Cytoplasmic streaming velocity as a plant size determinant

Actin network evolution as a key driver of eukaryotic diversification

Eukaryotic cells have been evolving for billions of years, giving rise to wildly diverse cell forms and functions. Despite their variability, all eukaryotic cells share key hallmarks, including membrane-bound organelles, heavily regulated cytoskeletal networks and complex signaling cascades. Because the actin cytoskeleton interfaces with each of these features, understanding how it evolved and diversified across eukaryotic phyla is essential to understanding the evolution and diversification of eukaryotic cells themselves. Here, we discuss what we know about the origin and diversity of actin networks in terms of their compositions, structures and regulation, and how actin evolution contributes to the diversity of eukaryotic form and function.

Emergent Spatiotemporal Organization in Stochastic Intracellular Transport Dynamics

The interior of a living cell is an active, fluctuating, and crowded environment, yet it maintains a high level of coherent organization. This dichotomy is readily apparent in the intracellular transport system of the cell. Membrane-bound compartments called endosomes play a key role in carrying cargo, in conjunction with myriad components including cargo adaptor proteins, membrane sculptors, motor proteins, and the cytoskeleton. These components coordinate to effectively navigate the crowded cell interior and transport cargo to specific intracellular locations, even though the underlying protein interactions and enzymatic reactions exhibit stochastic behavior. A major challenge is to measure, analyze, and understand how, despite the inherent stochasticity of the constituent processes, the collective outcomes show an emergent spatiotemporal order that is precise and robust. This review focuses on this intriguing dichotomy, providing insights into the known mechanisms of noise suppression and noise utilization in intracellular transport processes, and also identifies opportunities for future inquiry.

Transcriptome and functional analyses of phenotypic plasticity in sea grape Caulerpa okamurae

Caulerpa is a marine green macroalga distinguished by a large single cell with multiple nuclei. It also exhibits remarkable morphological intraspecies variations, in response to diverse environmental types. However, the molecular mechanisms underlying this phenotypic plasticity remain poorly understood. In this work, we compare the transcriptomes of Caulerpa okamurae Weber Bosse, 1897 displaying altered phenotypes of cultivation and natural phenotypes and investigate significantly regulated genes and their biological functions using differential expression analyses. We observe light-harvesting complex upregulation and cellular framework stability downregulation in altered phenotypes compared to the natural phenotypes. Intertidal macrophytes reduce light capture to avoid photodamage and regulate their morphology to protect against wave damage. In contrast, the lower light conditions and the cultivation environment augment light capture and increase a morphology prioritizing light trapping. Moreover, the addition of simulated wave-sweeping stimuli induces a return to the natural morphology under high-light conditions, showing how mechanical stress affects morphological organization in C. okamurae. We provide detailed gene expression patterns in C. okamurae under varying light intensities and water conditions, suggesting a distinct influence on its morphological traits.

Myosin XI, a model of its conserved role in plant cell tip growth

In eukaryotic cells, organelle and vesicle transport, positioning, and interactions play crucial roles in cytoplasmic organization and function. These processes are governed by intracellular trafficking mechanisms. At the core of that trafficking, the cytoskeleton and directional transport by motor proteins stand out as its key regulators. Plant cell tip growth is a well-studied example of cytoplasm organization by polarization. This polarization, essential for the cell's function, is driven by the cytoskeleton and its associated motors. This review will focus on myosin XI, a molecular motor critical for vesicle trafficking and polarized plant cell growth. We will center our discussion on recent data from the moss Physcomitrium patens and the liverwort Marchantia polymorpha. The biochemical properties and structure of myosin XI in various plant species are discussed, highlighting functional conservation across species. We further explore this conservation of myosin XI function in the process of vesicle transport in tip-growing cells. Existing evidence indicates that myosin XI actively organizes actin filaments in tip-growing cells by a mechanism based on vesicle clustering at their tips. A hypothetical model is presented to explain the essential function of myosin XI in polarized plant cell growth based on vesicle clustering at the tip. The review also provides insight into the in vivo localization and dynamics of myosin XI, emphasizing its role in cytosolic calcium regulation, which influences the polymerization of F-actin. Lastly, we touch upon the need for additional research to elucidate the regulation of myosin function.

Rediscovering Chara as a model organism for molecular and evo-devo studies

Chara has been used as a model for decades in the field of plant physiology, enabling the investigation of fundamental physiological processes. In electrophysiological studies, Chara has been utilized thanks to its large internodal cells that can be easily manipulated. Additionally, Chara played a pioneering role in elucidating the presence and function of the cytoskeleton in cytoplasmic streaming, predating similar findings in terrestrial plants. Its representation considerably declined following the establishment and routine application of genetic transformation techniques in Arabidopsis. Nevertheless, the recent surge in evo-devo studies can be attributed to the whole genome sequencing of the Chara braunii, which has shed light on ancestral traits prevalent in land plants. Surprisingly, the Chara braunii genome encompasses numerous genes that were previously regarded as exclusive to land plants, suggesting their acquisition prior to the colonization of terrestrial habitats. This review summarizes the established methods used to study Chara, while incorporating recent molecular data, to showcase its renewed importance as a model organism in advancing plant evolutionary developmental biology.

RAF-like protein kinases mediate a deeply conserved, rapid auxin response

The plant-signaling molecule auxin triggers fast and slow cellular responses across land plants and algae. The nuclear auxin pathway mediates gene expression and controls growth and development in land plants, but this pathway is absent from algal sister groups. Several components of rapid responses have been identified in Arabidopsis, but it is unknown if these are part of a conserved mechanism. We recently identified a fast, proteome-wide phosphorylation response to auxin. Here, we show that this response occurs across 5 land plant and algal species and converges on a core group of shared targets. We found conserved rapid physiological responses to auxin in the same species and identified rapidly accelerated fibrosarcoma (RAF)-like protein kinases as central mediators of auxin-triggered phosphorylation across species. Genetic analysis connects this kinase to both auxin-triggered protein phosphorylation and rapid cellular response, thus identifying an ancient mechanism for fast auxin responses in the green lineage.

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

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

The nuclear lamina is required for proper development and nuclear shape distortion in tomato

The nuclear lamina in plant cells is composed of plant-specific proteins, including nuclear matrix constituent proteins (NMCPs), which have been postulated to be functional analogs of lamin proteins that provide structural integrity to the organelle and help stabilize the three-dimensional organization of the genome. Using genomic editing, we generated alleles for the three genes encoding NMCPs in cultivated tomato (Solanum lycopersicum) to determine if the consequences of perturbing the nuclear lamina in this crop species were similar to or distinct from those observed in the model Arabidopsis thaliana. Loss of the sole NMCP2-class protein was lethal in tomato but is tolerated in Arabidopsis. Moreover, depletion of NMCP1-type nuclear lamina proteins leads to distinct developmental phenotypes in tomato, including leaf morphology defects and reduced root growth rate (in nmcp1b mutants), compared with cognate mutants in Arabidopsis. These findings suggest that the nuclear lamina interfaces with different developmental and signaling pathways in tomato compared with Arabidopsis. At the subcellular level, however, tomato nmcp mutants resembled their Arabidopsis counterparts in displaying smaller and more spherical nuclei in differentiated cells. This result argues that the plant nuclear lamina facilitates nuclear shape distortion in response to forces exerted on the organelle within the cell.

Mapping of the Classical Mutation rosette Highlights a Role for Calcium in Wound-Induced Rooting

Removal of the root system induces the formation of new roots from the remaining shoot. This process is primarily controlled by the phytohormone auxin, which interacts with other signals in a yet unresolved manner. Here, we study the classical tomato mutation rosette (ro), which lacks shoot-borne roots. ro mutants were severely inhibited in formation of wound-induced roots (WiRs) and had reduced auxin transport rates. We mapped ro to the tomato ortholog of the Arabidopsis thaliana BIG and the mammalians UBR4/p600. RO/BIG is a large protein of unknown biochemical function. In A. thaliana, BIG was implicated in regulating auxin transport and calcium homeostasis. We show that exogenous calcium inhibits WiR formation in tomato and A. thaliana ro/big mutants. Exogenous calcium antagonized the root-promoting effects of the auxin indole-3-acetic-acid but not of 2,4-dichlorophenoxyacetic acid, an auxin analog that is not recognized by the polar transport machinery, and accumulation of the auxin transporter PIN-FORMED1 (PIN1) was sensitive to calcium levels in the ro/big mutants. Consistent with a role for calcium in mediating auxin transport, both ro/big mutants and calcium-treated wild-type plants were hypersensitive to treatment with polar auxin transport inhibitors. Subcellular localization of BIG suggests that, like its mammalian ortholog, it is associated with the endoplasmic reticulum. Analysis of subcellular morphology revealed that ro/big mutants exhibited disruption in cytoplasmic streaming. We suggest that RO/BIG maintains auxin flow by stabilizing PIN membrane localization, possibly by attenuating the inhibitory effect of Ca2+ on cytoplasmic streaming.

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

Regulation of organelle size and organization during development

During early embryogenesis, as cells divide in the developing embryo, the size of intracellular organelles generally decreases to scale with the decrease in overall cell size. Organelle size scaling is thought to be important to establish and maintain proper cellular function, and defective scaling may lead to impaired development and disease. However, how the cell regulates organelle size and organization are largely unanswered questions. In this review, we summarize the process of size scaling at both the cell and organelle levels and discuss recently discovered mechanisms that regulate this process during early embryogenesis. In addition, we describe how some recently developed techniques and Xenopus as an animal model can be used to investigate the underlying mechanisms of size regulation and to uncover the significance of proper organelle size scaling and organization.

Cytoplasmic organization promotes protein diffusion in Xenopus extracts

The cytoplasm is highly organized. However, the extent to which this organization influences the dynamics of cytoplasmic proteins is not well understood. Here, we use Xenopus laevis egg extracts as a model system to study diffusion dynamics in organized versus disorganized cytoplasm. Such extracts are initially homogenized and disorganized, and self-organize into cell-like units over the course of tens of minutes. Using fluorescence correlation spectroscopy, we observe that as the cytoplasm organizes, protein diffusion speeds up by about a factor of two over a length scale of a few hundred nanometers, eventually approaching the diffusion time measured in organelle-depleted cytosol. Even though the ordered cytoplasm contained organelles and cytoskeletal elements that might interfere with diffusion, the convergence of protein diffusion in the cytoplasm toward that in organelle-depleted cytosol suggests that subcellular organization maximizes protein diffusivity. The effect of organization on diffusion varies with molecular size, with the effects being largest for protein-sized molecules, and with the time scale of the measurement. These results show that cytoplasmic organization promotes the efficient diffusion of protein molecules in a densely packed environment.

Cytoplasmic organization is a hallmark of living cells. Here, the authors make use of self-organizing cell extracts to examine how the emergence of large-scale organizations influences the microscopic diffusion of protein molecules in the cytoplasm.

Differential gene expression in tall fescue tissues in response to water deficit

Tall fescue (Festuca arundinacea Schreb.) is a popular pasture and turf grass particularly known for drought resistance, allowing for its persistence in locations that are unfavorable for other cool-season grasses. Also, its seed-borne fungal symbiont (endophyte) Epichloë coenophiala, which resides in the crown and pseudostem, can be a contributing factor in its drought tolerance. Because it contains the apical meristems, crown survival under drought stress is critical to plant survival as well as the endophyte. In this study, we subjected tall fescue plants with their endophyte to water-deficit stress or, as controls with normal watering, then compared plant transcriptome responses in four vegetative tissues: leaf blades, pseudostem, crown, and roots. A transcript was designated a differentially expressed gene (DEG) if it exhibited at least a twofold expression difference between stress and control samples with an adjusted p value of .001. Pathway analysis of the DEGs across all tissue types included photosynthesis, carbohydrate metabolism, phytohormone biosynthesis and signaling, cellular organization, and a transcriptional regulation. While no specific pathway was observed to be differentially expressed in the crown, genes encoding auxin response factors, nuclear pore anchors, structural maintenance of chromosomes, and class XI myosin proteins were more highly differentially expressed in crown than in the other vegetative tissues, suggesting that regulation in expression of these genes in the crown may aid in survival of the meristems in the crown.

Looking beyond the gene network - metabolic and mechanical cell drivers of leaf morphogenesis

The above-ground organs in plants display a rich diversity, yet they grow to characteristic sizes and shapes. Organ morphogenesis progresses through a sequence of key events, which are robustly executed spatiotemporally as an emerging property of intrinsic molecular networks while adapting to various environmental cues. This Review focuses on the multiscale control of leaf morphogenesis. Beyond the list of known genetic determinants underlying leaf growth and shape, we focus instead on the emerging novel mechanisms of metabolic and biomechanical regulations that coordinate plant cell growth non-cell-autonomously. This reveals how metabolism and mechanics are not solely passive outcomes of genetic regulation but play instructive roles in leaf morphogenesis. Such an integrative view also extends to fluctuating environmental cues and evolutionary adaptation. This synthesis calls for a more balanced view on morphogenesis, where shapes are considered from the standpoints of geometry, genetics, energy and mechanics, and as emerging properties of the cellular expression of these different properties.

Discovery of ultrafast myosin, its amino acid sequence, and structural features

Cytoplasmic streaming with extremely high velocity (∼70 μm s^-1) occurs in cells of the characean algae (Chara). Because cytoplasmic streaming is caused by myosin XI, it has been suggested that a myosin XI with a velocity of 70 μm s^-1, the fastest myosin measured so far, exists in Chara cells. However, the velocity of the previously cloned Chara corallina myosin XI (CcXI) was about 20 μm s^-1, one-third of the cytoplasmic streaming velocity in Chara Recently, the genome sequence of Chara braunii has been published, revealing that this alga has four myosin XI genes. We cloned these four myosin XI (CbXI-1, 2, 3, and 4) and measured their velocities. While the velocities of CbXI-3 and CbXI-4 motor domains (MDs) were similar to that of CcXI MD, the velocities of CbXI-1 and CbXI-2 MDs were 3.2 times and 2.8 times faster than that of CcXI MD, respectively. The velocity of chimeric CbXI-1, a functional, full-length CbXI-1 construct, was 60 μm s^-1 These results suggest that CbXI-1 and CbXI-2 would be the main contributors to cytoplasmic streaming in Chara cells and show that these myosins are ultrafast myosins with a velocity 10 times faster than fast skeletal muscle myosins in animals. We also report an atomic structure (2.8-Å resolution) of myosin XI using X-ray crystallography. Based on this crystal structure and the recently published cryo-electron microscopy structure of acto-myosin XI at low resolution (4.3-Å), it appears that the actin-binding region contributes to the fast movement of Chara myosin XI. Mutation experiments of actin-binding surface loops support this hypothesis.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

Here we review, from a quantitative point of view, the cell biology of protonemal tip growth in the model moss Physcomitrium patens. We focus on the role of the cytoskeleton, vesicle trafficking, and cell wall mechanics, including reviewing some of the existing mathematical models of tip growth. We provide a primer for existing cell biological tools that can be applied to the future study of tip growth in moss. Polarized cell growth is a ubiquitous process throughout the plant kingdom in which the cell elongates in a self-similar manner. This process is important for nutrient uptake by root hairs, fertilization by pollen, and gametophyte development by the protonemata of bryophytes and ferns. In this review, we will focus on the tip growth of moss cells, emphasizing the role of cytoskeletal organization, cytoplasmic zonation, vesicle trafficking, cell wall composition, and dynamics. We compare some of the existing knowledge on tip growth in protonemata against what is known in pollen tubes and root hairs, which are better-studied tip growing cells. To fully understand how plant cells grow requires that we deepen our knowledge in a variety of forms of plant cell growth. We focus this review on the model plant Physcomitrium patens, which uses tip growth as the dominant form of growth at its protonemal stage. Because mosses and vascular plants shared a common ancestor more than 450 million years ago, we anticipate that both similarities and differences between tip growing plant cells will provide mechanistic information of tip growth as well as of plant cell growth in general. Towards this mechanistic understanding, we will also review some of the existing mathematical models of plant tip growth and their applicability to investigate protonemal morphogenesis. We attempt to integrate the conclusions and data across cell biology and physical modeling to our current state of knowledge of polarized cell growth in P. patens and highlight future directions in the field.

Arabidopsis thaliana myosin XIK is recruited to the Golgi through interaction with a MyoB receptor

Plant cell organelles are highly mobile and their positioning play key roles in plant growth, development and responses to changing environmental conditions. Movement is acto-myosin dependent. Despite controlling the dynamics of several organelles, myosin and myosin receptors identified so far in Arabidopsis thaliana generally do not localise to the organelles whose movement they control, raising the issue of how specificity is determined. Here we show that a MyoB myosin receptor, MRF7, specifically localises to the Golgi membrane and affects its movement. Myosin XI-K was identified as a putative MRF7 interactor through mass spectrometry analysis. Co-expression of MRF7 and XI-K tail triggers the relocation of XI-K to the Golgi, linking a MyoB/myosin complex to a specific organelle in Arabidopsis. FRET-FLIM confirmed the in vivo interaction between MRF7 and XI-K tail on the Golgi and in the cytosol, suggesting that myosin/myosin receptor complexes perhaps cycle on and off organelle membranes. This work supports a traditional mechanism for organelle movement where myosins bind to receptors and adaptors on the organelle membranes, allowing them to actively move on the actin cytoskeleton, rather than passively in the recently proposed cytoplasmic streaming model.

Perico et al. use co-expression analysis and a FRET-FLIM approach to show that the Arabidopsis MyoB myosin receptor, MRF7, triggers the relocation of Myosin XI-K to the Golgi. As such, this study provides evidence for plant myosin recruitment and control of organelle movement.

Large DNA fragment deletion in lncRNA77580 regulates neighboring gene expression in soybean (Glycine max)

Long non-coding RNAs (lncRNAs) affect gene expressions via a wide range of mechanisms and are considered important regulators of numerous essential biological processes, including abiotic stress responses. However, the biological functions of most lncRNAs are yet to be determined. Moreover, to date, no effective methods have been developed to study the function of plant lncRNAs. We previously discovered a salt stress-related lncRNA, lncRNA77580 in soybean (Glycine max L.). In this study, we cloned the full-length lncRNA77580 and found that it shows nuclear-specific localisation. Furthermore, we employed CRISPR/Cas9 technology to induce large DNA fragment deletions in lncRNA77580 in soybean using a dual-single guide RNA/Cas9 design. As a result, we obtained deletion mutant soybean roots with targeted genomic fragment deletion in lncRNA77580. Deletion and overexpression of lncRNA77580 were found to alter the expression of several neighboring protein-coding genes associated with the response to salt stress. The longer the deleted DNA fragment in lncRNA77580, the greater the influence on the expression of lncRNA77580 itself and neighboring genes. Collectively, the findings of this study revealed that large DNA fragment deletion in lncRNAs using the CRISPR/Cas9 system is a powerful method to obtain functional mutations of soybean lncRNAs that could benefit future research on lncRNA function in soybean.

Vacuolar occupancy is crucial for cell elongation and growth regardless of the underlying mechanism

In the physiological range, the phytohormone auxin inhibits the growth of underground tissues. In the roots of Arabidopsis thaliana, cell size inhibition has been shown to be accompanied by auxin-mediated reduction of vacuole size. A tonoplast-localized protein family (Networked 4) with actin-binding capacity was demonstrated to modulate the compactness of the vacuole. Overexpression of NET4A led to smaller, more spherical and compact vacuoles, which occupied less cellular space compared to wild type. This reduction of vacuolar occupancy is similar to the observed auxin-induced decrease in occupancy, albeit there are enormous morphological differences. Here, we show that a net4a net4b double mutant and a NET4A overexpressor line are still sensitive to auxin-induced vacuolar constrictions. However, the overexpressor showed a partial auxin resistance accompanied by more compact vacuoles, thereby indicating an additional regulatory mechanism. Furthermore, we show that other NET superfamily members do not compensate for the loss of NET4A and NET4B expression on the transcriptional level. This leads us to hypothesize that regulation of vacuole size is a general mechanism to regulate cell expansion and that other players besides NET4 must participate in regulating the vacuole-cytoskeleton interface.

Structure of the class XI myosin globular tail reveals evolutionary hallmarks for cargo recognition in plants

The plant-specific class XI myosins (MyoXIs) play key roles at the molecular, cellular and tissue levels, engaging diverse adaptor proteins to transport cargoes along actin filaments. To recognize their cargoes, MyoXIs have a C-terminal globular tail domain (GTD) that is evolutionarily related to those of class V myosins (MyoVs) from animals and fungi. Despite recent advances in understanding the functional roles played by MyoXI in plants, the structure of its GTD, and therefore the molecular determinants for cargo selectivity and recognition, remain elusive. In this study, the first crystal structure of a MyoXI GTD, that of MyoXI-K from Arabidopsis thaliana, was elucidated at 2.35 Å resolution using a low-identity and fragment-based phasing approach in ARCIMBOLDO_SHREDDER. The results reveal that both the composition and the length of the α5-α6 loop are distinctive features of MyoXI-K, providing evidence for a structural stabilizing role for this loop, which is otherwise carried out by a molecular zipper in MyoV GTDs. The crystal structure also shows that most of the characterized cargo-binding sites in MyoVs are not conserved in plant MyoXIs, pointing to plant-specific cargo-recognition mechanisms. Notably, the main elements involved in the self-regulation mechanism of MyoVs are conserved in plant MyoXIs, indicating this to be an ancient ancestral trait.

Bundling up the Role of the Actin Cytoskeleton in Primary Root Growth

Primary root growth is required by the plant to anchor in the soil and reach out for nutrients and water, while dealing with obstacles. Efficient root elongation and bending depends upon the coordinated action of environmental sensing, signal transduction, and growth responses. The actin cytoskeleton is a highly plastic network that constitutes a point of integration for environmental stimuli and hormonal pathways. In this review, we present a detailed compilation highlighting the importance of the actin cytoskeleton during primary root growth and we describe how actin-binding proteins, plant hormones, and actin-disrupting drugs affect root growth and root actin. We also discuss the feedback loop between actin and root responses to light and gravity. Actin affects cell division and elongation through the control of its own organization. We remark upon the importance of longitudinally oriented actin bundles as a hallmark of cell elongation as well as the role of the actin cytoskeleton in protein trafficking and vacuolar reshaping during this process. The actin network is shaped by a plethora of actin-binding proteins; however, there is still a large gap in connecting the molecular function of these proteins with their developmental effects. Here, we summarize their function and known effects on primary root growth with a focus on their high level of specialization. Light and gravity are key factors that help us understand root growth directionality. The response of the root to gravity relies on hormonal, particularly auxin, homeostasis, and the actin cytoskeleton. Actin is necessary for the perception of the gravity stimulus via the repositioning of sedimenting statoliths, but it is also involved in mediating the growth response via the trafficking of auxin transporters and cell elongation. Furthermore, auxin and auxin analogs can affect the composition of the actin network, indicating a potential feedback loop. Light, in its turn, affects actin organization and hence, root growth, although its precise role remains largely unknown. Recently, fundamental studies with the latest techniques have given us more in-depth knowledge of the role and organization of actin in the coordination of root growth; however, there remains a lot to discover, especially in how actin organization helps cell shaping, and therefore root growth.

Micromanipulation of amyloplasts with optical tweezers in Arabidopsis stems

Intracellular sedimentation of highly dense, starch-filled amyloplasts toward the gravity vector is likely a key initial step for gravity sensing in plants. However, recent live-cell imaging technology revealed that most amyloplasts continuously exhibit dynamic, saltatory movements in the endodermal cells of Arabidopsis stems. These complicated movements led to questions about what type of amyloplast movement triggers gravity sensing. Here we show that a confocal microscope equipped with optical tweezers can be a powerful tool to trap and manipulate amyloplasts noninvasively, while simultaneously observing cellular responses such as vacuolar dynamics in living cells. A near-infrared (λ=1064 nm) laser that was focused into the endodermal cells at 1 mW of laser power attracted and captured amyloplasts at the laser focus. The optical force exerted on the amyloplasts was theoretically estimated to be up to 1 pN. Interestingly, endosomes and trans-Golgi network were trapped at 30 mW but not at 1 mW, which is probably due to lower refractive indices of these organelles than that of the amyloplasts. Because amyloplasts are in close proximity to vacuolar membranes in endodermal cells, their physical interaction could be visualized in real time. The vacuolar membranes drastically stretched and deformed in response to the manipulated movements of amyloplasts by optical tweezers. Our new method provides deep insights into the biophysical properties of plant organelles in vivo and opens a new avenue for studying gravity-sensing mechanisms in plants.

Characterization of ancestral myosin XI from Marchantia polymorpha by heterologous expression in Arabidopsis thaliana

Previous studies have revealed duplications and diversification of myosin XI genes between angiosperms and bryophytes; however, the functional differentiation and conservation of myosin XI between them remain unclear. Here, we identified a single myosin XI gene from the liverwort Marchantia polymorpha (Mp). The molecular properties of Mp myosin XI are similar to those of Arabidopsis myosin XIs responsible for cytoplasmic streaming, suggesting that the motor function of myosin XI is able to generate cytoplasmic streaming. In cultured Arabidopsis cells, transiently expressed green fluorescent protein (GFP)-fused Mp myosin XI was observed as some intracellular structures moving along the F-actin. These intracellular structures were co-localized with motile endoplasmic reticulum (ER) strands, suggesting that Mp myosin XI binds to the ER and generates intracellular transport in Arabidopsis cells. The tail domain of Mp myosin XI was co-localized with that of Arabidopsis myosin XI-2 and XI-K, suggesting that all these myosin XIs bind to common cargoes. Furthermore, expression of GFP-fused Mp myosin XI rescued the defects of growth, cytoplasmic streaming and actin organization in Arabidopsis multiple myosin XI knockout mutants. The heterologous expression experiments demonstrated the cellular and physiological competence of Mp myosin XI in Arabidopsis. However, the average velocity of organelle transport in Marchantia rhizoids was 0.04 ± 0.01 μm s-1 , which is approximately one-hundredth of that in Arabidopsis cells. Taken together, our results suggest that the molecular properties of myosin XI are conserved, but myosin XI-driven intracellular transport in vivo would be differentiated from bryophytes to angiosperms.

Heterologous transformation of Camelina sativa with high-speed chimeric myosin XI-2 promotes plant growth and leads to increased seed yield

Camelina sativa is a Brassicaceae oilseed plant used as a biotechnology platform for biofuel and healthy vegetable oil. As Camelina is closely related to the model plant Arabidopsis, the genetic tools of Arabidopsis are considered useful when applied to Camelina. Myosin XI-2 is one of the major motive forces driving cytoplasmic streaming in Arabidopsis. In our previous study, high-speed chimeric myosin XI-2, a myosin XI-2 artificially modified by genetically exchanging the motor domain of Arabidopsis myosin XI-2 with the faster Chara myosin XI, was shown to accelerate cytoplasmic streaming and promote plant growth in Arabidopsis. Here, we heterologously transformed this high-speed Chara-Arabidopsis chimeric myosin XI-2 gene in Camelina. The transgenic plants exhibited not only enhancement of leaf development and main stem elongation but also early flowering and seed setting, indicating that the high-speed chimeric myosin XI-2 can improve plant growth in Camelina. Interestingly, total seed yield was significantly increased in the transgenic plants as the total seed number increased. Our results suggest that the high-speed myosin XI system might also be effective to improve the growth of other closely related plant species.

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.

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

In vivo interactions between myosin XI, vesicles and filamentous actin are fast and transient in Physcomitrella patens

The actin cytoskeleton and active membrane trafficking machinery are essential for polarized cell growth. To understand the interactions between myosin XI, vesicles and actin filaments in vivo, we performed fluorescence recovery after photobleaching and showed that the dynamics of myosin XIa at the tip of the spreading earthmoss Physcomitrella patens caulonemal cells are actin-dependent and that 50% of myosin XI is bound to vesicles. To obtain single-particle information, we used variable-angle epifluorescence microscopy in protoplasts to demonstrate that protein myosin XIa and VAMP72-labeled vesicles localize in time and space over periods lasting only a few seconds. By tracking data with Hidden Markov modeling, we showed that myosin XIa and VAMP72-labeled vesicles exhibit short runs of actin-dependent directed transport. We also found that the interaction of myosin XI with vesicles is short-lived. Together, this vesicle-bound fraction, fast off-rate and short average distance traveled seem be crucial for the dynamic oscillations observed at the tip, and might be vital for regulation and recycling of the exocytosis machinery, while simultaneously promoting vesicle focusing and vesicle secretion at the tip, necessary for cell wall expansion.

Spatio-temporal control of post-Golgi exocytic trafficking in plants

A complex and dynamic endomembrane system is a hallmark of eukaryotic cells and underpins the evolution of specialised cell types in multicellular organisms. Endomembrane system function critically depends on the ability of the cell to (1) define compartment and pathway identity, and (2) organise compartments and pathways dynamically in space and time. Eukaryotes possess a complex molecular machinery to control these processes, including small GTPases and their regulators, SNAREs, tethering factors, motor proteins, and cytoskeletal elements. Whereas many of the core components of the eukaryotic endomembrane system are broadly conserved, there have been substantial diversifications within different lineages, possibly reflecting lineage-specific requirements of endomembrane trafficking. This Review focusses on the spatio-temporal regulation of post-Golgi exocytic transport in plants. It highlights recent advances in our understanding of the elaborate network of pathways transporting different cargoes to different domains of the cell surface, and the molecular machinery underpinning them (with a focus on Rab GTPases, their interactors and the cytoskeleton). We primarily focus on transport in the context of growth, but also highlight how these pathways are co-opted during plant immunity responses and at the plant-pathogen interface.

Cellular perspectives for improving mesophyll conductance

After entering the leaf, CO2 faces an intricate pathway to the site of photosynthetic fixation embedded within the chloroplasts. The efficiency of CO2 flux is hindered by a number of structural and biochemical barriers which, together, define the ease of flow of the gas within the leaf, termed mesophyll conductance. Previous authors have identified the key elements of this pathway, raising the prospect of engineering the system to improve CO2 flux and, thus, to increase leaf photosynthetic efficiency. In this review, we provide a perspective on the potential for improving the individual elements that contribute to this complex parameter. We lay particular emphasis on generation of the cellular architecture of the leaf which sets the initial boundaries of a number of mesophyll conductance parameters, incorporating an overview of the molecular transport processes which have been proposed as major facilitators of CO2 flux across structural boundaries along the pathway. The review highlights the research areas where future effort might be invested to increase our fundamental understanding of mesophyll conductance and leaf function and, consequently, to enable translation of these findings to improve the efficiency of crop photosynthesis.

Organelle size scaling over embryonic development

Cell division without growth results in progressive cell size reductions during early embryonic development. How do the sizes of intracellular structures and organelles scale with cell size and what are the functional implications of such scaling relationships? Model organisms, in particular Caenorhabditis elegans worms, Drosophila melanogaster flies, Xenopus laevis frogs, and Mus musculus mice, have provided insights into developmental size scaling of the nucleus, mitotic spindle, and chromosomes. Nuclear size is regulated by nucleocytoplasmic transport, nuclear envelope proteins, and the cytoskeleton. Regulators of microtubule dynamics and chromatin compaction modulate spindle and mitotic chromosome size scaling, respectively. Developmental scaling relationships for membrane-bound organelles, like the endoplasmic reticulum, Golgi, mitochondria, and lysosomes, have been less studied, although new imaging approaches promise to rectify this deficiency. While models that invoke limiting components and dynamic regulation of assembly and disassembly can account for some size scaling relationships in early embryos, it will be exciting to investigate the contribution of newer concepts in cell biology such as phase separation and interorganellar contacts. With a growing understanding of the underlying mechanisms of organelle size scaling, future studies promise to uncover the significance of proper scaling for cell function and embryonic development, as well as how aberrant scaling contributes to disease. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Early Embryonic Development > Fertilization to Gastrulation Comparative Development and Evolution > Model Systems.

Re-evaluation of physical interaction between plant peroxisomes and other organelles using live-cell imaging techniques

The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.

Could vesicular transport of Na+ and Cl- be a feature of salt tolerance in halophytes?

Background

Halophytes tolerate external salt concentrations of 200 mm and more, accumulating salt concentrations of 500 mm and more in their shoots; some, recretohalophytes, excrete salt through glands on their leaves. Ions are accumulated in central vacuoles, but the pathway taken by these ions from the outside of the roots to the vacuoles inside the cells is poorly understood. Do the ions cross membranes through ion channels and transporters or move in vesicles, or both? Vesicular transport from the plasma membrane to the vacuole would explain how halophytes avoid the toxicity of high salt concentrations on metabolism. There is also a role for vesicles in the export of ions via salt glands.

Scope and Methods

We have collected data on the fluxes of sodium and chloride ions in halophytes, based on the weight of the transporting organs and on the membrane area across which the flux occurs; the latter range from 17 nmol m-2 s-1 to 4.2 μmol m-2 s-1 and values up to 1 μmol m-2 s-1 need to be consistent with whatever transport system is in operation. We have summarized the sizes and rates of turnover of vesicles in plants, where clathrin-independent vesicles are 100 nm or more in diameter and can merge with the plasma membrane at rates of 100 s-1. We gathered evidence for vesicular transport of ions in halophytes and evaluated whether vesicular transport could account for the observable fluxes.

Conclusions

There is strong evidence in favour of vesicular transport in plants and circumstantial evidence in favour of the movement of ions in vesicles. Estimated rates of vesicle turnover could account for ion transport at the lower reported fluxes (around 20 nmol m-2 s-1), but the higher fluxes may require vesicles of the order of 1 μm or more in diameter. The very high fluxes reported in some salt glands might be an artefact of the way they were measured.

Boolean AND and OR logic for cell signalling gateways: a communication perspective

Cell signalling plays a vital role in development, sustaining, differentiation, and reproduction of cells. Pathways involved in signalling networks are quite interwoven and complex. Complexity encountered in understanding these pathways is often reduced with the help of Boolean circuit representation. In this study, the authors provide communication aspect of the signalling pathways that have two input Boolean logic AND/OR implemented at the rear effector protein. Communication is assumed to be taking place in extracellular and intracellular environment. The two environments are connected using a receptor protein acting as relay between a molecular source and effector protein. Each relay detects molecules from outside environment and stimulates the production of signals in the intracellular space. These signals/molecules further activate the effector protein which acts as a Boolean switch driven by AND/OR logic. Assuming Poisson reception at the relay as well as at the receiver, the authors provide probability of error of the AND and OR Boolean logic communication systems. Furthermore, reliability and some capacity bounds are deduced for the given Boolean communication system.

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.

Oscillatory fluid flow drives scaling of contraction wave with system size

Flows over remarkably long distances are crucial to the functioning of many organisms, across all kingdoms of life. Coordinated flows are fundamental to power deformations, required for migration or development, or to spread resources and signals. A ubiquitous mechanism to generate flows, particularly prominent in animals and amoebas, is actomyosin cortex-driven mechanical deformations that pump the fluid enclosed by the cortex. However, it is unclear how cortex dynamics can self-organize to give rise to coordinated flows across the largely varying scales of biological systems. Here, we develop a mechanochemical model of actomyosin cortex mechanics coupled to a contraction-triggering, soluble chemical. The chemical itself is advected with the flows generated by the cortex-driven deformations of the tubular-shaped cell. The theoretical model predicts a dynamic instability giving rise to stable patterns of cortex contraction waves and oscillatory flows. Surprisingly, simulated patterns extend beyond the intrinsic length scale of the dynamic instability-scaling with system size instead. Patterns appear randomly but can be robustly generated in a growing system or by flow-generating boundary conditions. We identify oscillatory flows as the key for the scaling of contraction waves with system size. Our work shows the importance of active flows in biophysical models of patterning, not only as a regulating input or an emergent output, but also as a full part of a self-organized machinery. Contractions and fluid flows are observed in all kinds of organisms, so this concept is likely to be relevant for a broad class of systems.

Plant organelle dynamics: cytoskeletal control and membrane contact sites

Contents Summary 381 I. Introduction 381 II. Basic movement characteristics 382 III. Actin and associated motors, myosins, play a primary role in plant organelle movement and positioning 382 IV. Mechanisms of myosin recruitment: a tightly regulated system? 384 V. Microtubules, associated motors and interplay with actin 386 VI. Role of organelle interactions: tales of tethers 387 VII. Summary model to describe organelle movement in higher plants 390 VIII. Why is organelle movement important? 390 IX. Conclusions and future perspectives 391 Acknowledgements 391 References 391 SUMMARY: Organelle movement and positioning are correlated with plant growth and development. Movement characteristics are seemingly erratic yet respond to external stimuli including pathogens and light. Given these clear correlations, we still do not understand the specific roles that movement plays in these processes. There are few exceptions including organelle inheritance during cell division and photorelocation of chloroplasts to prevent photodamage. The molecular and biophysical components that drive movement can be broken down into cytoskeletal components, motor proteins and tethers, which allow organelles to physically interact with one another. Our understanding of these components and concepts has exploded over the past decade, with recent technological advances allowing an even more in-depth profiling. Here, we provide an overview of the cytoskeletal and tethering components and discuss the mechanisms behind organelle movement in higher plants.

Myosin XI-K is involved in root organogenesis, polar auxin transport, and cell division

The interplay between myosin- and auxin-mediated processes was investigated by following root development in the triple myosin knockout mutant xi-k xi-1 xi-2 (3KO). It was found that the 3KO plants generated significantly more lateral and adventitious roots than the wild-type plants or the rescued plant line expressing functional myosin XI-K:yellow fluorescent protein (YFP; 3KOR). Using the auxin-dependent reporter DR5:venus, a significant change in the auxin gradient toward the root tip was found in 3KO plants, which correlated with the loss of polar localization of the auxin transporter PIN1 in the stele and with the increased number of stele cells with oblique cell walls. Interestingly, myosin XI-K:YFP was localized to the cell division apparatus in the root and shoot meristems. In anaphase and early telophase, XI-K:YFP was concentrated in the midzone and the forming cell plate. In late telophase, XI-K:YFP formed a ring that overlapped with the growing phragmoplast. Myosin receptors MyoB1 and MyoB2 that are highly expressed throughout the plant were undetectable in dividing cells, suggesting that the myosin function in cell division relies on distinct adaptor proteins. These results suggest that myosin XIs are involved in orchestrating root organogenesis via effects on polar distribution of auxin responses and on cell division.

Kinesins and Myosins: Molecular Motors that Coordinate Cellular Functions in Plants

Kinesins and myosins are motor proteins that can move actively along microtubules and actin filaments, respectively. Plants have evolved a unique set of motors that function as regulators and organizers of the cytoskeleton and as drivers of long-distance transport of various cellular components. Recent progress has established the full complement of motors encoded in plant genomes and has revealed valuable insights into the cellular functions of many kinesin and myosin isoforms. Interestingly, several of the motors were found to functionally connect the two cytoskeletal systems and thereby to coordinate their activities. In this review, we discuss the available genetic, cell biological, and biochemical data for each of the plant kinesin and myosin families from the context of their subcellular mechanism of action as well as their physiological function in the whole plant. We particularly emphasize work that illustrates mechanisms by which kinesins and myosins coordinate the activities of the cytoskeletal system.

Myosin-Driven Intracellular Transport

The delivery of intracellular material within cells is crucial for maintaining normal function. Myosins transport a wide variety of cargo, ranging from vesicles to ribonuclear protein particles (RNPs), in plants, fungi, and metazoa. The properties of a given myosin transporter are adapted to move on different actin filament tracks, either on the disordered actin networks at the cell cortex or along highly organized actin bundles to distribute their cargo in a localized manner or move it across long distances in the cell. Transport is controlled by selective recruitment of the myosin to its cargo that also plays a role in activation of the motor.

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.

Ethylene promotes pollen tube growth by affecting actin filament organization via the cGMP-dependent pathway in Arabidopsis thaliana

Ethylene and cGMP are key regulators of plant developmental processes. In this study, we demonstrate that ethylene or cGMP promote pollen tube growth in a dose-dependent manner. The etr1-1 mutant was found to be insensitive to ethylene with regard to pollen tube growth, while the growth-promoting effect of ethylene in etr2-2, ein4-4, or ein4-7 did not change, suggesting that ethylene signaling was mainly perceived by ETR1. However, the function of cGMP was not inhibited in etr1-1 and pollen tubes became insensitive to ethylene when the endogenous cGMP level was artificially decreased. This shows that cGMP is necessary for the control of pollen tube growth and that it might be a downstream component of ETR1 in the ethylene signaling pathway. Our study also found that ethylene or cGMP increase the actin bundles and elevated the percentage of relative amount of F-actin, while removal of cGMP decreased actin bundles abundance and altered the ratio of F-actin in the tip and base regions of pollen tubes. In conclusion, our data suggests that ethylene functions as the upstream signal of cGMP, and that both signals promote pollen germination and tube growth by regulating F-actin, which is essential for vesicular transport and cytoplasmic streaming.

Measurement of enzymatic and motile activities of Arabidopsis myosins by using Arabidopsis actins

There are two classes of myosin, XI and VIII, in higher plants. Myosin XI moves actin filaments at high speed and its enzyme activity is also very high. In contrast, myosin VIII moves actin filaments very slowly with very low enzyme activity. Because most of these enzymatic and motile activities were measured using animal skeletal muscle α-actin, but not plant actin, they would not accurately reflect the actual activities in plant cells. We thus measured enzymatic and motile activities of the motor domains of two Arabidopsis myosin XI isoforms (MYA2, XI-B), and one Arabidopsis myosin VIII isoform (ATM1), by using three Arabidopsis actin isoforms (ACT1, ACT2, and ACT7). The measured activities were different from those measured by using muscle actin. Moreover, Arabidopsis myosins showed different enzymatic and motile activities when using different Arabidopsis actin isoforms. Our results suggest that plant actin should be used for measuring enzymatic and motile activities of plant myosins and that different actin isoforms in plant cells might function as different tracks along which affinities and velocities of each myosin isoform are modulated.

Plasmolysis-deplasmolysis causes changes in endoplasmic reticulum form, movement, flow, and cytoskeletal association

Plasmolysis of hypocotyl cells of transgenic Arabidopsis thaliana and Nicotiana benthamiana diminishes the dynamics of the remodeling of the endoplasmic reticulum (ER) in the central protoplast, namely that withdrawn from the cell wall, and more persistent cisternae are formed, yet little change in the actin network in the protoplast occurs. Also, protein flow within the ER network in the protoplast, as detected with fluorescence recovery after photobleaching (FRAP), is not affected by plasmolysis. After plasmolysis, another network of strictly tubular ER remains attached to the plasma membrane-wall interface and is contained within the Hechtian strands and reticulum. FRAP studies indicate that protein flow within these ER tubules diminishes. Actin is largely absent from the Hechtian reticulum and the ER becomes primarily associated with altered, branched microtubules. The smaller volume of the central protoplast is accompanied by decreased movement rates of tubules, cisternae, and spheroid organelles, but this reduced movement is not readily reversed by the increase in volume that accompanies deplasmolysis.

Myosin XI is associated with fitness and adaptation to aridity in wild pearl millet

Phenotypic changes in plants can be observed along many environmental gradients and are determined by both environmental and genetic factors. The identification of alleles associated with phenotypic variations is a rapidly developing area of research. We studied the genetic basis of phenotypic variations in 11 populations of wild pearl millet (Pennisetum glaucum) on two North-South aridity gradients, one in Niger and one in Mali. Most of the 11 phenotypic traits assessed in a common garden experiment varied between the populations studied. Moreover, the size of the inflorescence, the number of flowers and aboveground dry mass co-varied positively with a decrease in rainfall. To decipher the genetic basis of these phenotypes, we used an association mapping strategy with a mixed model. We found two SNPs on the same myosin XI contig significantly associated with variations in the average number of flowers. Both the allele frequency of the two SNPs and the average number of flowers co-varied with the rainfall gradient on the two gradients. Interestingly, this gene was also a target of selection during domestication. The Myosin XI gene is thus a good candidate for fitness-related adaptation in wild populations.

Myosin-driven transport network in plants

We investigate the myosin XI-driven transport network in Arabidopsis using protein-protein interaction, subcellular localization, gene knockout, and bioinformatics analyses. The two major groups of nodes in this network are myosins XI and their membrane-anchored receptors (MyoB) that, together, drive endomembrane trafficking and cytoplasmic streaming in the plant cells. The network shows high node connectivity and is dominated by generalists, with a smaller fraction of more specialized myosins and receptors. We show that interaction with myosins and association with motile vesicles are common properties of the MyoB family receptors. We identify previously uncharacterized myosin-binding proteins, putative myosin adaptors that belong to two unrelated families, with four members each (MadA and MadB). Surprisingly, MadA1 localizes to the nucleus and is rapidly transported to the cytoplasm, suggesting the existence of myosin XI-driven nucleocytoplasmic trafficking. In contrast, MadA2 and MadA3, as well as MadB1, partition between the cytosolic pools of motile endomembrane vesicles that colocalize with myosin XI-K and diffuse material that does not. Gene knockout analysis shows that MadB1-4 contribute to polarized root hair growth, phenocopying myosins, whereas MadA1-4 are redundant for this process. Phylogenetic analysis reveals congruent evolutionary histories of the myosin XI, MyoB, MadA, and MadB families. All these gene families emerged in green algae and show concurrent expansions via serial duplication in flowering plants. Thus, the myosin XI transport network increased in complexity and robustness concomitantly with the land colonization by flowering plants and, by inference, could have been a major contributor to this process.

Constitutive Expression of miR408 Improves Biomass and Seed Yield in Arabidopsis

miR408 is highly conserved among different plant species and targets transcripts encoding copper-binding proteins. The function of miR408 in reproductive development remains largely unclear despite it being known to play important roles during vegetative development in Arabidopsis. Here, we show that transgenic Arabidopsis plants overexpressing MIR408 have altered morphology including significantly increased leaf area, petiole length, plant height, flower size, and silique length, resulting in enhanced biomass and seed yield. The increase in plant size was primarily due to cell expansion rather than cell proliferation, and was consistent with higher levels of myosin gene expression and gibberellic acid (GA) measured in transgenic plants. In addition, photosynthetic rate was significantly increased in the MIR408-overexpressing plants, as manifested by higher levels of chloroplastic copper content and plastocyanin (PC) expression. In contrast, overexpression of miR408-regulated targets, Plantacyanin and Laccase 13, resulted in reduced biomass production and seed yield. RNA-sequencing revealed that genes involved in primary metabolism and stress response were preferentially enriched in the genes upregulated in MIR408-overexpressing plants. These results indicate that miR408 plays an important role in regulating biomass and seed yield and that MIR408 may be a potential candidate gene involved in the domestication of agricultural crops.

Myosin XI-I is Mechanically and Enzymatically Unique Among Class-XI Myosins in Arabidopsis

Arabidopsis possesses 13 genes encoding class-XI myosins. Among these, myosin XI-I is phylogenetically distant. To examine the molecular properties of Arabidopsis thaliana myosin XI-I (At myosin XI-I), we performed in vitro mechanical and enzymatic analyses using recombinant constructs of At myosin XI-I. Unlike other biochemically studied class-XI myosins, At myosin XI-I showed extremely low actin-activated ATPase activity (Vmax = 3.7 Pi s(-1) head(-1)). The actin-sliding velocity of At myosin XI-I was 0.25 µm s(-1), >10 times lower than those of other class-XI myosins. The ADP dissociation rate from acto-At myosin XI-I was 17 s(-1), accounting for the low actin-sliding velocity. In contrast, the apparent affinity for actin in the presence of ATP, estimated from Kapp (0.61 µM) of actin-activated ATPase, was extremely high. The equilibrium dissociation constant for actin was very low in both the presence and absence of ATP, indicating a high affinity for actin. To examine At myosin XI-I motility in vivo, green fluorescent protein-fused full-length At myosin XI-I was expressed in cultured Arabidopsis cells. At myosin XI-I localized not only on the nuclear envelope but also on small dots moving slowly (0.23 µm s(-1)) along actin filaments. Our results show that the properties of At myosin XI-I differ from those of other Arabidopsis class-XI myosins. The data suggest that At myosin XI-I does not function as a driving force for cytoplasmic streaming but regulates the organelle velocity, supports processive organelle movement or acts as a tension generator.