The Arabidopsis profilin gene family. Evidence for an ancient split between constitutive and pollen-specific profilin genes

Redox dynamics in seeds of Acer spp: unraveling adaptation strategies of different seed categories

Background

Seeds of woody plant species, such as those in the Acer genus like Norway maple (Acer platanoides L.) and sycamore (Acer pseudoplatanus L.), exhibit unique physiological traits and responses to environmental stress. Thioredoxins (Trxs) play a central role in the redox regulation of cells, interacting with other redox-active proteins such as peroxiredoxins (Prxs), and contributing to plant growth, development, and responses to biotic and abiotic stresses. However, there is limited understanding of potential variations in this system between seeds categorized as recalcitrant and orthodox, which could provide insights into adaptive strategies.

Methods

Using proteomic analysis and DDA methods we investigated the Trx-h1 target proteins in seed axes. We complemented the results of the proteomic analysis with gene expression analysis of the Trx-h1, 1-Cys-Prx, and TrxR NTRA genes in the embryonic axes of maturing, mature, and stored seeds from two Acer species.

Results and discussion

The expression of Trx-h1 and TrxR NTRA throughout seed maturation in both species was low. The expression of 1-Cys-Prx remained relatively stable throughout seed maturation. In stored seeds, the expression levels were minimal, with slightly higher levels in sycamore seeds, which may confirm that recalcitrant seeds remain metabolically active during storage. A library of 289 proteins interacting with Trx-h1 was constructed, comprising 68 from Norway maple and 221 from sycamore, with distinct profiles in each seed category. Recalcitrant seed axes displayed a wide array of metabolic, stress response, and signaling proteins, suggesting sustained metabolic activity during storage and the need to address oxidative stress. Conversely, the orthodox seed axes presented a protein profile, reflecting efficient metabolic shutdown, which contributes to their extended viability. The results of the study provide new insights into seed viability and storage longevity mechanisms. They enhance the understanding of seed biology and lay the foundation for further evolutionary research on seeds of different categories.

Evolutionary dynamics of the cytoskeletal profilin gene family in Brassica juncea L. reveal its roles in silique development and stress resilience

Essential to plant adaptation, cell wall (CW) integrity is maintained by CW-biosynthesis genes. Cytoskeletal actin-(de)polymerizing, phospholipid-binding profilin (PRF) proteins play important roles in maintaining cellular homeostasis across kingdoms. However, evolutionary selection of PRF genes and their systematic characterization in family Brassicaceae, especially in Brassica juncea remain unexplored. Here, a comprehensive analysis of genome-wide identification of BjPRFs, their phylogenetic association, genomic localization, gene structure, and transcriptional profiling were performed in an evolutionary framework. Identification of 23 BjPRFs in B. juncea indicated an evolutionary conservation within Brassicaceae. The BjPRFs evolved through paralogous and orthologous gene formation in Brassica genomes. Evolutionary divergence of BjPRFs indicated purifying selection, with nonsynonymous (dN)/synonymous (dS) value of 0.090 for orthologous gene-pairs. Hybrid homology-modeling identified evolutionary distinct and conserved domains in BjPRFs which suggested that these proteins evolved following the divergence of monocot and eudicot plants. RNA-seq profiles of BjPRFs revealed their functional evolution in spatiotemporal manner during plant-development and stress-conditions in diploid/amphidiploid Brassica species. Real-Time PCR experiments in seedling, vegetative, floral and silique tissues of B. juncea suggested their essential roles in systematic plant development. These observations underscore the expansion of BjPRFs in B. juncea, and offer valuable evolutionary insights for exploring cellular mechanisms, and stress resilience.

Functional Characterization of an Arabidopsis Profilin Protein as a Molecular Chaperone under Heat Shock Stress

Profilins (PFNs) are actin monomer-binding proteins that function as antimicrobial agents in plant phloem sap. Although the roles of Arabidopsis thaliana profilin protein isoforms (AtPFNs) in regulating actin polymerization have already been described, their biochemical and molecular functions remain to be elucidated. Interestingly, a previous study indicated that AtPFN2 with high molecular weight (HMW) complexes showed lower antifungal activity than AtPFN1 with low molecular weight (LMW). These were bacterially expressed and purified to characterize the unknown functions of AtPFNs with different structures. In this study, we found that AtPFN1 and AtPFN2 proteins have LMW and HMW structures, respectively, but only AtPFN2 has a potential function as a molecular chaperone, which has never been reported elsewhere. AtPFN2 has better protein stability than AtPFN1 due to its higher molecular weight under heat shock conditions. The function of AtPFN2 as a holdase chaperone predominated in the HMW complexes, whereas the chaperone function of AtPFN1 was not observed in the LMW forms. These results suggest that AtPFN2 plays a critical role in plant tolerance by increasing hydrophobicity due to external heat stress.

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

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

iTRAQ-Based Quantitative Proteomic Analysis of the Arabidopsis Mutant opr3-1 in Response to Exogenous MeJA

Jasmonates (JAs) regulate the defense of biotic and abiotic stresses, growth, development, and many other important biological processes in plants. The comprehensive proteomic profiling of plants under JAs treatment provides insights into the regulation mechanism of JAs. Isobaric tags for relative and absolute quantification (iTRAQ)-based quantitative proteomic analysis was performed on the Arabidopsis wild type (Ws) and JA synthesis deficiency mutant opr3-1. The effects of exogenous MeJA treatment on the proteome of opr3-1, which lacks endogenous JAs, were investigated. A total of 3683 proteins were identified and 126 proteins were differentially regulated between different genotypes and treatment groups. The functional classification of these differentially regulated proteins showed that they were involved in metabolic processes, responses to abiotic stress or biotic stress, the defense against pathogens and wounds, photosynthesis, protein synthesis, and developmental processes. Exogenous MeJA treatment induced the up-regulation of a large number of defense-related proteins and photosynthesis-related proteins, it also induced the down-regulation of many ribosomal proteins in opr3-1. These results were further verified by a quantitative real-time PCR (qRT-PCR) analysis of 15 selected genes. Our research provides the basis for further understanding the molecular mechanism of JAs’ regulation of plant defense, photosynthesis, protein synthesis, and development.

Analysis of Pollen Allergens in Lily by Transcriptome and Proteome Data

The lily (Lilium spp.) anther contains a lot of pollen. It is not known if lily pollen contains allergens, and therefore screening pollen allergy-related proteins and genes is necessary. The pollen development period of lily 'Siberia' was determined by microscope observation. Early mononuclear microspores and mature pollens were used as sequencing materials. The analysis of the pollen transcriptome identified differentially expressed genes (DEGs), e.g., Profilin, Phl p 7 (Polcalcin), Ole e 1, and Phl p 11, which are associated with pollen allergens. The proteome analysis positively verified a significant increase in pollen allergenic protein content. The expression levels of LoProfiilin and LoPolcalcin, annotated as allergen proteins, gradually increased in mature pollen. LoProfiilin and LoPolcalcin were cloned and their open reading frame lengths were 396 bp and 246 bp, which encoded 131 and 81 amino acids, respectively. Amino acid sequence and structure alignment indicated that the protein sequences of LoProfilin and LoPolcalcin were highly conserved. Subcellular localization analysis showed that LoProfilin protein was localized in the cell cytoplasm and nucleus. LoProfilin and LoPolcalcin were highly expressed in mature pollen at the transcriptional and protein levels. A tertiary structure prediction analysis identified LoProfilin and LoPolcalcin as potential allergens in lily pollen.

Cofilin and profilin: partners in cancer aggressiveness

This review covers aspects of cofilin and profilin regulations and their influence on actin polymerisation responsible for cell motility and metastasis. The regulation of their activity by phosphorylation and nitration, miRs, PI(4,5)P₂ binding, pH, oxidative stress and post-translational modification is described. In this review, we have highlighted selected similarities, complementarities and differences between the two proteins and how their interplay affects actin filament dynamics.

Profilin Negatively Regulates Formin-Mediated Actin Assembly to Modulate PAMP-Triggered Plant Immunity

Profilin functions with formin in actin assembly, a process that regulates multiple aspects of plant development and immune responses. High-level eukaryotes contain multiple isoforms of profilin, formin, and actin, whose partner-specific interactions in actin assembly are not completely understood in plant development and defense responses. To examine the functionally distinct interactions between profilin and formin, we studied all five Arabidopsis profilins and their interactions with formin by using both in vitro biochemical and in vivo cell biology approaches. Unexpectedly, we found a previously undescribed negative regulatory function of AtPRF3 in AtFH1-mediated actin polymerization. The N-terminal 37 residues of AtPRF3 were identified to play a predominant role in inhibiting formin-mediated actin nucleation via their high affinity for the formin polyproline region and their triggering of the oligomerization of AtPRF3. Both in vivo and in vitro mechanistic studies of AtPRF3 revealed a universal mechanism in which the weak interaction between profilin and formin positively regulates actin assembly by ensuring rapid recycling of profilin, whereas profilin oligomerization negatively regulates actin polymerization. Upon recognition of the pathogen-associated molecular pattern, the gene transcription and protein degradation of AtPRF3 are modulated for actin assembly during plant innate immunity. The prf3 Arabidopsis plants show higher sensitivity to the bacterial flagellum peptide in both the plant growth and ROS responses. These findings demonstrate a profilin-mediated actin assembly mechanism underlying the plant immune responses.

A profilin gene promoter from switchgrass (Panicum virgatum L.) directs strong and specific transgene expression to vascular bundles in rice

A switchgrass vascular tissue-specific promoter (PvPfn2) and its 5'-end serial deletions drive high levels of vascular bundle transgene expression in transgenic rice. Constitutive promoters are widely used for crop genetic engineering, which can result in multiple off-target effects, including suboptimal growth and epigenetic gene silencing. These problems can be potentially avoided using tissue-specific promoters for targeted transgene expression. One particularly urgent need for targeted cell wall modification in bioenergy crops, such as switchgrass (Panicum virgatum L.), is the development of vasculature-active promoters to express cell wall-affective genes only in the specific tissues, i.e., xylem and phloem. From a switchgrass expression atlas we identified promoter sequence upstream of a vasculature-specific switchgrass profilin gene (PvPfn2), especially in roots, nodes and inflorescences. When the putative full-length (1715 bp) and 5'-end serial deletions of the PvPfn2 promoter (shortest was 413 bp) were used to drive the GUS reporter expression in stably transformed rice (Oryza sativa L.), strong vasculature-specificity was observed in various tissues including leaves, leaf sheaths, stems, and flowers. The promoters were active in both phloem and xylem. It is interesting to note that the promoter was active in many more tissues in the heterologous rice system than in switchgrass. Surprisingly, all four 5'-end promoter deletions, including the shortest fragment, had the same expression patterns as the full-length promoter and with no attenuation in GUS expression in rice. These results indicated that the PvPfn2 promoter variants are new tools to direct transgene expression specifically to vascular tissues in monocots. Of special interest is the very compact version of the promoter, which could be of use for vasculature-specific genetic engineering in monocots.

A Processive Arabidopsis Formin Modulates Actin Filament Dynamics in Association with Profilin

Formins are conserved regulators of actin cytoskeletal organization and dynamics that have been implicated to be important for cell division and cell polarity. The mechanism by which diverse formins regulate actin dynamics in plants is still not well understood. Using in vitro single-molecule imaging technology, we directly observed that the FH1-FH2 domain of an Arabidopsis thaliana formin, AtFH14, processively attaches to the barbed end of actin filaments as a dimer and slows their elongation rate by 90%. The attachment persistence of FH1-FH2 is concentration dependent. Furthermore, by use of the triple-color total internal reflection fluorescence microscopy, we found that ABP29, a barbed-end capping protein, competes with FH1-FH2 at the filament barbed end, where its binding is mutually exclusive with AtFH14. In the presence of different plant profilin isoforms, FH1-FH2 enhances filament elongation rates from about 10 to 42 times. Filaments buckle when FH1-FH2 is anchored specifically to cover slides, further indicating that AtFH14 moves processively on the elongating barbed end. At high concentration, AtFH14 bundles actin filaments randomly into antiparallel or parallel spindle-like structures; however, the FH1-FH2-mediated bundles become thinner and longer in the presence of plant profilins. This is the direct demonstration of a processive formin from plants. Our results also illuminate the molecular mechanism of AtFH14 in regulating actin dynamics via association with profilin.

Domestication-driven Gossypium profilin 1 (GhPRF1) gene transduces early flowering phenotype in tobacco by spatial alteration of apical/floral-meristem related gene expression

BACKGROUND

Plant profilin genes encode core cell-wall structural proteins and are evidenced for their up-regulation under cotton domestication. Notwithstanding striking discoveries in the genetics of cell-wall organization in plants, little is explicit about the manner in which profilin-mediated molecular interplay and corresponding networks are altered, especially during cellular signalling of apical meristem determinacy and flower development.

RESULTS

Here we show that the ectopic expression of GhPRF1 gene in tobacco resulted in the hyperactivation of apical meristem and early flowering phenotype with increased flower number in comparison to the control plants. Spatial expression alteration in CLV1, a key meristem-determinacy gene, is induced by the GhPRF1 overexpression in a WUS-dependent manner and mediates cell signalling to promote flowering. But no such expression alterations are recorded in the GhPRF1-RNAi lines. The GhPRF1 transduces key positive flowering regulator AP1 gene via coordinated expression of FT4, SOC1, FLC1 and FT1 genes involved in the apical-to-floral meristem signalling cascade which is consistent with our in silico profilin interaction data. Remarkably, these positive and negative flowering regulators are spatially controlled by the Actin-Related Protein (ARP) genes, specifically ARP4 and ARP6 in proximate association with profilins. This study provides a novel and systematic link between GhPRF1 gene expression and the flower primordium initiation via up-regulation of the ARP genes, and an insight into the functional characterization of GhPRF1 gene acting upstream to the flowering mechanism. Also, the transgenic plants expressing GhPRF1 gene show an increase in the plant height, internode length, leaf size and plant vigor.

CONCLUSIONS

Overexpression of GhPRF1 gene induced early and increased flowering in tobacco with enhanced plant vigor. During apical meristem determinacy and flower development, the GhPRF1 gene directly influences key flowering regulators through ARP-genes, indicating for its role upstream in the apical-to-floral meristem signalling cascade.

Insights into the Ecology and Evolution of Polyploid Plants through Network Analysis

Polyploidy is a widespread phenomenon throughout eukaryotes, with important ecological and evolutionary consequences. Although genes operate as components of complex pathways and networks, polyploid changes in genes and gene expression have typically been evaluated as either individual genes or as a part of broad-scale analyses. Network analysis has been fruitful in associating genomic and other 'omic'-based changes with phenotype for many systems. In polyploid species, network analysis has the potential not only to facilitate a better understanding of the complex 'omic' underpinnings of phenotypic and ecological traits common to polyploidy, but also to provide novel insight into the interaction among duplicated genes and genomes. This adds perspective to the global patterns of expression (and other 'omic') change that accompany polyploidy and to the patterns of recruitment and/or loss of genes following polyploidization. While network analysis in polyploid species faces challenges common to other analyses of duplicated genomes, present technologies combined with thoughtful experimental design provide a powerful system to explore polyploid evolution. Here, we demonstrate the utility and potential of network analysis to questions pertaining to polyploidy with an example involving evolution of the transgressively superior cotton fibres found in polyploid Gossypium hirsutum. By combining network analysis with prior knowledge, we provide further insights into the role of profilins in fibre domestication and exemplify the potential for network analysis in polyploid species.

Profilin-Dependent Nucleation and Assembly of Actin Filaments Controls Cell Elongation in Arabidopsis

Actin filaments in plant cells are incredibly dynamic; they undergo incessant remodeling and assembly or disassembly within seconds. These dynamic events are choreographed by a plethora of actin-binding proteins, but the exact mechanisms are poorly understood. Here, we dissect the contribution of Arabidopsis (Arabidopsis thaliana) PROFILIN1 (PRF1), a conserved actin monomer-binding protein, to actin organization and single filament dynamics during axial cell expansion of living epidermal cells. We found that reduced PRF1 levels enhanced cell and organ growth. Surprisingly, we observed that the overall frequency of nucleation events in prf1 mutants was dramatically decreased and that a subpopulation of actin filaments that assemble at high rates was reduced. To test whether profilin cooperates with plant formin proteins to execute actin nucleation and rapid filament elongation in cells, we used a pharmacological approach. Here, we used Small Molecule Inhibitor of Formin FH2 (SMIFH2), after validating its mode of action on a plant formin in vitro, and observed a reduced nucleation frequency of actin filaments in live cells. Treatment of wild-type epidermal cells with SMIFH2 mimicked the phenotype of prf1 mutants, and the nucleation frequency in prf1-2 mutant was completely insensitive to these treatments. Our data provide compelling evidence that PRF1 coordinates the stochastic dynamic properties of actin filaments by modulating formin-mediated actin nucleation and assembly during plant cell expansion.

Profilin Regulates Apical Actin Polymerization to Control Polarized Pollen Tube Growth

Pollen tube growth is an essential step during flowering plant reproduction, whose growth depends on a population of dynamic apical actin filaments. Apical actin filaments were thought to be involved in the regulation of vesicle fusion and targeting in the pollen tube. However, the molecular mechanisms that regulate the construction of apical actin structures in the pollen tube remain largely unclear. Here, we identify profilin as an important player in the regulation of actin polymerization at the apical membrane in the pollen tube. Downregulation of profilin decreased the amount of filamentous actin and induced disorganization of apical actin filaments, and reduced tip-directed vesicle transport and accumulation in the pollen tube. Direct visualization of actin dynamics revealed that the elongation of actin filaments originating at the apical membrane decreased in profilin mutant pollen tubes. Mutant profilin that is defective in binding poly-L-proline only partially rescues the actin polymerization defect in profilin mutant pollen tubes, although it fully rescues the actin turnover phenotype. We propose that profilin controls the construction of actin structures at the pollen tube tip, presumably by favoring formin-mediated actin polymerization at the apical membrane.

Arabidopsis plants deficient in constitutive class profilins reveal independent and quantitative genetic effects

BACKGROUND

The actin cytoskeleton is involved in an array of integral structural and developmental processes throughout the cell. One of actin's best-studied binding partners is the small ubiquitously expressed protein, profilin. Arabidopsis thaliana is known to encode a family of five profilin sequence variants: three vegetative (also constitutive) profilins that are predominantly expressed in all vegetative tissues and ovules, and two reproductive profilins that are specifically expressed in pollen. This paper analyzes the roles of the three vegetative profilin members, PRF1, PRF2, and PRF3, in plant cell and organ development.

RESULTS

Using a collection of knockout or severe knockdown T-DNA single mutants, we found that defects in each of the three variants gave rise to specific developmental deficiencies. Plants lacking PRF1 or PRF2 had defects in rosette leaf morphology and inflorescence stature, while those lacking PRF3 led to plants with slightly elongated petioles. To further examine these effects, double mutants and double and triple gene-silenced RNAi epialleles were created. These plants displayed significantly compounded developmental defects, as well as distinct lateral root growth morphological phenotypes.

CONCLUSION

These results suggest that having at least one vegetative profilin gene is essential to viability. Evidence is presented that combinations of independent function, quantitative genetic effects, and functional redundancy have preserved the three vegetative profilin genes in the Arabidopsis lineage.

The ARID-HMG DNA-binding protein AtHMGB15 is required for pollen tube growth in Arabidopsis thaliana

In flowering plants, male gametes (sperm cells) develop within male gametophytes (pollen grains) and are delivered to female gametes for double fertilization by pollen tubes. Therefore, pollen tube growth is crucial for reproduction. The mechanisms that control pollen tube growth remain poorly understood. In this study, we demonstrated that the ARID-HMG DNA-binding protein AtHMGB15 plays an important role in pollen tube growth. This protein is preferentially expressed in pollen grains and pollen tubes and is localized in the vegetative nuclei of the tricellular pollen grains and pollen tubes. Knocking down AtHMGB15 expression via a Ds insertion caused retarded pollen tube growth, leading to a significant reduction in the seed set. The athmgb15-1 mutation affected the expression of 1686 genes in mature pollen, including those involved in cell wall formation and modification, cell signaling and cellular transport during pollen tube growth. In addition, it was observed that AtHMGB15 binds to DNA in vitro and interacts with the transcription factors AGL66 and AGL104, which are required for pollen maturation and pollen tube growth. These results suggest that AtHMGB15 functions in pollen tube growth through the regulation of gene expression.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

Molecular and cellular characterization of the tomato pollen profilin, LePro1

Profilin is an actin-binding protein involved in the dynamic turnover and restructuring of the actin cytoskeleton in all eukaryotic cells. We previously cloned a profilin gene, designated as LePro1 from tomato pollen. To understand its biological role, in the present study, we investigated the temporal and spatial expression of LePro1 during pollen development and found that the transcript was only detected at late stages during microsporogenesis and pollen maturation. Using antisense RNA, we successfully knocked down the expression of LePro1 in tomato plants using stable transformation, and obtained two antisense lines, A2 and A3 showing significant down-regulation of LePro1 in pollen resulting in poor pollen germination and abnormal pollen tube growth. A disorganized F-actin distribution was observed in the antisense pollen. Down-regulation of LePro1 also appeared to affect hydration of pollen deposited on the stigma and arrested pollen tube elongation in the style, thereby affecting fertilization. Our results suggest that LePro1 in conjunction with perhaps other cytoskeletal proteins, plays a regulatory role in the proper organization of F-actin in tomato pollen tubes through promoting actin assembly. Down-regulation of LePro1 leads to interruption of actin assembly and disorganization of the actin cytoskeleton thus arresting pollen tube growth. Based on the present and previous studies, it is likely that a single transcript of profilin gives rise to multiple forms displaying multifunctionality in tomato pollen.

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.

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.

Control of the actin cytoskeleton in root hair development

The development of root hair includes four stages: bulge site selection, bulge formation, tip growth, and maturation. The actin cytoskeleton is involved in all of these stages and is organized into distinct arrangements in the different stages. In addition to the actin configuration, actin isoforms also play distinct roles in the different stages. The actin cytoskeleton is regulated by actin-binding proteins, such as formin, Arp2/3 complex, profilin, actin depolymerizing factor, and villin. Some upstream signals, i.e. calcium, phospholipids, and small GTPase regulate the activity of these actin-binding proteins to produce the proper actin configuration. We constructed a working model on how the actin cytoskeleton is controlled by actin-binding proteins and upstream signaling in root hair development based on the current literature: at the tip of hairs, actin polymerization appears to be facilitated by Arp2/3 complex that is activated by small GTPase, and profilin that is regulated by phosphatidylinositol 4,5-bisphosphate. Meanwhile, actin depolymerization and turnover are likely mediated by villin and actin depolymerizing factor, which are stimulated by calcium. At the shank, actin cables are produced by formin and villin. Under the complicated interaction, the actin cytoskeleton is controlled spatially and temporally during root hair development.

Characterization of profilin polymorphism in pollen with a focus on multifunctionality

Profilin, a multigene family involved in actin dynamics, is a multiple partners-interacting protein, as regard of the presence of at least of three binding domains encompassing actin, phosphoinositide lipids, and poly-L-proline interacting patches. In addition, pollen profilins are important allergens in several species like Olea europaea L. (Ole e 2), Betula pendula (Bet v 2), Phleum pratense (Phl p 12), Zea mays (Zea m 12) and Corylus avellana (Cor a 2). In spite of the biological and clinical importance of these molecules, variability in pollen profilin sequences has been poorly pointed out up until now. In this work, a relatively high number of pollen profilin sequences have been cloned, with the aim of carrying out an extensive characterization of their polymorphism among 24 olive cultivars and the above mentioned plant species. Our results indicate a high level of variability in the sequences analyzed. Quantitative intra-specific/varietal polymorphism was higher in comparison to inter-specific/cultivars comparisons. Multi-optional posttranslational modifications, e.g. phosphorylation sites, physicochemical properties, and partners-interacting functional residues have been shown to be affected by profilin polymorphism. As a result of this variability, profilins yielded a clear taxonomic separation between the five plant species. Profilin family multifunctionality might be inferred by natural variation through profilin isovariants generated among olive germplasm, as a result of polymorphism. The high variability might result in both differential profilin properties and differences in the regulation of the interaction with natural partners, affecting the mechanisms underlying the transmission of signals throughout signaling pathways in response to different stress environments. Moreover, elucidating the effect of profilin polymorphism in adaptive responses like actin dynamics, and cellular behavior, represents an exciting research goal for the future.

Multiple actin isotypes in plants: diverse genes for diverse roles?

Plant actins are encoded by a gene family. Despite the crucial significance of the actin cytoskeleton for plant structure and function, the importance of individual actin isotypes and their specific roles in various plant tissues or even single cells is rather poorly understood. This review summarizes our current knowledge about the plant actin gene family including its evolution, gene and protein structure, and the expression profiles and regulation. Based on this background information, we review mutant and complementation analyses in Arabidopsis to draw an emerging picture of overlapping and specific roles of plant actin isotypes. Finally, we examine hypotheses explaining the mechanisms of isotype-specific functions.

Parallel up-regulation of the profilin gene family following independent domestication of diploid and allopolyploid cotton (Gossypium)

Cotton is remarkable among our major crops in that four species were independently domesticated, two allopolyploids and two diploids. In each case thousands of years of human selection transformed sparsely flowering, perennial shrubs into highly productive crops with seeds bearing the vastly elongated and abundant single-celled hairs that comprise modern cotton fiber. The genetic underpinnings of these transformations are largely unknown, but comparative gene expression profiling experiments have demonstrated up-regulation of profilin accompanying domestication in all three species for which wild forms are known. Profilins are actin monomer binding proteins that are important in cytoskeletal dynamics and in cotton fiber elongation. We show that Gossypium diploids contain six profilin genes (GPRF1-GPRF6), located on four different chromosomes (eight chromosomes in the allopolyploid). All but one profilin (GPRF6) are expressed during cotton fiber development, and both homeologs of GPRF1-GPRF5 are expressed in fibers of the allopolyploids. Remarkably, quantitative RT-PCR and RNAseq data demonstrate that GPRF1-GPRF5 are all up-regulated, in parallel, in the three independently domesticated cottons in comparison with their wild counterparts. This result was additionally supported by iTRAQ proteomic data. In the allopolyploids, there This usage of novel should be fine, since it refers to a novel evolutionary process, not a novel discovery has been novel recruitment of the sixth profilin gene (GPRF6) as a result of domestication. This parallel up-regulation of an entire gene family in multiple species in response to strong directional selection is without precedent and suggests unwitting selection on one or more upstream transcription factors or other proteins that coordinately exercise control over profilin expression.

Characterization of PROFILIN genes from allotetraploid (Gossypium hirsutum) cotton and its diploid progenitors and expression analysis in cotton genotypes differing in fiber characteristics

The actin-binding protein profilin (PRF) plays an important role in cell growth and expansion by regulating the organization of the actin filaments. Recent studies have reported association between fiber elongation in cultivated cotton (Gossypium hirsutum) and PRF expression. In the present study, we cloned four genomic clones from allotetraploid cotton (G. hirsutum) and its putative diploid progenitors (G. arboreum and G. raimondii) designated GhPRF1_A, GhPRF1_D, GaPRF1, and GrPRF1 encoding cotton PRF and characterized their genomic structure, phylogenetic relationships and promoter structure. Sequence analysis of the coding regions of all clones resulted in a single protein product which revealed more than 80% similarity to most plant PRFs and a typical organization with an actin-binding and a polybasic phospholipid binding motif at the carboxy terminus. DNA blot hybridization suggested that PRF gene is present with more than one copy in the allotetraploid species G. hirsutum. Expression analysis performed in various organs of cultivated cotton revealed that the PRF gene was preferentially expressed in cotton fibers. Very low levels of expression were observed in whole flowers, while PRF transcripts were not detected in other organs examined. Furthermore, higher levels of expression were observed at the early stages of cotton fiber development (at 10 days post anthesis), indicative that this gene may play a major role in the early stages of cotton fiber development. Quantitation of the expression by real-time PCR revealed higher expression levels in a G. hirsutum variety with higher fiber percentage compared to a variety with lower percentage. In addition, higher levels of expression were found in cultivated allotetraploid G. barbadense cotton species with higher fiber length in comparison to cultivated allotetraploid G. hirsutum.

Arabidopsis VILLIN1 and VILLIN3 have overlapping and distinct activities in actin bundle formation and turnover

Actin filament bundles are higher-order cytoskeletal structures that are crucial for the maintenance of cellular architecture and cell expansion. They are generated from individual actin filaments by the actions of bundling proteins like fimbrins, LIMs, and villins. However, the molecular mechanisms of dynamic bundle formation and turnover are largely unknown. Villins belong to the villin/gelsolin/fragmin superfamily and comprise at least five isovariants in Arabidopsis thaliana. Different combinations of villin isovariants are coexpressed in various tissues and cells. It is not clear whether these isovariants function together and act redundantly or whether they have unique activities. VILLIN1 (VLN1) is a simple filament-bundling protein and is Ca(2+) insensitive. Based on phylogenetic analyses and conservation of Ca(2+) binding sites, we predict that VLN3 is a Ca(2+)-regulated villin capable of severing actin filaments and contributing to bundle turnover. The bundling activity of both isovariants was observed directly with time-lapse imaging and total internal reflection fluorescence (TIRF) microscopy in vitro, and the mechanism mimics the "catch and zipper" action observed in vivo. Using time-lapse TIRF microscopy, we observed and quantified the severing of individual actin filaments by VLN3 at physiological calcium concentrations. Moreover, VLN3 can sever actin filament bundles in the presence of VLN1 when calcium is elevated to micromolar levels. Collectively, these results demonstrate that two villin isovariants have overlapping and distinct activities.

Comparative transcript analyses of the ovule, microspore, and mature pollen in Brassica napus

Transcriptome data for plant reproductive organs/cells currently is very limited as compared to sporophytic tissues. Here, we constructed cDNA libraries and obtained ESTs for Brassica napus pollen (4,864 ESTs), microspores (i.e., early stage pollen development; 6,539 ESTs) and ovules (10,468 ESTs). Clustering and assembly of the 21,871 ESTs yielded a total of 10,782 unigenes, with 3,362 contigs and 7,420 singletons. The pollen transcriptome contained high levels of polygalacturonases and pectinesterases, which are involved in cell wall synthesis and expansion, and very few transcription factors or transcripts related to protein synthesis. The set of genes expressed in mature pollen showed little overlap with genes expressed in ovules or in microspores, suggesting in the latter case that a marked differentiation had occurred from the early microspore stages through to pollen development. Remarkably, the microspores and ovules exhibited a high number of co-expressed genes (N = 1,283) and very similar EST functional profiles, including high transcript numbers for transcriptional and translational processing genes, protein modification genes and unannotated genes. In addition, examination of expression values for genes co-expressed among microspores and ovules revealed a highly statistically significant correlation among these two tissues (R = 0.360, P = 1.2 x 10(-40)) as well as a lack of differentially expressed genes. Overall, the results provide new insights into the transcriptional profile of rarely studied ovules, the transcript changes during pollen development, transcriptional regulation of pollen tube growth and germination, and describe the parallels in the transcript populations of microspore and ovules which could have implications for understanding the molecular foundation of microspore totipotency in B. napus.

Germination behavior, biochemical features and sequence analysis of the RACK1/arcA homolog from Phaseolus vulgaris

Partial peptide sequence of a 36 kDa protein from common bean embryo axes showed 100% identity with a reported beta-subunit of a heterotrimeric G protein from soybean. Analysis of the full sequence showed 96.6% identity with the reported soybean G(beta)-subunit, 86% with RACK1B and C from Arabidopsis and 66% with human and mouse RACK1, at the amino acid level. In addition, it showed 85.5, 85 and 83% identities with arcA from Solanum lycopersicum, Arabidopsis (RACK1A) and Nicotiana tabacum, respectively. The amino acid sequence displayed seven WD40 domains and two sites for activated protein kinase C binding. The protein showed a constant expression level but the mRNA had a maximum at 32 h post-imbibition. Western immunoblotting showed the protein in vegetative plant tissues, and in both microsomal and soluble fractions from embryo axes. Synthetic auxin treatment during germination delayed the peak of RACK1 mRNA expression to 48 h but did not affect the protein expression level while the polar auxin transport inhibitor, naphtylphtalamic acid had no effect on either mRNA or protein expression levels. Southern blot and genomic DNA amplification revealed a small gene family with at least one member without introns in the genome. Thus, the RACK1/arcA homolog from common bean has the following features: (1) it is highly conserved; (2) it is both soluble and insoluble within the embryo axis; (3) it is encoded by a small gene family; (4) its mRNA has a peak of expression at the time point of germination stop and (5) its expression is only slightly affected by auxin but unaffected by an auxin transport blocker.

Expression analysis and characteristics of profilin gene from silkworm, Bombyx mori

A recombinant Bombyx mori profilin protein (rBmPFN) was overexpressed in Escherichia coli BL21. Purified rBmPFN was used to generate anti-BmPFN polyclonal antibody, which were used to determine the subcellular localization of BmPFN. Immunostaining indicated that profilin can be found in both the nucleus and cytoplasm but is primarily located in the cytoplasm. Real-time RT-PCR and Western blot analyses indicated that, during the larvae stage, profilin expression levels are highest in the silk gland, followed by the gonad, and are lowest in the fatty body. Additionally, BmPFN expression begins during the egg stage, increases during the larvae stage, reaches a peak during the pupa stage, and decreases significantly in the moth. Therefore, we propose that BmPFN may play an important role during larva stage development, especially in the silk gland.

Different subcellular localizations and functions of Arabidopsis myosin VIII

BACKGROUND

Myosins are actin-activated ATPases that use energy to generate force and move along actin filaments, dragging with their tails different cargos. Plant myosins belong to the group of unconventional myosins and Arabidopsis myosin VIII gene family contains four members: ATM1, ATM2, myosin VIIIA and myosin VIIIB.

RESULTS

In transgenic plants expressing GFP fusions with ATM1 (IQ-tail truncation, lacking the head domain), fluorescence was differentially distributed: while in epidermis cells at the root cap GFP-ATM1 equally distributed all over the cell, in epidermal cells right above this region it accumulated in dots. Further up, in cells of the elongation zone, GFP-ATM1 was preferentially positioned at the sides of transversal cell walls. Interestingly, the punctate pattern was insensitive to brefeldin A (BFA) while in some cells closer to the root cap, ATM1 was found in BFA bodies. With the use of different markers and transient expression in Nicotiana benthamiana leaves, it was found that myosin VIII co-localized to the plasmodesmata and ER, colocalized with internalized FM4-64, and partially overlapped with the endosomal markers ARA6, and rarely with ARA7 and FYVE. Motility of ARA6 labeled organelles was inhibited whenever associated with truncated ATM1 but motility of FYVE labeled organelles was inhibited only when associated with large excess of ATM1. Furthermore, GFP-ATM1 and RFP-ATM2 (IQ-tail domain) co-localized to the same spots on the plasma membrane, indicating a specific composition at these sites for myosin binding.

CONCLUSION

Taken together, our data suggest that myosin VIII functions differently in different root cells and can be involved in different steps of endocytosis, BFA-sensitive and insensitive pathways, ER tethering and plasmodesmatal activity.

Global analysis of gene expression in flower buds of Ms-cd1 Brassica oleracea conferring male sterility by using an Arabidopsis microarray

The dominant male sterility gene Ms-cd1 is identified in Brassica oleracea. Electron microscopical observations revealed that abortion of pollen development starts after tetrad formation. This important male sterility phenotype is characterized by lack of degradation of the primary pollen mother cell (PMC) wall and delayed degradation of callose surrounding the tetrads and thus arrest of microspore release. Gene expression of the male sterile and fertile buds was analyzed by heterologous hybridization of Brassica oleracea cRNA onto an Arabidopsis whole genome oligonucleotide microarray. A total of 277 suppressed genes including 40 kinase-, 32 cell wall modification and 29 transport related genes were found to be significantly down regulated >3-fold in the male sterile mutant. The vast majority of the differentially expressed transcripts are found to present late pollen stage specific genes. Kinase genes, cell wall modification genes and ion transport genes were greatly over-represented when compared to their percentage of all flower bud expressed genes and represent 36.5% of the genes suppressed by Ms-cd1. Our results also suggest that Ms-cd1 may blocks an anther developmental pathway with a small number of genes suppressed in tapetum cells which prevent the degradation of callose and PMC wall, which further leads to the suppression of a large number of genes involved in signaling pathways, cell wall modification and ion transport in pollen grains.

Profilin is essential for tip growth in the moss Physcomitrella patens

The actin cytoskeleton is critical for tip growth in plants. Profilin is the main monomer actin binding protein in plant cells. The moss Physcomitrella patens has three profilin genes, which are monophyletic, suggesting a single ancestor for plant profilins. Here, we used RNA interference (RNAi) to determine the loss-of-function phenotype of profilin. Reduction of profilin leads to a complete loss of tip growth and a partial inhibition of cell division, resulting in plants with small rounded cells and fewer cells. We silenced all profilins by targeting their 3' untranslated region sequences, enabling complementation analyses by expression of profilin coding sequences. We show that any moss or a lily (Lilium longiflorum) profilin support tip growth. Profilin with a mutation in its actin binding site is unable to rescue profilin RNAi, while a mutation in the poly-l-proline binding site weakly rescues. We show that moss tip growing cells contain a prominent subapical cortical F-actin structure composed of parallel actin cables. Cells lacking profilin lose this structure; instead, their F-actin is disorganized and forms polarized cortical patches. Plants expressing the actin and poly-l-proline binding mutants exhibited similar F-actin disorganization. These results demonstrate that profilin and its binding to actin are essential for tip growth. Additionally, profilin is not needed for formation of F-actin, but profilin and its interactions with actin and poly-l-proline ligands are required to properly organize F-actin.

Evolutionary biology of plant food allergens

The majority of plant food allergens can be grouped into just 4 protein families. This review summarizes the evolutionary relationships of allergenic and nonallergenic members of these families. Proteins from the prolamin superfamily have been described in vascular plants. This superfamily contains several allergenic (2S albumins, nonspecific lipid transfer proteins, and cereal amylase and protease inhibitors) and nonallergenic (hybrid proline-rich proteins, cereal indolines, and alpha-globulins) member families. The cupin superfamily comprises numerous functionally highly diverse protein families from all groups of organisms. However, allergenicity within the cupins is confined to the vicilin and legumin seed storage proteins. Profilins are ubiquitous eukaryotic proteins that are nonallergenic, with the exception of profilins from flowering plants. Finally, the Bet v 1 superfamily contains the pathogenesis-related proteins 10 family, the family of major latex proteins and ripening-related proteins, the norcoclaurine synthases, and the cytokinin-binding proteins, with pathogenesis-related proteins 10 family members from certain taxa being the only allergenic members. The study of the distribution of allergenic and nonallergenic members of protein families will provide new insights into the evolution of allergenicity and the factors that make proteins allergenic.

Inositol-1 (or 4)-monophosphatase from Glycine max embryo axes is a phosphatase with broad substrate specificity that includes phytate dephosphorylation

A phosphate-hydrolyzing activity from Glycine max embryo axes was purified by a series of chromatographic steps and electroelution from activity gels, and demonstrated to be an inositol-1 (or 4)-monophosphatase by partial internal amino acid sequence. This enzyme hydrolyzed ATP, sodium pyrophosphate (NaPPi), inositol hexakisphosphate, and inositol 1-monophosphate, but not p-nitrophenyl phosphate, ADP, AMP or glucose 6-P. Using NaPPi as substrate, the highly purified protein hydrolyzed up to 0.4 mmol phosphate min(-1) mg(-1) protein and had a Km(avg) of 235 microM for NaPPi. Since NaPPi is relatively inexpensive and readily available, we used this as substrate for the subsequent characterization. We observed the following: (a) specific inhibition by Li and NaF but not by butanedione monoxime, or orthovanadate; (b) activation by Cu(2+) and Mg(2+); (c) optimum activity at pH 7.4; and (d) temperature stability after 1-h incubations at 37-80 degrees C, with maximum activity at 37 degrees C. The partially purified protein was detected by in-gel activity assays and the band was electroeluted to yield a highly purified protein. Analysis by SDS-PAGE and native IEF-PAGE yielded a single major polypeptide of 29 kDa and pI approximately 5.9, respectively. In addition, in-gel activity from embryo axes and whole hypocotyls at early germination times revealed one high and one intermediate molecular weight isoform, but only the intermediate one corresponded to IMPase. Throughout the post-imbibition period, the activity of the high molecular weight isoform disappeared and IMPase increased, indicating an increasing expression of the enzyme as germination and growth proceeded. These data indicate that the inositol-1 (or 4)-monophosphatase present in the embryo axis of G. max has a wide phosphate substrate specificity, and may play an important role in phosphate metabolism during the germination process.

The function of actin-binding proteins in pollen tube growth

Pollen tube growth is a key step in sexual reproduction of higher plants. The pollen tube is a typical example of tip-growing cells and shows a polarized cytoplasm. To develop and maintain polarized growth, pollen tubes need a carefully regulated actin cytoskeleton. It is well known that actin-binding proteins are responsible for the direct control of dynamic actin filaments and serve as a link between signal transduction pathways and dynamic actin changes in determining cellular architecture. Several of these classes have been identified in pollen tubes and their detailed characterisation is progressing rapidly. Here, we aim to survey what is known about the major actin-binding proteins that affect actin assembly and dynamics, and their higher-order organisation in pollen tube growth.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

Profilin and Rop GTPases are localized at infection sites of plant cells

We have found 5 profilin cDNAs in cultured parsley cells, representing a small gene family of about 5 members in parsley. Specific antibodies were produced using heterologously expressed parsley profilin as antigen. Western blot analysis revealed the occurrence of similar amounts of profilin in roots and green parts of parsley plants. Immunocytochemical staining of parsley cells infected with the oomycetous plant pathogen Phytophthora infestans clearly revealed that profilin accumulates at the site on the plasma membrane subtending the oomycetous appressorium, where the actin cables focus. We also observed the accumulation of Rop GTPases around this site, which might point to a potential function in signaling to the cytoskeleton.

Organization and function of the actin cytoskeleton in developing root cells

The actin cytoskeleton is a highly dynamic structure, which mediates various cellular functions in large part through accessory proteins that tilt the balance between monomeric G-actin and filamentous actin (F-actin) or by facilitating interactions between actin and the plasma membrane, microtubules, and other organelles. Roots have become an attractive model to study actin in plant development because of their simple anatomy and accessibility of some root cell types such as root hairs for microscopic analyses. Roots also exhibit a remarkable developmental plasticity and possess a delicate sensory system that is easily manipulated, so that one can design experiments addressing a range of important biological questions. Many facets of root development can be regulated by the diverse actin network found in the various root developmental regions. Various molecules impinge on this actin scaffold to define how a particular root cell type grows or responds to a specific environmental signal. Although advances in genomics are leading the way toward elucidating actin function in roots, more significant strides will be realized when such tools are combined with improved methodologies for accurately depicting how actin is organized in plant cells.

Distinct roles of the first introns on the expression of Arabidopsis profilin gene family members

Profilin is a small actin-binding protein that regulates cellular dynamics of the actin cytoskeleton. In Arabidopsis (Arabidopsis thaliana), five profilins were identified. The vegetative class profilins, PRF1, PRF2, and PRF3, are expressed in vegetative organs. The reproductive class profilins, PRF4 and PRF5, are mainly expressed in pollen. In this study, we examined the role of the first intron in the expression of the Arabidopsis profilin gene family using transgenic plants and a transient expression system. In transgenic plants, we examined PRF2 and PRF5, which represent vegetative and reproductive profilins. The expression of the PRF2 promoter fused with the beta-glucuronidase (GUS) gene was observed in the vascular bundles, but transgenic plants carrying the PRF2 promoter-GUS with its first intron showed constitutive expression throughout the vegetative tissues. However, the first intron of PRF5 had little effect on the reporter gene expression pattern. Transgenic plants containing PRF5 promoter-GUS fusion with or without its first intron showed reproductive tissue-specific expression. To further investigate the different roles of the first two introns on gene expression, the first introns were exchanged between PRF2 and PRF5. The first intron of PRF5 had no apparent effect on the expression pattern of the PRF2 promoter. But, unlike the intron of PRF5, the first intron of PRF2 greatly affected the reproductive tissue-specific expression of the PRF5 promoter, confirming a different role for these introns. The results of a transient expression assay indicated that the first intron of PRF1 and PRF2 enhances gene expression, whereas PRF4 and PRF5 do not. These results suggest that the first introns of profilin genes are functionally distinctive and the first introns are required for the strong and constitutive gene expression of PRF1 and PRF2 in vegetative tissues.

Arabidopsis group Ie formins localize to specific cell membrane domains, interact with actin-binding proteins and cause defects in cell expansion upon aberrant expression

The closely related proteins AtFH4 and AtFH8 represent the group Ie clade of Arabidopsis formin homologues. The subcellular localization of these proteins and their ability to affect the actin cytoskeleton were examined. AtFH4 protein activity was identified using fluorimetric techniques. Interactions between Arabidopsis profilin isoforms and AtFH4 were assayed in vitro and in vivo using pull-down assays and yeast-2-hybrid. The subcellular localization of group Ie formins was observed with indirect immunofluorescence (AtFH4) and an ethanol-inducible green fluorescent protein (GFP) fusion construct (AtFH8). AtFH4 protein affected actin dynamics in vitro, and yeast-2-hybrid assays suggested isoform-specific interactions with the actin-binding protein profilin in vivo. Indirect immunofluorescence showed that AtFH4 localized specifically to the cell membrane at borders between adjoining cells. Expression of an AtFH8 fusion protein resulted in GFP localization to cell membrane zones, similar to AtFH4. Furthermore, aberrant expression of AtFH8 resulted in the inhibition of root hair elongation. Taken together, these data suggest that the group Ie formins act with profilin to regulate actin polymerization at specific sites associated with the cell membrane.

Inverted repeat PCR for the rapid assembly of constructs to induce RNA interference

Expressing stem-loop RNAs in plants, fungi, and animals efficiently silences homologous target gene expression. We devised a novel PCR strategy, called inverted repeat PCR (IR-PCR), which allows rapid assembly and cloning of stem-loop-containing constructs in any vector. IR-PCR relies on differentially tagging antisense and sense copies of the target in one round of PCR and assembling them in a second. We used IR-PCR to assemble constructs targeting profilin, actin, and actin-related protein (ARP) transcripts from Arabidopsis. Immunoblotting of lines expressing a profilin PRF1 3' untranslated region (UTR)-specific construct demonstrated a 77 to 97% reduction in PRF1 protein, but not other profilin isovariants.

Genomic characterization and linkage mapping of the apple allergen genes Mal d 2 (thaumatin-like protein) and Mal d 4 (profilin)

Four classes of apple allergens (Mal d 1, -2, -3 and -4) have been reported. By using PCR cloning and sequencing approaches, we obtained genomic sequences of Mal d 2 (thaumatin-like protein) and Mal d 4 (profilin) from the cvs Prima and Fiesta, the two parents of a European reference mapping population. Two copies of the Mal d 2 gene (Mal d 2.01 A and Mal d 2.01 B) were identified, which primarily differed in the length of a single intron (378 or 380 nt) and in one amino acid in the signal peptide. Both Mal d 2.01 A and Mal d 2.01 B were mapped at identical position on linkage group 9. Genomic characterization of four Mal d 4 genes (Mal d 4.01 A and B, Mal d 4.02 A and Mal d 4.03 A) revealed their complete gDNA sequences which varied among genes in length from 862 to 2,017 nt. They all contained three exons of conserved length: 123, 138, and 135 nt. Mal d 4.01 appeared to be duplicated in two copies and located on linkage group 9. Mal d 4.02 A and Mal d 4.03 A were single copy genes located on linkage group 2 and 8, respectively.

The role of the actin cytoskeleton in plant cell signaling

The plant actin cytoskeleton provides a dynamic cellular component which is involved in the maintenance of cell shape and structure. It has been demonstrated recently that the actin cytoskeleton and its associated elements provide a key target in many signaling events. In addition to acting as a target, the actin cytoskeleton can also act as a transducer of signal information. In this review we describe some newly discovered aspects of the roles of the actin cytoskeleton in plant cell signaling. In addition to a summary of the roles played by actin-binding proteins, we also briefly review the progress made in understanding how the actin cytoskeleton participates in the self-incompatibility response in pollen tubes. Finally, the emerging importance of the actin cytoskeleton in the perception and responses to stimuli such as gravity, touch and cold stress exposure are discussed. Contents I. Introduction - the actin cytoskeleton 13 II. Actin-binding proteins 14 III. The actin cytoskeleton as a target and mediator of plant cell signaling 20 IV. Summary and conclusion 25 References 25 Acknowledgements 25.

Genome-wide analysis of spatial gene expression in Arabidopsis flowers

We have compared the gene expression profiles of inflorescences of the floral homeotic mutants apetala1, apetala2, apetala3, pistillata, and agamous with that of wild-type plants using a flower-specific cDNA microarray and a whole genome oligonucleotide array. By combining the data sets from the individual mutant/wild type comparisons, we were able to identify a large number of genes that are, within flowers, predicted to be specifically or at least predominantly expressed in one type of floral organ. We have analyzed the expression patterns of several of these genes by in situ hybridization and found that they match the predictions that were made based on the microarray experiments. Moreover, genes with known floral organ-specific expression patterns were correctly assigned by our analysis. The vast majority of the identified transcripts are found in stamens or carpels, whereas few genes are predicted to be expressed specifically or predominantly in sepals or petals. These findings indicate that spatially limited expression of a large number of genes is part of flower development and that its extent differs significantly between the reproductive organs and the organs of the perianth.

The Arabidopsis cytoskeletal genome

In the past decade the first Arabidopsis genes encoding cytoskeletal proteins were identified. A few dozen genes in the actin and tubulin cytoskeletal systems have been characterized thoroughly, including gene families encoding actins, profilins, actin depolymerizing factors, α-tubulins, and β-tubulins. Conventional molecular genetics have shown these family members to be differentially expressed at the temporal and spatial levels with an ancient split separating those genes expressed in vegetative tissues from those expressed in reproductive tissues. A few members of other cytoskeletal gene families have also been partially characterized, including an actin-related protein, annexins, fimbrins, kinesins, myosins, and villins. In the year 2001 the Arabidopsis genome sequence was completed. Based on sequence homology with well-characterized animal, fungal, and protist sequences, we find candidate cytoskeletal genes in the Arabidopsis database: more than 150 actin-binding proteins (ABPs), including monomer binding, capping, cross-linking, attachment, and motor proteins; more than 200 microtubule-associated proteins (MAPs); and, surprisingly, 10 to 40 potential intermediate filament (IF) proteins. Most of these sequences are uncharacterized and were not identified as related to cytoskeletal proteins. Several Arabidopsis ABPs, MAPs, and IF proteins are represented by individual genes and most were represented as as small gene families. However, several classes of cytoskeletal genes including myosin, eEF1α, CLIP, tea1, and kinesin are part of large gene families with 20 to 70 potential gene members each. This treasure trove of data provides an unprecedented opportunity to make rapid advances in understanding the complex plant cytoskeletal proteome. However, the functional analysis of these proposed cytoskeletal proteins and their mutants will require detailed analysis at the cell biological, molecular genetic, and biochemical levels. New approaches will be needed to move more efficiently and rapidly from this mass of DNA sequence to functional studies on cytoskeletal proteins.

Cloning and molecular and immunological characterisation of two new food allergens, Cap a 2 and Lyc e 1, profilins from bell pepper (Capsicum annuum) and Tomato (Lycopersicon esculentum)

BACKGROUND

Profilins are recognised by IgE of about 20% of patients allergic to birch pollen and plant foods. They are ubiquitous intracellular proteins highly cross-reactive among plant species. Therefore, they were called panallergens and are made responsible for cross-sensitisation between plant pollen and food.

OBJECTIVES

The aim of the present study was to clone the cDNAs encoding profilins from bell pepper and tomato, to produce and purify the recombinant proteins and to compare their IgE-binding capacities to those of the natural proteins.

METHODS

cDNA clones coding for profilin were obtained by RT-PCR from total RNA of tomato and bell pepper fruits, sequenced and expressed as non-fusion proteins in ESCHERICHIA COLI. The recombinant profilins were subsequently purified and tested for IgE-binding and inhibition capacity with sera from 34 food-allergic patients. Possible oligomerisation of recombinant profilins was investigated by HPLC analysis and its influence on IgE binding assayed by ELISA.

RESULTS

The open reading frame from both profilins encompasses 393 bp with a predicted molecular mass of 14,184 kD and a pI of 4.44 for bell pepper profilin (Cap a 2) and 14,257 kD and a pI of 4.46 for the profilin from tomato (Lyc e 1). The two protein sequences display 91% identity, whereas tomato profilin from pollen shares only 75% identity with tomato fruit profilin. Eleven out of 34 food-allergic patients (32%) display IgE binding to both purified profilins. Preincubation of a serum pool with either purified rCap a 2 or rLyc e 1 nearly abolished IgE binding to natural Cap a 2 and Lyc e 1, respectively. In addition, purified recombinant Cap a 2 was able to inhibit IgE-binding to rLyc e 1 by approximately 50%, whereas rLyc e 1 completely blocked IgE-binding to rCap a 2 in cross-inhibition assays. HPLC analysis showed that in solution Cap a 2 and Lyc e 1 can be found predominantly as dimers, which can be partially reduced to monomers by addition of dithiothreitol (DTT). In ELISA DTT-treated Lyc e 1 displayed a clearly lower IgE-binding capacity than untreated profilin.

CONCLUSIONS

Purified rCap a 2 and rLyc e 1 proved to be valuable tools for studying cross-reactivity to profilins in patients allergic to pollen and food.

Comparative analysis of the Arabidopsis pollen transcriptome

We present a genome-wide view of the male gametophytic transcriptome in Arabidopsis based on microarray analysis. In comparison with the transcriptome of the sporophyte throughout development, the pollen transcriptome showed reduced complexity and a unique composition. We identified 992 pollen-expressed mRNAs, nearly 40% of which were detected specifically in pollen. Analysis of the functional composition of the pollen transcriptome revealed the over-representation of mRNAs encoding proteins involved in cell wall metabolism, cytoskeleton, and signaling and under-representation of mRNAs involved in transcription and protein synthesis. For several gene families, we observed a common pattern of mutually exclusive gene expression between pollen and sporophytic tissues for different gene family members. Our results provide a 50-fold increase in the knowledge of genes expressed in Arabidopsis pollen. Moreover, we also detail the extensive overlap (61%) of the pollen transcriptome with that of the sporophyte, which provides ample potential to influence sporophytic fitness through gametophytic selection.