Identification and Molecular Characterization of Myosin Gene Family in Oryza sativa Genome

QTLs and Candidate Loci Associated with Drought Tolerance Traits of Kaybonnet x ZHE733 Recombinant Inbred Lines Rice Population

Rice is the most important staple crop for the sustenance of the world's population, and drought is a major factor limiting rice production. Quantitative trait locus (QTL) analysis of drought-resistance-related traits was conducted on a recombinant inbred line (RIL) population derived from the self-fed progeny of a cross between the drought-resistant tropical japonica U.S. adapted cultivar Kaybonnet and the drought-sensitive indica cultivar ZHE733. K/Z RIL population of 198 lines was screened in the field at Fayetteville (AR) for three consecutive years under controlled drought stress (DS) and well-watered (WW) treatment during the reproductive stage. The effects of DS were quantified by measuring morphological traits, grain yield components, and root architectural traits. A QTL analysis using a set of 4133 single nucleotide polymorphism (SNP) markers and the QTL IciMapping identified 41 QTLs and 184 candidate genes for drought-related traits within the DR-QTL regions. RT-qPCR in parental lines was used to confirm the putative candidate genes. The comparison between the drought-resistant parent (Kaybonnet) and the drought-sensitive parent (ZHE733) under DS conditions revealed that the gene expression of 15 candidate DR genes with known annotations and two candidate DR genes with unknown annotations within the DR-QTL regions was up-regulated in the drought-resistant parent (Kaybonnet). The outcomes of this research provide essential information that can be utilized in developing drought-resistant rice cultivars that have higher productivity when DS conditions are prevalent.

A myosin XI adaptor, TAPE, is essential for pollen tube elongation in rice

Pollen tube (PT) elongation is important for double fertilization in angiosperms and affects the seed-setting rate and, therefore, crop productivity. Compared to Arabidopsis (Arabidopsis thaliana L.), information on PT elongation in rice (Oryza sativa L.) is limited by the difficulty in obtaining homozygous mutants. In a screen of T-DNA insertional mutants, we identified a mutant in the Tethering protein of actomyosin transport in pollen tube elongation (TAPE) gene with an unusual segregation ratio by genotyping analysis. A CRISPR/Cas9 knockout mutant of TAPE that produced a short PT was sterile, and TAPE was expressed specifically in pollen grains. TAPE is a homolog of a myosin XI adaptor in Arabidopsis with three tetratricopeptide repeat and Phox and Bem1 protein domains. TAPE showed latrunculin B-sensitive, actin-dependent localization to the endoplasmic reticulum. Yeast two-hybrid screening and transcriptome analysis revealed that TAPE interacted with pollen-specific LIM protein 2b and elongation factor 1-alpha. Loss of TAPE affected transcription of 1,259 genes, especially genes related to cell organization, which were downregulated. In summary, TAPE encodes a myosin XI adaptor essential for rice PT elongation.

Genome-Wide Identification and Comparative Analysis of Myosin Gene Family in Four Major Cotton Species

Myosin protein as a molecular motor, binding with Actin, plays a significant role in various physiological activities such as cell division, movement, migration, and morphology; however, there are only a few studies on plant Myosin gene family, particularly in cotton. A total of 114 Myosin genes were found in Gossypium hirsutum, Gossypium barbadense, Gossypium raimondii, and Gossypium arboreum. All Myosins could be grouped into six groups, and for each group of these genes, similar gene structures are found. Study of evolution suggested that the whole genome duplications event occurring about 13-20 MYA (millions of years ago) is the key explanation for Myosins expanse in cotton. Cis-element and qPCR analysis revealed that plant hormones such as abscisic acid, methyl jasmonate, and salicylic acid can control the expression of Myosins. This research provides useful information on the function of Myosin genes in regulating plant growth, production, and fiber elongation for further studies.

Functional Diversity of Class XI Myosins in Arabidopsis thaliana

Plant myosin XI acts as a motive force for cytoplasmic streaming through interacting with actin filaments within the cell. Arabidopsis thaliana (At) has 13 genes belonging to the myosin XI family. Previous reverse genetic approaches suggest that At myosin XIs are partially redundant, but are functionally diverse for their specific tasks within the plant. However, the tissue-specific expression and enzymatic properties of myosin XIs have to date been poorly understood, primarily because of the difficulty in cloning and expressing large myosin XI genes and proteins. In this study, we cloned full-length cDNAs and promoter regions for all 13 At myosin XIs and identified tissue-specific expression (using promoter-reporter assays) and motile and enzymatic activities (using in vitro assays). In general, myosins belonging to the same class have similar velocities and ATPase activities. However, the velocities and ATPase activities of the 13 At myosin XIs are significantly different and are classified broadly into three groups based on velocity (high group, medium group and low group). Interestingly, the velocity groups appear roughly correlated with the tissue-specific expression patterns. Generally, ubiquitously expressed At myosin XIs belong to the medium-velocity group, pollen-specific At myosin XIs belong to the high-velocity group and only one At myosin XI (XI-I) is classified as belonging to the low-velocity group. In this study, we demonstrated the diversity of the 13 myosin XIs in Arabidopsis at the molecular and tissue levels. Our results indicate that myosin XIs in higher plants have distinct motile and enzymatic activities adapted for their specific roles.

Actin-myosin XI: an intracellular control network in plants

Actin is one of the three major cytoskeletal components in eukaryotic cells. Myosin XI is an actin-based motor protein in plant cells. Organelles are attached to myosin XI and translocated along the actin filaments. This dynamic actin-myosin XI system plays a major role in subcellular organelle transport and cytoplasmic streaming. Previous studies have revealed that myosin-driven transport and the actin cytoskeleton play essential roles in plant cell growth. Recent data have indicated that the actin-myosin XI cytoskeleton is essential for not only cell growth but also reproductive processes and responses to the environment. In this review, we have summarized previous reports regarding the role of the actin-myosin XI cytoskeleton in cytoplasmic streaming and plant development and recent advances in the understanding of the functions of actin-myosin XI cytoskeleton in Arabidopsis thaliana.

Plant ER geometry and dynamics: biophysical and cytoskeletal control during growth and biotic response

The endoplasmic reticulum (ER) is an intricate and dynamic network of membrane tubules and cisternae. In plant cells, the ER 'web' pervades the cortex and endoplasm and is continuous with adjacent cells as it passes through plasmodesmata. It is therefore the largest membranous organelle in plant cells. It performs essential functions including protein and lipid synthesis, and its morphology and movement are linked to cellular function. An emerging trend is that organelles can no longer be seen as discrete membrane-bound compartments, since they can physically interact and 'communicate' with one another. The ER may form a connecting central role in this process. This review tackles our current understanding and quantification of ER dynamics and how these change under a variety of biotic and developmental cues.

The molecular mechanism and physiological role of cytoplasmic streaming

Cytoplasmic streaming occurs widely in plants ranging from algae to angiosperms. However, the molecular mechanism and physiological role of cytoplasmic streaming have long remained unelucidated. Recent molecular genetic approaches have identified specific myosin members (XI-2 and XI-K as major and XI-1, XI-B, and XI-I as minor motive forces) for the generation of cytoplasmic streaming among 13 myosin XIs in Arabidopsis thaliana. Simultaneous knockout of these myosin XI members led to a reduced velocity of cytoplasmic streaming and marked defects of plant development. Furthermore, the artificial modifications of myosin XI-2 velocity changed plant and cell sizes along with the velocity of cytoplasmic streaming. Therefore, we assume that cytoplasmic streaming is one of the key regulators in determining plant size.

Genome-wide identification, splicing, and expression analysis of the myosin gene family in maize (Zea mays)

The actin-based myosin system is essential for the organization and dynamics of the endomembrane system and transport network in plant cells. Plants harbour two unique myosin groups, class VIII and class XI, and the latter is structurally and functionally analogous to the animal and fungal class V myosin. Little is known about myosins in grass, even though grass includes several agronomically important cereal crops. Here, we identified 14 myosin genes from the genome of maize (Zea mays). The relatively larger sizes of maize myosin genes are due to their much longer introns, which are abundant in transposable elements. Phylogenetic analysis indicated that maize myosin genes could be classified into class VIII and class XI, with three and 11 members, respectively. Apart from subgroup XI-F, the remaining subgroups were duplicated at least in one analysed lineage, and the duplication events occurred more extensively in Arabidopsis than in maize. Only two pairs of maize myosins were generated from segmental duplication. Expression analysis revealed that most maize myosin genes were expressed universally, whereas a few members (XI-1, -6, and -11) showed an anther-specific pattern, and many underwent extensive alternative splicing. We also found a short transcript at the O1 locus, which conceptually encoded a headless myosin that most likely functions at the transcriptional level rather than via a dominant-negative mechanism at the translational level. Together, these data provide significant insights into the evolutionary and functional characterization of maize myosin genes that could transfer to the identification and application of homologous myosins of other grasses.

Understanding myosin functions in plants: are we there yet?

Myosins are motor proteins that drive movements along actin filaments and have long been assumed to be responsible for cytoplasmic streaming in plant cells. This conjecture is now firmly established by genetic analysis in the reference species, Arabidopsis thaliana. This work and similar approaches in the moss, Physcomitrella patens, also established that myosin-driven movements are necessary for cell growth and polarity, organelle distribution and shape, and actin organization and dynamics. Identification of a mechanistic link between intracellular movements and cell expansion has proven more challenging, not the least because of the high level of apparent genetic redundancy among myosin family members. Recent progress in the creation of functional complementation constructs and identification of interaction partners promises a way out of this dilemma.

Myosin XIK of Arabidopsis thaliana accumulates at the root hair tip and is required for fast root hair growth

Myosin motor proteins are thought to carry out important functions in the establishment and maintenance of cell polarity by moving cellular components such as organelles, vesicles, or protein complexes along the actin cytoskeleton. In Arabidopsis thaliana, disruption of the myosin XIK gene leads to reduced elongation of the highly polar root hairs, suggesting that the encoded motor protein is involved in this cell growth. Detailed live-cell observations in this study revealed that xik root hairs elongated more slowly and stopped growth sooner than those in wild type. Overall cellular organization including the actin cytoskeleton appeared normal, but actin filament dynamics were reduced in the mutant. Accumulation of RabA4b-containing vesicles, on the other hand, was not significantly different from wild type. A functional YFP-XIK fusion protein that could complement the mutant phenotype accumulated at the tip of growing root hairs in an actin-dependent manner. The distribution of YFP-XIK at the tip, however, did not match that of the ER or several tip-enriched markers including CFP-RabA4b. We conclude that the myosin XIK is required for normal actin dynamics and plays a role in the subapical region of growing root hairs to facilitate optimal growth.

Whole genome duplication events in plant evolution reconstructed and predicted using myosin motor proteins

Background

The evolution of land plants is characterized by whole genome duplications (WGD), which drove species diversification and evolutionary novelties. Detecting these events is especially difficult if they date back to the origin of the plant kingdom. Established methods for reconstructing WGDs include intra- and inter-genome comparisons, K_S age distribution analyses, and phylogenetic tree constructions.

Results

By analysing 67 completely sequenced plant genomes 775 myosins were identified and manually assembled. Phylogenetic trees of the myosin motor domains revealed orthologous and paralogous relationships and were consistent with recent species trees. Based on the myosin inventories and the phylogenetic trees, we have identified duplications of the entire myosin motor protein family at timings consistent with 23 WGDs, that had been reported before. We also predict 6 WGDs based on further protein family duplications. Notably, the myosin data support the two recently reported WGDs in the common ancestor of all extant angiosperms. We predict single WGDs in the Manihot esculenta and Nicotiana benthamiana lineages, two WGDs for Linum usitatissimum and Phoenix dactylifera, and a triplication or two WGDs for Gossypium raimondii. Our data show another myosin duplication in the ancestor of the angiosperms that could be either the result of a single gene duplication or a remnant of a WGD.

Conclusions

We have shown that the myosin inventories in angiosperms retain evidence of numerous WGDs that happened throughout plant evolution. In contrast to other protein families, many myosins are still present in extant species. They are closely related and have similar domain architectures, and their phylogenetic grouping follows the genome duplications. Because of its broad taxonomic sampling the dataset provides the basis for reliable future identification of further whole genome duplications.

Plant-Specific Myosin XI, a Molecular Perspective

In eukaryotic cells, organelle movement, positioning, and communications are critical for maintaining cellular functions and are highly regulated by intracellular trafficking. Directional movement of motor proteins along the cytoskeleton is one of the key regulators of such trafficking. Most plants have developed a unique actin-myosin system for intracellular trafficking. Although the composition of myosin motors in angiosperms is limited to plant-specific myosin classes VIII and XI, there are large families of myosins, especially in class XI, suggesting functional diversification among class XI members. However, the molecular properties and regulation of each myosin XI member remains unclear. To achieve a better understanding of the plant-specific actin-myosin system, the characterization of myosin XI members at the molecular level is essential. In the first half of this review, we summarize the molecular properties of tobacco 175-kDa myosin XI, and in the later half, we focus on myosin XI members in Arabidopsis thaliana. Through detailed comparison of the functional domains of these myosins with the functional domain of myosin V, we look for possible diversification in enzymatic and mechanical properties among myosin XI members concomitant with their regulation.

Recent advances in understanding plant myosin function: life in the fast lane

Plant myosins are required for organelle movement, and a role in actin organization has recently come to light. Myosin mutants display several gross morphological phenotypes, the most severe being dwarfism and reduced fecundity, and there is a correlation between reduced organelle movement and morphological defects. This review aims to discuss recent findings in plants relating to the role of myosins in actin dynamics, development, and organelle movement, more specifically the endoplasmic reticulum (ER). One overarching theme is that there still appear to be more questions than answers relating to plant myosin function and regulation.

Cold stress contributes to aberrant cytokinesis during male meiosis I in a wheat thermosensitive genic male sterile line

The male sterility of a wheat thermosensitive genic male sterile (TGMS) line is strictly controlled by temperature. When the TGMS line BS366 was exposed to 10 °C from the pollen mother cell stage to the meiosis stage, a few pollen grains were formed and devoid of starch. We report here a large-scale transcriptomic study using the Affymetrix wheat GeneChip to follow gene expression in BS366 line anthers in response to cold stress. Notably, many cytoskeletal signaling components were gradually induced in response to cold stress in BS366 line anthers. However, the cytoskeleton-associated genes that play key roles in the dynamic organization of the cytoskeleton were dramatically repressed. Histological studies revealed that the separation of dyads occurred abnormally during male meiosis I, indicating defective male meiotic cytokinesis. Fluorescence labelling and subcellular histological observations revealed that the phragmoplast was defectively formed and the cell plate was abnormally assembled during meiosis I under cold stress. Based on the transcriptomic analysis and observations of characterized histological changes, our results suggest that cold stress repressed transcription of cytoskeleton dynamic factors and subsequently caused the defective cytokinesis during meiosis I. The results may explain the male sterility caused by low temperature in wheat TGMS lines.

Motoring around the plant cell: insights from plant myosins

Organelle movement in plants cells is extremely dynamic. Movement is driven by the acto-myosin system. Higher plant myosins fall into two classes: classes XI and VIII. Localization studies have highlighted that myosins are present throughout the cytosol, label motile puncta and decorate the nuclear envelope and plasma membrane. Functional studies through expression of dominant-negative myosin variants, RNAi (RNA interference) and T-DNA insertional analysis have shown that class XI myosins are required for organelle movement. Intriguingly, organelle movement is also linked to Arabidopsis growth and development. The present review tackles current findings relating to plant organelle movement and the role of myosins.

Genome-wide analysis of cyclins in maize (Zea mays)

Cyclins are primary regulators of the activity of cyclin-dependent kinases and play crucial roles in cell cycle progression in eukaryotes. Although extensive studies have revealed the roles of some cyclins and underlying mechanisms in plants, relatively few cyclins have been functionally analyzed in maize. We identified 59 cyclins in the maize genome, distributed on 10 chromosomes; these were grouped into six types by phylogenetic analysis. The cyclin genes in the maize genome went through numerous tandem gene duplications on five chromosomes. However, no segmental duplications, which occur in rice, were found on maize chromosomes. This information allows us to assess the position of plant cyclin genes in terms of evolution and classification, which will be useful for functional studies of maize cyclins.

Natural and artificial mutants as valuable resources for functional genomics and molecular breeding

With the completion of rice genome sequencing, large collection of expression data and the great efforts in annotating rice genomes, the next challenge is to systematically assign functions to all predicted genes in the genome. The generations and collections of mutants at the genome-wide level form technological platform of functional genomics. In this study, we have reviewed currently employed tools to generate such mutant populations. These tools include natural, physical, chemical, tissue culture, T-DNA, transposon or gene silencing based mutagenesis. We also reviewed how these tools were used to generate a large collection of mutants and how these mutants can be screened and detected for functional analysis of a gene. The data suggested that the current population of mutants might be large enough to tag all predicted genes. However, the collection of flanking sequencing tags (FSTs) is limited due to the relatively higher cost. Thus, we have proposed a new strategy to generate gene-silencing mutants at the genome-wide level. Due to the large collection of insertion mutants, the next step to rice functional genomics should be focusing on functional characterization of tagged genes by detailed survey of corresponding mutants. Additionally, we also evaluated the utilization of these mutants as valuable resources for molecular breeding.

Peroxisome biogenesis and function

Peroxisomes are small and single membrane-delimited organelles that execute numerous metabolic reactions and have pivotal roles in plant growth and development. In recent years, forward and reverse genetic studies along with biochemical and cell biological analyses in Arabidopsis have enabled researchers to identify many peroxisome proteins and elucidate their functions. This review focuses on the advances in our understanding of peroxisome biogenesis and metabolism, and further explores the contribution of large-scale analysis, such as in sillco predictions and proteomics, in augmenting our knowledge of peroxisome function In Arabidopsis.

Organelle motility in the pollen tube: a tale of 20 years

Organelle movement is an evident feature of pollen tubes and is essential for the process of tube growth because it enables the proper distribution of organelles and the accumulation of secretory vesicles in the tube apex. Organelles move along the actin filaments through dynamic interactions with myosin but other proteins are probably responsible for control of this activity. The role of microtubules and microtubule-based motors is less clear and somewhat enigmatic. Nevertheless, the pollen tube is an excellent cell model in which to study and analyse the molecular mechanisms that drive and control organelle motility in relation to plant cell expansion. Current knowledge and the main scientific discoveries in this field of research over the last 20 years are summarized here. Future prospects in the study of the molecular mechanisms that mediate organelle transport and vesicle accumulation during pollen tube elongation are also discussed.

Subnuclear compartmentalization and function of actin and nuclear myosin I in plants

Actins are highly conserved proteins that serve as the basic building blocks of cytoskeletal microfilaments. In animal cells, specific nuclear actin adopts unconventional conformations that are involved in multiple nuclear functions and that associate with nuclear actin binding proteins. However, there is practically no information available about nuclear actin in plants. Indeed, actin has not been detected in the nuclear proteomes of many plants, and orthologs of the main structural nuclear actin-binding proteins have yet to be identified. Here, we have investigated the characteristics, intranuclear compartmentalization, and function of actin in isolated Allium cepa nuclei as well as that of its motor protein nuclear myosin I (NMI). Using conformation-specific antibodies for nuclear actin isoforms, ss-actin, and NMI, the distribution of these proteins was studied in Western blots and by immunocytochemistry. Moreover, the participation of nuclear actin in transcription was analyzed in run on in situ assays and inhibition of RNA polymerases I and II. We show that actin isoforms with distinct solubilities are present in onion nuclei with a consistent subnuclear compartmentalization. Actin and NMI are highly enriched in foci that are similar to transcription foci, although actin is also distributed diffusely in the nucleus and nucleolus as well as accumulating in a subset of the Cajal bodies. Immunogold labeling identified both proteins in the nuclear transcription subdomains and in other subnuclear compartments. In addition, actin and NMI were diffusely distributed in the nuclear matrix.

Inter-dependence of dimerization and organelle binding in myosin XI

Cytoplasmic streaming is a ubiquitous process in plant cells that is thought to be driven by the active movement of myosin XI motor proteins along actin filaments. These myosin motors bind to organelles through their C-terminal globular tail domain, although recent studies have also suggested a role for the central coiled-coil region during organelle binding. Here we have investigated the relationship between these two protein domains of MYA1, an Arabidopsis myosin XI, in a series of in vivo experiments demonstrating that dimerization of the coiled-coil region stabilizes organelle binding of the globular tail. Surprisingly, yeast two-hybrid assays, bimolecular fluorescence complementation, Förster resonance energy transfer and in vitro pull-down experiments all demonstrated that dimerization of the 174-residue MYA1 coiled coils by themselves was unstable. Furthermore, only the first of the two major coiled-coil segments in MYA1 contributed significantly to dimer formation. Interestingly, dimerization of myosin tail constructs that included the organelle-binding globular tail was stable, although the globular tails by themselves did not interact. This suggests an inter-dependent relationship between dimerization and organelle binding in myosin XI, whereby each process synergistically stimulates the other.

Class VIII myosins are required for plasmodesmatal localization of a closterovirus Hsp70 homolog

The Hsp70 homolog (Hsp70h) of Beet yellows virus (BYV) functions in virion assembly and cell-to-cell movement and is autonomously targeted to plasmodesmata in association with the actomyosin motility system (A. I. Prokhnevsky, V. V. Peremyslov, and V. V. Dolja, J. Virol. 79:14421-14428, 2005). Myosins are a diverse category of molecular motors that possess a motor domain and a tail domain involved in cargo binding. Plants have two classes of myosins, VIII and XI, whose specific functions are poorly understood. We used dominant negative inhibition to identify myosins required for Hsp70h localization to plasmodesmata. Six full-length myosin cDNAs from the BYV host plant Nicotiana benthamiana were sequenced and shown to encode apparent orthologs of the Arabidopsis thaliana myosins VIII-1, VIII-2, VIII-B, XI-2, XI-F, and XI-K. We found that the ectopic expression of the tail domains of each of the class VIII, but not the class XI, myosins inhibited the plasmodesmatal localization of Hsp70h. In contrast, the overexpression of the motor domains or the entire molecules of the class VIII myosins did not affect Hsp70h targeting. Further mapping revealed that the minimal cargo-binding part of the myosin VIII tails was both essential and sufficient for the inhibition of the proper Hsp70h localization. Interestingly, plasmodesmatal localization of the Tobacco mosaic virus movement protein and Arabidopsis protein RGP2 was not affected by myosin VIII tail overexpression. Collectively, our data implicate class VIII myosins in protein delivery to plasmodesmata and suggest that more than one mechanism of such delivery exist in plants.

The tail that wags the dog: the globular tail domain defines the function of myosin V/XI

Actin-based organelle movements are driven by the related multifunctional myosin motors of class V in animals and fungi and class XI in plants. The versatility of these motors depends critically on their C-terminal globular tail domain that allows them to bind to a broad variety of cargo molecules. Regulation of this motor-cargo attachment is frequently employed to modulate organelle movement. While the overall structure of the cargo-binding globular tail appears to be conserved between myosin V and XI, it has become apparent that the motor-cargo interactions differ widely even within a single organism and involve protein complexes with different architecture and completely unrelated protein domains. At the same time, indirect evidence suggests that adaptor or receptor dimerization could facilitate efficient myosin capture. Comparison of myosin V and XI across the large evolutionary distance between animals and plants will likely reveal more fundamental insights into these important motors.

Comparative transcriptional profiling and evolutionary analysis of the GRAM domain family in eukaryotes

The GRAM domain was found in glucosyltransferases, myotubularins and other membrane-associated proteins. So far, functions for majority of these proteins are yet to be uncovered. In order to address the evolutionary and functional significance of this family members, we have performed a comprehensive investigation on their genome-wide identification, phylogenetic relationship and expression divergence in five different organisms representing monocot/dicot plants, vertebrate/invertebrate animals and yeast, namely, Oryza sativa, Arabidopsis thaliana, Mus musculus, Drosophila melanogaster and Saccharomyces cerevisiae, respectively. We have identified 65 members of GRAM domain family from these organisms. Our data revealed that this family was an ancient group and various organisms had evolved into different family sizes. Large-scale genome duplication and divergence in both expression patterns and functions were significantly contributed to the expansion and retention of this family. Mouse and Drosophila members showed higher divergences in their proteins as indicated by higher Ka/Ks ratios and possessed multiple domains in various combinations. However, in plants, their protein functions were possibly retained with a relatively low divergence as signified by lower Ka/Ks ratios and only one additional domain was combined during evolution. On the other hand, this family in all five organisms exhibited high divergence in their expression patterns both at tissue level and under various biotic and abiotic stresses. These highly divergent expression patterns unraveled the complexity of functions of GRAM domain family. Each member may play specialized roles in a specific tissue or stress condition and may function as regulators of environmental and hormonal signaling.

Actin microfilament dynamics and actin side-binding proteins in plants

Actin microfilaments are highly organized and essential intracellular components of organelle movement and cell morphogenesis in plants. The organization of these microfilaments undergoes dynamic changes during cell division, elongation, and differentiation. Recent live-cell imaging of plant actin microfilaments has revealed their native organization and remarkable dynamics. In addition, characterization of plant actin side-binding proteins has progressed rapidly by genetic, biochemical, and bioinformatic approaches. The gathering and integration of microscopy-based information from actin microfilament dynamics and the molecular identification of actin side-binding proteins have provided considerable insights into actin microfilament-dependent events and actin microfilament organization in plants.

Expression of unconventional myosin genes during neuronal development in zebrafish

Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (1, 2, 3, 5, 6, 7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

Comparison of gene expression between upland and lowland rice cultivars under water stress using cDNA microarray

To elucidate the differences in the regulation of water stress tolerance between two genotypes of rice, upland-rice (UR, resistant to water stress) and lowland-rice (LR, susceptible to water stress), we constructed subtracted cDNA libraries from polyethyleneglycol (PEG)-treated and non-treated rice seedlings (IRAT109, an upland-rice variety) by suppression subtractive hybridization (SSH), from which about 2,000 recombinant colonies were picked and amplified. Then, a cDNA microarray containing these expressed sequence tags (ESTs) was used to analyze the gene expression profiles in UR and LR in response to PEG treatment. Microarray data revealed that the majority of genes expressed in UR and LR are almost identical and Student's t test showed that 13% of all the ESTs detected in leaves and 7% of that in roots expressed differentially in transcripts abundance between the two genotypes. After sequencing, it was found that 64 and 79 unique ESTs expressed at higher levels in UR and LR, respectively. Many of the ESTs that showed higher expression in UR upon PEG treatment represented genes for transcription factors, genes playing roles in detoxification or protection against oxidative stress, and genes that help in maintaining cell turgor. In contrast, some ESTs that showed higher expression in LR were genes functioning in the degradation of cellular components. Based on data from this study and previous reports, we suggest that overexpression of some genes that expressed at higher level in UR may improve water stress tolerance in LR and other plant species.

Organelle targeting of myosin XI is mediated by two globular tail subdomains with separate cargo binding sites

Myosin XI are actin-based molecular motors that are thought to drive organelle movements in plants, analogous to myosin V in animals and fungi. Similar domain structure of these myosins suggests that binding to organelles may occur via the globular tail domain in both types of motors, even though sequence similarity is low. To address this hypothesis, we developed a structure homology model for the globular tail of MYA1, a myosin XI from Arabidopsis, based on the known structure of yeast myosin V (Myo2p) globular tail. This model suggested an interaction between two subdomains of the globular tail which was verified by yeast two-hybrid assay and by in vivo bimolecular fluorescence complementation (BiFC). Interface mapping demonstrated that this subdomain interaction depends critically on the C terminus of helix H6 as well as three specific residues in helices H3 and H15, consistent with the structural prediction. The reconstituted globular tails of several Arabidopsis myosin XIs in BiFC assays targeted to peroxisomes in plant cells, identifying this domain as sufficient for cargo binding. Unlike myosin V, either subdomain of myosin XI alone was targeting-competent and responsible for association with different organelles. In addition, our data suggest that organelle binding is regulated by an allosteric interaction between two tail subdomains. We conclude that the globular tail of myosin XI shares a similar structure with that of myosin V, but has evolved plant-specific cargo binding mechanisms.

ORYZA SATIVA MYOSIN XI B controls pollen development by photoperiod-sensitive protein localizations

Myosins are actin-based motor proteins responsible for various motility and signal transduction. Only a small set of myosin classes is present inplants, and little is known about their functions. Here we showed how a rice myosin gene controlled pollen development by sensing changed environmental factors. The analysis is based on a gene-trapped Ds insertion mutant Oryza sativa myosin XI B (osmyoXIB). This mutant showed male sterility under short day length (SD) conditions and fertility under long day length (LD) conditions. Under both SD and LD conditions, the OSMYOXIB transcript was detected in whole anthers. However, under SD conditions, the OSMYOXIB-GUS fusion protein was localized only in the epidermal layer of anthers due to the lack of 3'-untranslated region (3'-UTR) and to dilute (DIL) domain sequences following the Ds insertion. As a result, mutant pollen development was affected, leading to male sterility. By contrast, under LD conditions, the fusion protein was localized normally in anthers. Despite normal localization, the protein was only partially functional due to the lack of DIL domain sequences, resulting in limited recovery of pollen fertility. This study also provides a case for a novel molecular aspect of gene expression, i.e., cell layer-specific translation in anthers.

Association of six YFP-myosin XI-tail fusions with mobile plant cell organelles

BACKGROUND

Myosins are molecular motors that carry cargo on actin filaments in eukaryotic cells. Seventeen myosin genes have been identified in the nuclear genome of Arabidopsis. The myosin genes can be divided into two plant-specific subfamilies, class VIII with four members and class XI with 13 members. Class XI myosins are related to animal and fungal myosin class V that are responsible for movement of particular vesicles and organelles. Organelle localization of only one of the 13 Arabidopsis myosin XI (myosin XI-6; At MYA2), which is found on peroxisomes, has so far been reported. Little information is available concerning the remaining 12 class XI myosins.

RESULTS

We investigated 6 of the 13 class XI Arabidopsis myosins. cDNAs corresponding to the tail region of 6 myosin genes were generated and incorporated into a vector to encode YFP-myosin tail fusion proteins lacking the motor domain. Chimeric genes incorporating tail regions of myosin XI-5 (At MYA1), myosin XI-6 (At MYA2), myosin XI-8 (At XI-B), myosin XI-15 (At XI-I), myosin XI-16 (At XI-J) and myosin XI-17 (At XI-K) were expressed transiently. All YFP-myosin-tail fusion proteins were targeted to small organelles ranging in size from 0.5 to 3.0 mum. Despite the absence of a motor domain, the fluorescently-labeled organelles were motile in most cells. Tail cropping experiments demonstrated that the coiled-coil region was required for specific localization and shorter tail regions were inadequate for targeting. Myosin XI-6 (At MYA2), previously reported to localize to peroxisomes by immunofluorescence, labeled both peroxisomes and vesicles when expressed as a YFP-tail fusion. None of the 6 YFP-myosin tail fusions interacted with chloroplasts, and only one YFP-tail fusion appeared to sometimes co-localize with fluorescent proteins targeted to Golgi and mitochondria.

CONCLUSION

6 myosin XI tails, extending from the coiled-coil region to the C-terminus, label specific vesicles and/or organelles when transiently expressed as YFP fusions in plant cells. Although comparable constructs lacking the motor domain result in a dominant negative effect on organelle motility in animal systems, the plant organelles remained motile. YFP-myosin tail fusions provide specific labeling for vesicles of unknown composition, whose identity can be investigated in future studies.

Arabidopsis thaliana myosin XIK is involved in root hair as well as trichome morphogenesis on stems and leaves

Myosins form a large superfamily of molecular motors that move along actin filaments. The functions of myosins in plant cells are thought to be related to various processes: cell division, movement of mitochondria and chloroplasts, cytoplasmic streaming, rearrangement of transvacuolar strands, and statolith positioning. Class VIII and XI myosins are represented in the Arabidopsis thaliana genome by 4 and 13 potential genes, respectively. The roles of individual class XI myosins and their cellular targets in A. thaliana are still unclear. In this work we implemented a reverse genetic approach to analyse the loss-of-function mutants of XIK, a representative of class XI myosins in A. thaliana. Three different T-DNA insertion mutants in the myosin XIK gene showed similar phenotypes: impaired growth of root hair cells, twisted shape of stem trichomes, and irregular size, branch positioning, and branch expansion of leaf trichomes. Morphometric analysis of mutant seedlings showed that the average length of root hairs was reduced up to 50% in comparison with wild-type root hairs, suggesting an involvement of the class XI myosin XIK in tip growth. On leaves, the proportion of trichomes with short branches was doubleed in mutant plants, and the mutant trichomes possessed a mildly twisted shape. Therefore, we concluded that myosin XIK is involved also in the elongation of stalks and branches of trichomes.

Isolation of myosin XI genes from the Closterium peracerosum-strigosum-littorale complex and analysis of their expression during sexual reproduction

Myosins comprise a large superfamily of molecular motors that generate mechanical force in ATP-dependent interactions with actin filaments. On the basis of their conserved motor domain sequences, myosins can be divided into at least 17 classes, 3 of which (VIII, XI, XIII) are found in plants. Although full sequences of myosins are available from several species of green plants, little is known about the functions of these proteins. Additionally, sequence information for algal myosin is incomplete, and little attention has been given to the molecular evolution of myosin from green plants. In the present study, the Closterium peracerosum-strigosum-littorale complex was used as a model system for investigating a unicellular basal charophycean alga. This organism has been well studied with respect to sexual reproduction between its two mating types. Three types of partial sequences belonging to class XI myosins were obtained using degenerate primers designed to amplify motor domain sequences. Real-time polymerase chain reaction analysis of the respective myosin genes during various stages of the algal life cycle showed that one of the genes was more highly expressed during sexual reproduction, and that expression was cell-cycle-dependent in vegetatively grown cells.

Genome-wide analysis of cyclin family in rice (Oryza Sativa L.)

The cyclins together with highly conserved cyclin-dependent kinases regulate cell cycle progression in plants. Although extensive and systematic study on cell cycle mechanisms and cyclin functions in yeasts and animals has been carried out, only a small number of plant cyclins have been characterized and classified functionally and phylogenetically. We identified several types of cyclin genes in the rice genome and characterized them by phylogenetic, tandem and segmental duplications analyses. Our results indicated that there were at least 49 predicted rice cyclin genes in the rice genome, and they were distributed on 12 chromosomes. Of these cyclins, one possessed only cyclin_C domain and no cyclin_N domain, and the remaining 48 cyclins with cyclin_N domains were classified as nine types based on evolutionary relationships. Eight of these nine types were common between rice and Arabidopsis, whereas only one, known as F-type cyclins, was unique to rice. No homologues of the F-type cyclins in plants could be retrieved from the public databases, and reverse transcription-PCR analysis supported an existence of the F-type cyclin genes. Sequence alignment suggested that the cyclin genes in the rice genome experienced a mass of gene tandem and segmental duplications occurred on seven chromosomes related to the origins of new cyclin genes. Our study provided an opportunity to facilitate assessment and classification of new members, serving as a guide for further functional elucidation of rice cyclins.