important in many biological processes such as plant growth, development, and interaction with the environment. The plant cell surface continuum can be defined as the space that contains and interlinks the cell wall, plasma membrane and cytoskeleton compartments. In this review, we provide an updated view of cell surface proteins that include modular domains with an extensin (EXT)-motif followed by a cytoplasmic kinase-like domain, known as PERKs (for proline-rich extensin-like receptor kinases); with an EXT-motif and an actin binding domain, known as formins; and with extracellular hybrid-EXTs. We focus our attention on the EXT-motifs with the short sequence Ser-Pro(3-5), which is found in several different protein contexts within the same extracellular space, highlighting a putative conserved structural and functional role. A closer understanding of the dynamic regulation of plant cell surface continuum and its relationship with the downstream signalling cascade is a crucial forthcoming challenge.

Quantitative analyses on dynamic changes in the organization of host Arabidopsis thaliana actin microfilaments surrounding the infection organ of the powdery mildew fungus Golovinomyces orontii

Obligate biotrophic fungi that cause powdery mildew on host plants form a specialized infection organ called the haustorium in the host apoplast. It was previously reported that the haustorium is surrounded by host actin microfilaments (AFs). The previous study used fixed cells, in which AFs were stained with fluorescently labeled phalloidine, therefore the structural dynamics of haustorium-surrounding AFs has not been examined. In the present study, we performed a live imaging analysis to examine the dynamics and developmental changes in the organization of haustorium-surrounding host AFs using host Arabidopsis thaliana and A. thaliana-adapted powdery mildew fungus Golovinomyces orontii. Image correlation-based velocimetry analysis suggested that AFs around haustorium are rather static compared to the dynamicity of AFs at the cell surface. Quantification of AF density and bundling showed that the density, but not the level of bundling, of haustorium-surrounding AFs increased as the haustorium matures. The possible role of AFs around haustoria is discussed.

Gall-forming root-knot nematodes hijack key plant cellular functions to induce multinucleate and hypertrophied feeding cells

Among plant-parasitic nematodes, the root-knot nematodes (RKNs) of the Meloidogyne spp. are the most economically important genus. RKN are root parasitic worms able to infect nearly all crop species and have a wide geographic distribution. During infection, RKNs establish and maintain an intimate relationship with the host plant. This includes the creation of a specialized nutritional structure composed of multinucleate and hypertrophied giant cells, which result from the redifferentiation of vascular root cells. Giant cells constitute the sole source of nutrients for the nematode and are essential for growth and reproduction. Hyperplasia of surrounding root cells leads to the formation of the gall or root-knot, an easily recognized symptom of plant infection by RKNs. Secreted effectors produced in nematode salivary glands and injected into plant cells through a specialized feeding structure called the stylet play a critical role in the formation of giant cells. Here, we describe the complex interactions between RKNs and their host plants. We highlight progress in understanding host plant responses, focusing on how RKNs manipulate key plant processes and functions, including cell cycle, defence, hormones, cellular scaffold, metabolism and transport.

The connection of cytoskeletal network with plasma membrane and the cell wall

The cell wall provides external support of the plant cells, while the cytoskeletons including the microtubules and the actin filaments constitute an internal framework. The cytoskeletons contribute to the cell wall biosynthesis by spatially and temporarily regulating the transportation and deposition of cell wall components. This tight control is achieved by the dynamic behavior of the cytoskeletons, but also through the tethering of these structures to the plasma membrane. This tethering may also extend beyond the plasma membrane and impact on the cell wall, possibly in the form of a feedback loop. In this review, we discuss the linking components between the cytoskeletons and the plasma membrane, and/or the cell wall. We also discuss the prospective roles of these components in cell wall biosynthesis and modifications, and aim to provide a platform for further studies in this field.

Maturation of nematode-induced galls in Medicago truncatula is related to water status and primary metabolism modifications

Root-knot nematodes are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, these nematodes induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells (GCs). These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. We analyzed the modifications of water status, ionic content and accumulation of metabolites in development and mature galls induced by Meloidogyne incognita and in uninfected roots of Medicago truncatula plants. Water potential and osmotic pressure are significantly modified in mature galls compared to developing galls and control roots. Ionic content is significantly modified in galls compared to roots. Principal component analyses of metabolite content showed that mature gall metabolism is significantly modified compared to developing gall metabolism. The most striking differences were the three-fold increase of trehalose content associated to the five-fold diminution in glucose concentration in mature galls. Gene expression analysis showed that trehalose accumulation was, at least, partially linked to a significantly lower expression of the trehalase gene in mature galls. Our results point to significant modifications of gall physiology during maturation.

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.

Formins: linking cytoskeleton and endomembranes in plant cells

The cytoskeleton plays a central part in spatial organization of the plant cytoplasm, including the endomebrane system. However, the mechanisms involved are so far only partially understood. Formins (FH2 proteins), a family of evolutionarily conserved proteins sharing the FH2 domain whose dimer can nucleate actin, mediate the co-ordination between actin and microtubule cytoskeletons in multiple eukaryotic lineages including plants. Moreover, some plant formins contain transmembrane domains and participate in anchoring cytoskeletal structures to the plasmalemma, and possibly to other membranes. Direct or indirect membrane association is well documented even for some fungal and metazoan formins lacking membrane insertion motifs, and FH2 proteins have been shown to associate with endomembranes and modulate their dynamics in both fungi and metazoans. Here we summarize the available evidence suggesting that formins participate in membrane trafficking and endomembrane, especially ER, organization also in plants. We propose that, despite some methodological pitfalls inherent to in vivo studies based on (over)expression of truncated and/or tagged proteins, formins are beginning to emerge as candidates for the so far somewhat elusive link between the plant cytoskeleton and the endomembrane system.

Heterodimeric capping protein from Arabidopsis is a membrane-associated, actin-binding protein

The actin cytoskeleton is a major regulator of cell morphogenesis and responses to biotic and abiotic stimuli. The organization and activities of the cytoskeleton are choreographed by hundreds of accessory proteins. Many actin-binding proteins are thought to be stimulus-response regulators that bind to signaling phospholipids and change their activity upon lipid binding. Whether these proteins associate with and/or are regulated by signaling lipids in plant cells remains poorly understood. Heterodimeric capping protein (CP) is a conserved and ubiquitous regulator of actin dynamics. It binds to the barbed end of filaments with high affinity and modulates filament assembly and disassembly reactions in vitro. Direct interaction of CP with phospholipids, including phosphatidic acid, results in uncapping of filament ends in vitro. Live-cell imaging and reverse-genetic analyses of cp mutants in Arabidopsis (Arabidopsis thaliana) recently provided compelling support for a model in which CP activity is negatively regulated by phosphatidic acid in vivo. Here, we used complementary biochemical, subcellular fractionation, and immunofluorescence microscopy approaches to elucidate CP-membrane association. We found that CP is moderately abundant in Arabidopsis tissues and present in a microsomal membrane fraction. Sucrose density gradient separation and immunoblotting with known compartment markers were used to demonstrate that CP is enriched on membrane-bound organelles such as the endoplasmic reticulum and Golgi. This association could facilitate cross talk between the actin cytoskeleton and a wide spectrum of essential cellular functions such as organelle motility and signal transduction.

Robust in vitro assay system for quantitative analysis of parasitic root-knot nematode infestation using Lotus japonicus

Root-knot nematodes are sedentary endoparasites that induce permanent infestation sites inside the roots of a broad range of crop plants. The development of effective control strategies require understanding the root-knot nematode parasitic process, however, the key molecular determinants for host manipulation during infestation remain elusive. One limiting factor has been the lack of a standardized conventional method for quantitative measurement of host parasitism by root-knot nematodes, particularly one that enables efficient downstream analyses and is free from other biological sources of variability. We report here a robust, highly reproducible system for quantitative analysis of all stages of root-knot nematode infestation using the legume Lotus japonicus as the plant host. This system provides a high quality nematode inoculum that maintains consistency in juvenile age and viability even between independently prepared populations. An optimized root transformation protocol was also developed for L. japonicus to facilitate downstream molecular studies in conjunction with the quantitative assay. Hairy root transformation efficiencies up to 91% were achieved. Root-knot nematodes formed egg masses at the root surface of both intact plants and transgenic hairy root cultures within eight weeks, confirming the assay conditions support an efficient completion of the infestation cycle. The in vitro assay system described here is compatible with other plant hosts and will benefit agricultural biotechnology research as it now enables specific high-throughput screening of nematode resistance traits together with subsequent mechanistic elucidation of the causative factors.

Transcriptomic signatures of transfer cells in early developing nematode feeding cells of Arabidopsis focused on auxin and ethylene signaling

Phyto-endoparasitic nematodes induce specialized feeding cells (NFCs) in their hosts, termed syncytia and giant cells (GCs) for cyst and root-knot nematodes (RKNs), respectively. They differ in their ontogeny and global transcriptional signatures, but both develop cell wall ingrowths (CIs) to facilitate high rates of apoplastic/symplastic solute exchange showing transfer cell (TC) characteristics. Regulatory signals for TC differentiation are not still well-known. The two-component signaling system (2CS) and reactive oxygen species are proposed as inductors of TC identity, while, 2CSs-related genes are not major contributors to differential gene expression in early developing NFCs. Transcriptomic and functional studies have assigned a major role to auxin and ethylene as regulatory signals on early developing TCs. Genes encoding proteins with similar functions expressed in both early developing NFCs and typical TCs are putatively involved in upstream or downstream responses mediated by auxin and ethylene. Yet, no function directly associated to the TCs identity of NFCs, such as the formation of CIs is described for most of them. Thus, we reviewed similarities between transcriptional changes observed during the early stages of NFCs formation and those described during differentiation of TCs to hypothesize about putative signals leading to TC-like differentiation of NFCs with particular emphasis on auxin an ethylene.

Profilin as a regulator of the membrane-actin cytoskeleton interface in plant cells

Membrane structures and cytoskeleton dynamics are intimately inter-connected in the eukaryotic cell. Recently, the molecular mechanisms operating at this interface have been progressively addressed. Many experiments have revealed that the actin cytoskeleton can interact with membranes through various discrete membrane domains. The actin-binding protein, profilin has been proven to inhibit actin polymerization and to promote F-actin elongation. This is dependent on many factors, such as the profilin/G-actin ratio and the ionic environment of the cell. Additionally, profilin has specific domains that interact with phosphoinositides and poly-L-proline rich proteins; theoretically, this gives profilin the opportunity to interact with membranes, and a large number of experiments have confirmed this possibility. In this article, we summarize recent findings in plant cells, and discuss the evidence of the connections among actin cytoskeleton, profilin and biomembranes through direct or indirect relationships.

Formins and membranes: anchoring cortical actin to the cell wall and beyond

Formins are evolutionarily conserved eukaryotic proteins participating in actin and microtubule organization. Land plants have three formin clades, with only two - Class I and II - present in angiosperms. Class I formins are often transmembrane proteins, residing at the plasmalemma and anchoring the cortical cytoskeleton across the membrane to the cell wall, while Class II formins possess a PTEN-related membrane-binding domain. Lower plant Class III and non-plant formins usually contain domains predicted to bind RHO GTPases that are membrane-associated. Thus, some kind of membrane anchorage appears to be a common formin feature. Direct interactions between various non-plant formins and integral or peripheral membrane proteins have indeed been reported, with varying mechanisms and biological implications. Besides of summarizing new data on Class I and Class II formin-membrane relationships, this review surveys such "non-classical" formin-membrane interactions and examines which, if any, of them may be evolutionarily conserved and operating also in plants. FYVE, SH3 and BAR domain-containing proteins emerge as possible candidates for such conserved membrane-associated formin partners.

Formin homology 1 (OsFH1) regulates root-hair elongation in rice (Oryza sativa)

The outgrowth of root hairs from the epidermal cell layer is regulated by a strict genetic regulatory system and external growth conditions. Rice plants cultivated in water-logged paddy land are exposed to a soil ecology that differs from the environment surrounding upland plants, such as Arabidopsis and maize. To identify genes that play important roles in root-hair growth, a forward genetics approach was used to screen for short-root-hair mutants. A short-root-hair mutant was identified, and the gene was isolated using map-based cloning and sequencing. The mutant harbored a point mutation at a splicing acceptor site, which led to truncation of OsFH1 (rice formin homology 1). Subsequent analysis of two additional T-DNA mutants verified that OsFH1 is important for root-hair elongation. Further studies revealed that the action of OsFH1 on root-hair growth is dependent on growth conditions. The mutant Osfh1 exhibited root-hair defects when roots were grown submerged in solution, and mutant roots produced normal root hairs in the air. However, root-hair phenotypes of mutants were not influenced by the external supply of hormones or carbohydrates, a deficiency of nutrients, such as Fe or P i , or aeration. This study shows that OsFH1 plays a significant role in root-hair elongation in a growth condition-dependent manner.

Plant formins: membrane anchors for actin polymerization

In plants, the actin cytoskeleton plays a fundamental role in intracellular transport, cell growth, and morphology. Formins are central regulators of actin polymerization and actin-based processes in many eukaryotes. Plants have a diverse family of formins and this diversity arose early in land plant evolution, probably deriving from family expansion and domain acquisition. Recently, formins from different plant lineages have been studied and the focus of these studies is beginning to shift from biochemical characterization to in vivo function. In vivo studies have shown that distinct biochemical activities confer specific cellular functions. Despite these differences, many plant formins have in common a direct link to the plasma membrane, suggesting that formins in plants are important links between the plasma membrane and actin remodeling.

Analysis of tomato gene promoters activated in syncytia induced in tomato and potato hairy roots by Globodera rostochiensis

The potato cyst nematode (Globodera rostochiensis) induces feeding sites (syncytia) in tomato and potato roots. In a previous study, 135 tomato genes up-regulated during G. rostochiensis migration and syncytium development were identified. Five genes (CYP97A29, DFR, FLS, NIK and PMEI) were chosen for further study to examine their roles in plant-nematode interactions. The promoters of these genes were isolated and potential cis regulatory elements in their sequences were characterized using bioinformatics tools. Promoter fusions with the β-glucuronidase gene were constructed and introduced into tomato and potato genomes via transformation with Agrobacterium rhizogenes to produce hairy roots. The analysed promoters displayed different activity patterns in nematode-infected and uninfected transgenic hairy roots.

An Arabidopsis class II formin, AtFH19, nucleates actin assembly, binds to the barbed end of actin filaments, and antagonizes the effect of AtFH1 on actin dynamics

Formin is a major protein responsible for regulating the nucleation of actin filaments, and as such, it permits the cell to control where and when to assemble actin arrays. It is encoded by a multigene family comprising 21 members in Arabidopsis thaliana. The Arabidopsis formins can be separated into two phylogenetically-distinct classes: there are 11 class I formins and 10 class II formins. Significant questions remain unanswered regarding the molecular mechanism of actin nucleation and elongation stimulated by each formin isovariant, and how the different isovariants coordinate to regulate actin dynamics in cells. Here, we characterize a class II formin, AtFH19, biochemically. We found that AtFH19 retains all general properties of the formin family, including nucleation and barbed end capping activity. It can also generate actin filaments from a pool of actin monomers bound to profilin. However, both the nucleation and barbed end capping activities of AtFH19 are less efficient compared to those of another well-characterized formin, AtFH1. Interestingly, AtFH19 FH1FH2 competes with AtFH1 FH1FH2 in binding actin filament barbed ends, and inhibits the effect of AtFH1 FH1FH2 on actin. We thus propose a mechanism in which two quantitatively different formins coordinate to regulate actin dynamics by competing for actin filament barbed ends.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Bundling actin filaments from membranes: some novel players

Progress in live-cell imaging of the cytoskeleton has significantly extended our knowledge about the organization and dynamics of actin filaments near the plasma membrane of plant cells. Noticeably, two populations of filamentous structures can be distinguished. On the one hand, fine actin filaments which exhibit an extremely dynamic behavior basically characterized by fast polymerization and prolific severing events, a process referred to as actin stochastic dynamics. On the other hand, thick actin bundles which are composed of several filaments and which are comparatively more stable although they constantly remodel as well. There is evidence that the actin cytoskeleton plays critical roles in trafficking and signaling at both the cell cortex and organelle periphery but the exact contribution of actin bundles remains unclear. A common view is that actin bundles provide the long-distance tracks used by myosin motors to deliver their cargo to growing regions and accordingly play a particularly important role in cell polarization. However, several studies support that actin bundles are more than simple passive highways and display multiple and dynamic roles in the regulation of many processes, such as cell elongation, polar auxin transport, stomatal and chloroplast movement, and defense against pathogens. The list of identified plant actin-bundling proteins is ever expanding, supporting that plant cells shape structurally and functionally different actin bundles. Here I review the most recently characterized actin-bundling proteins, with a particular focus on those potentially relevant to membrane trafficking and/or signaling.