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

Myosin XI is required for boron transport under boron limitation via maintenance of endocytosis and polar localization of the boric acid channel AtNIP5;1

Myosin XI plays a major role in cytoplasmic streaming and is essential for intracellular transport. Here, we investigated the physiological roles of myosin XI in nutrient transport using double (2ko) and triple (3ko) myosin XI knockout mutants of Arabidopsis thaliana. The results revealed that the mutants exhibited more severe boron deficiency phenotypes under boron-limiting conditions, and the boron concentration in the aerial parts of mutant plants was lower than that in the wild-type. Microscopic analysis demonstrated a reduction in general endocytosis and abolishment of NIP5; 1's polar localization in 2ko and 3ko plants. Overall, these results indicate that myosin XI is necessary for proper boron transport via the maintenance of the endocytic pathway and NIP5; 1's polar localization.

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

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

Transposon-triggered epigenetic chromatin dynamics modulate EFR-related pathogen response

Infectious diseases drive wild plant evolution and impact crop yield. Plants, like animals, sense biotic threats through pattern recognition receptors (PRRs). Overly robust immune responses can harm plants; thus, understanding the tuning of defense response mechanisms is crucial for developing pathogen-resistant crops. In this study, we found that an inverted-repeat transposon (EFR-associated IR, Ea-IR) located between the loci encoding PRRs ELONGATION FACTOR-TU RECEPTOR (EFR) and myosin XI-k (XI-k) in Arabidopsis affects chromatin organization, promoting the formation of a repressive chromatin loop. Upon pathogen infection, chromatin changes around EFR and XI-k correlate with increased EFR transcription. Pathogen-induced chromatin opening causes RNA polymerase II readthrough, producing a longer, Ea-IR-containing XI-k transcript, processed by Dicer-like enzymes into small RNAs, which reset chromatin to a repressive state attenuating the immune response after infection. Arabidopsis accessions lacking Ea-IR have higher basal EFR levels and resistance to pathogens. We show a scenario in which a transposon, chromatin organization and gene expression interact to fine-tune immune responses, during both the course of infection and the course of evolution.

Raffinose catabolism enhances maize waterlogging tolerance by stimulating adventitious root growth and development

Raffinose mitigates plant heat, drought, and cold stresses; however, whether raffinose contributes to plant waterlogging tolerance is unknown. The maize raffinose synthase mutant zmrafs-1 had seedlings that lack raffinose, generated fewer and shorter adventitious roots, and were more sensitive to waterlogging stress, while overexpression of the raffinose synthase gene, ZmRAFS, increased raffinose content, stimulated adventitious root formation, and enhanced waterlogging tolerance of maize seedlings. Transcriptome analysis of null segregant seedlings compared with zmrafs-1, particularly when waterlogged, revealed that the expression of genes related to galactose metabolism and the auxin biosynthetic pathway were up-regulated by raffinose. Additionally, indole-3-acetic acid content was significantly decreased in zmrafs-1 seedlings and increased in ZmRAFS-overexpressing seedlings. Inhibition of the hydrolysis of raffinose by 1-deoxygalactonojirimycin decreased the waterlogging tolerance of maize seedlings, the expression of genes encoding proteins related to auxin transport-related genes, and the indole-3-acetic acid level in the seedlings, indicating that the hydrolysis of raffinose is necessary for maize waterlogging tolerance. These data demonstrate that raffinose catabolism stimulates adventitious root formation via the auxin signaling pathway to enhance maize waterlogging tolerance.

The microtubular preprophase band recruits Myosin XI to the cortical division site to guide phragmoplast expansion during plant cytokinesis

In plant vegetative tissues, cell division employs a mitotic microtubule array called the preprophase band (PPB) that marks the cortical division site. This transient cytoskeletal array imprints the spatial information to be read by the cytokinetic phragmoplast at later stages of mitotic cell division. In Arabidopsis thaliana, we discovered that the PPB recruited the Myosin XI motor MYA1/Myo11F to the cortical division site, where it joined microtubule-associated proteins and motors to form a ring of prominent cytoskeletal assemblies that received the expanding phragmoplast. Such a myosin localization pattern at the cortical division site was dependent on the POK1/2 Kinesin-12 motors. This regulatory function of MYA1/Myo11F in phragmoplast guidance was dependent on intact actin filaments. The discovery of these cytoskeletal motor assemblies pinpoints a mechanism underlying how two dynamic cytoskeletal networks work in concert to govern PPB-dependent division plane orientation in flowering plants.

Functional characterization of calmodulin-like proteins, CML13 and CML14, as novel light chains of Arabidopsis class VIII myosins

Myosins are important motor proteins that associate with the actin cytoskeleton. Structurally, myosins function as heteromeric complexes where smaller light chains, such as calmodulin (CaM), bind to isoleucine-glutamine (IQ) domains in the neck region to facilitate mechano-enzymatic activity. We recently identified Arabidopsis CaM-like (CML) proteins CML13 and CML14 as interactors of proteins containing multiple IQ domains, including a myosin VIII. Here, we demonstrate that CaM, CML13, and CML14 bind the neck region of all four Arabidopsis myosin VIII isoforms. Among CMLs tested for binding to myosins VIIIs, CaM, CML13, and CML14 gave the strongest signals using in planta split-luciferase protein interaction assays. In vitro, recombinant CaM, CML13, and CML14 showed specific, high-affinity, calcium-independent binding to the IQ domains of myosin VIIIs. CaM, CML13, and CML14 co-localized to plasma membrane-bound puncta when co-expressed with red fluorescent protein-myosin fusion proteins containing IQ and tail domains of myosin VIIIs. In vitro actin motility assays using recombinant myosin VIIIs demonstrated that CaM, CML13, and CML14 function as light chains. Suppression of CML13 or CML14 expression using RNA silencing resulted in a shortened-hypocotyl phenotype, similar to that observed in a quadruple myosin mutant, myosin viii4KO. Collectively, our data indicate that Arabidopsis CML13 and CML14 are novel myosin VIII light chains.

The interplay between cell wall integrity and cell cycle progression in plants

Plant cell walls are dynamic structures that play crucial roles in growth, development, and stress responses. Despite our growing understanding of cell wall biology, the connections between cell wall integrity (CWI) and cell cycle progression in plants remain poorly understood. This review aims to explore the intricate relationship between CWI and cell cycle progression in plants, drawing insights from studies in yeast and mammals. We provide an overview of the plant cell cycle, highlight the role of endoreplication in cell wall composition, and discuss recent findings on the molecular mechanisms linking CWI perception to cell wall biosynthesis and gene expression regulation. Furthermore, we address future perspectives and unanswered questions in the field, such as the identification of specific CWI sensing mechanisms and the role of CWI maintenance in the growth-defense trade-off. Elucidating these connections could have significant implications for crop improvement and sustainable agriculture.

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 OPAQUE1/DISCORDIA2 myosin XI is required for phragmoplast guidance during asymmetric cell division in maize

Formative asymmetric divisions produce cells with different fates and are critical for development. We show the maize (Zea mays) myosin XI protein, OPAQUE1 (O1), is necessary for asymmetric divisions during maize stomatal development. We analyzed stomatal precursor cells before and during asymmetric division to determine why o1 mutants have abnormal division planes. Cell polarization and nuclear positioning occur normally in the o1 mutant, and the future site of division is correctly specified. The defect in o1 becomes apparent during late cytokinesis, when the phragmoplast forms the nascent cell plate. Initial phragmoplast guidance in o1 is normal; however, as phragmoplast expansion continues o1 phragmoplasts become misguided. To understand how O1 contributes to phragmoplast guidance, we identified O1-interacting proteins. Maize kinesins related to the Arabidopsis thaliana division site markers PHRAGMOPLAST ORIENTING KINESINs (POKs), which are also required for correct phragmoplast guidance, physically interact with O1. We propose that different myosins are important at multiple steps of phragmoplast expansion, and the O1 actin motor and POK-like microtubule motors work together to ensure correct late-stage phragmoplast guidance.

Kaempferol-3-rhamnoside overaccumulation in flavonoid 3'-hydroxylase tt7 mutants compromises seed coat outer integument differentiation and seed longevity

Seeds slowly accumulate damage during storage, which ultimately results in germination failure. The seed coat protects the embryo from the external environment, and its composition is critical for seed longevity. Flavonols accumulate in the outer integument. The link between flavonol composition and outer integument development has not been explored. Genetic, molecular and ultrastructural assays on loss-of-function mutants of the flavonoid biosynthesis pathway were used to study the effect of altered flavonoid composition on seed coat development and seed longevity. Controlled deterioration assays indicate that loss of function of the flavonoid 3' hydroxylase gene TT7 dramatically affects seed longevity and seed coat development. Outer integument differentiation is compromised from 9 d after pollination in tt7 developing seeds, resulting in a defective suberin layer and incomplete degradation of seed coat starch. These distinctive phenotypes are not shared by other mutants showing abnormal flavonoid composition. Genetic analysis indicates that overaccumulation of kaempferol-3-rhamnoside is mainly responsible for the observed phenotypes. Expression profiling suggests that multiple cellular processes are altered in the tt7 mutant. Overaccumulation of kaempferol-3-rhamnoside in the seed coat compromises normal seed coat development. This observation positions TRANSPARENT TESTA 7 and the UGT78D1 glycosyltransferase, catalysing flavonol 3-O-rhamnosylation, as essential players in the modulation of seed longevity.

Cortical microtubules contribute to division plane positioning during telophase in maize

Cell divisions are accurately positioned to generate cells of the correct size and shape. In plant cells, the new cell wall is built in the middle of the cell by vesicles trafficked along an antiparallel microtubule and a microfilament array called the phragmoplast. The phragmoplast expands toward a specific location at the cell cortex called the division site, but how it accurately reaches the division site is unclear. We observed microtubule arrays that accumulate at the cell cortex during the telophase transition in maize (Zea mays) leaf epidermal cells. Before the phragmoplast reaches the cell cortex, these cortical-telophase microtubules transiently interact with the division site. Increased microtubule plus end capture and pausing occur when microtubules contact the division site-localized protein TANGLED1 or other closely associated proteins. Microtubule capture and pausing align the cortical microtubules perpendicular to the division site during telophase. Once the phragmoplast reaches the cell cortex, cortical-telophase microtubules are incorporated into the phragmoplast primarily by parallel bundling. The addition of microtubules into the phragmoplast promotes fine-tuning of the positioning at the division site. Our hypothesis is that division site-localized proteins such as TANGLED1 organize cortical microtubules during telophase to mediate phragmoplast positioning at the final division plane.

Redundant mechanisms in division plane positioning

Redundancies in plant cell division contribute to the maintenance of proper division plane orientation. Here we highlight three types of redundancy: 1) Temporal redundancy, or correction of earlier defects that results in proper final positioning, 2) Genetic redundancy, or functional compensation by homologous genes, and 3) Synthetic redundancy, or redundancy within or between pathways that contribute to proper division plane orientation. Understanding the types of redundant mechanisms involved provides insight into current models of division plane orientation and opens up new avenues for exploration.

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.

An Evidence Theory and Fuzzy Logic Combined Approach for the Prediction of Potential ARF-Regulated Genes in Quinoa

Quinoa constitutes among the tolerant plants to the challenging and harmful abiotic environmental factors. Quinoa was selected as among the model crops destined for bio-saline agriculture that could contribute to the staple food security for an ever-growing worldwide population under various climate change scenarios. The auxin response factors (ARFs) constitute the main contributors in the plant adaptation to severe environmental conditions. Thus, the determination of the ARF-binding sites represents the major step that could provide promising insights helping in plant breeding programs and improving agronomic traits. Hence, determining the ARF-binding sites is a challenging task, particularly in species with large genome sizes. In this report, we present a data fusion approach based on Dempster-Shafer evidence theory and fuzzy set theory to predict the ARF-binding sites. We then performed an "In-silico" identification of the ARF-binding sites in Chenopodium quinoa. The characterization of some known pathways implicated in the auxin signaling in other higher plants confirms our prediction reliability. Furthermore, several pathways with no or little available information about their functions were identified to play important roles in the adaptation of quinoa to environmental conditions. The predictive auxin response genes associated with the detected ARF-binding sites may certainly help to explore the biological roles of some unknown genes newly identified in quinoa.

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

Subcellular trafficking and post-translational modification regulate PIN polarity in plants

Auxin regulates plant growth and tropism responses. As a phytohormone, auxin is transported between its synthesis sites and action sites. Most natural auxin moves between cells via a polar transport system that is mediated by PIN-FORMED (PIN) auxin exporters. The asymmetrically localized PINs usually determine the directionality of intercellular auxin flow. Different internal cues and external stimuli modulate PIN polar distribution and activity at multiple levels, including transcription, protein stability, subcellular trafficking, and post-translational modification, and thereby regulate auxin-distribution-dependent development. Thus, the different regulation levels of PIN polarity constitute a complex network. For example, the post-translational modification of PINs can affect the subcellular trafficking of PINs. In this review, we focus on subcellular trafficking and post-translational modification of PINs to summarize recent progress in understanding PIN polarity.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Myosin XI drives polarized growth by vesicle focusing and local enrichment of F-actin in Physcomitrium patens

In tip-growing plant cells, growth results from myosin XI and F-actin-mediated deposition of cell wall polysaccharides contained in secretory vesicles. Previous evidence showed that myosin XI anticipates F-actin accumulation at the cell's tip, suggesting a mechanism where vesicle clustering via myosin XI increases F-actin polymerization. To evaluate this model, we used a conditional loss-of-function strategy by generating moss (Physcomitrium patens) plants harboring a myosin XI temperature-sensitive allele. We found that loss of myosin XI function alters tip cell morphology, vacuolar homeostasis, and cell viability but not following F-actin depolymerization. Importantly, our conditional loss-of-function analysis shows that myosin XI focuses and directs vesicles at the tip of the cell, which induces formin-dependent F-actin polymerization, increasing F-actin's local concentration. Our findings support the role of myosin XI in vesicle focusing, possibly via clustering and F-actin organization, necessary for tip growth, and deepen our understanding of additional myosin XI functions.

Myosin XI-B is involved in the transport of vesicles and organelles in pollen tubes of Arabidopsis thaliana

The movement of organelles and vesicles in pollen tubes depends on F-actin. However, the molecular mechanism through which plant myosin XI drives the movement of organelles is still controversial, and the relationship between myosin XI and vesicle movement in pollen tubes is also unclear. In this study, we found that the siliques of the myosin xi-b/e mutant were obviously shorter than those of the wild-type (WT) and that the seed set of the mutant was severely deficient. The pollen tube growth of myosin xi-b/e was significantly inhibited both in vitro and in vivo. Fluorescence recovery after photobleaching showed that the velocity of vesicle movement in the pollen tube tip of the myosin xi-b/e mutant was lower than that of the WT. It was also found that peroxisome movement was significantly inhibited in the pollen tubes of the myosin xi-b/e mutant, while the velocities of the Golgi stack and mitochondrial movement decreased relatively less in the pollen tubes of the mutant. The endoplasmic reticulum streaming in the pollen tube shanks was not significantly different between the WT and the myosin xi-b/e mutant. In addition, we found that myosin XI-B-GFP colocalized obviously with vesicles and peroxisomes in the pollen tubes of Arabidopsis. Taken together, these results indicate that myosin XI-B may bind mainly to vesicles and peroxisomes, and drive their movement in pollen tubes. These results also suggest that the mechanism by which myosin XI drives organelle movement in plant cells may be evolutionarily conserved compared with other eukaryotic cells.

Rab-E and its interaction with myosin XI are essential for polarised cell growth

The fundamental process of polarised exocytosis requires the interconnected activity of molecular motors trafficking vesicular cargo within a dynamic cytoskeletal network. In plants, few mechanistic details are known about how molecular motors, such as myosin XI, associate with their secretory cargo to support the ubiquitous processes of polarised growth and cell division. Live-cell imaging coupled with targeted gene knockouts and a high-throughput RNAi assay enabled the first characterisation of the loss of Rab-E function. Yeast two-hybrid and subsequent in silico structural prediction uncovered a specific interaction between Rab-E and myosin XI that is conserved between P. patens and A. thaliana. Rab-E co-localises with myosin XI at sites of active exocytosis, and at the growing tip both proteins are spatiotemporally coupled. Rab-E is required for normal plant growth in P. patens and the rab-E and myosin XI phenotypes are rescued by A. thaliana's Rab-E1c and myosin XI-K/E, respectively. Both PpMyoXI and AtMyoXI-K interact with PpRabE14, and the interaction is specifically mediated by PpMyoXI residue V1422. This interaction is required for polarised growth. Our results suggest that the interaction of Rab-E and myosin XI is a conserved feature of polarised growth in plants.

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.

Organelle movement and apical accumulation of secretory vesicles in pollen tubes of Arabidopsis thaliana depend on class XI myosins

F-actin and myosin XI play important roles in plant organelle movement. A few myosin XI genes in the genome of Arabidopsis are mainly expressed in mature pollen, which suggests that they may play a crucial role in pollen germination and pollen tube tip growth. In this study, a genetic complementation assay was conducted in a myosin xi-c (myo11c1) myosin xi-e (myo11c2) double mutant, and fluorescence labeling combined with microscopic observation was applied. We found that myosin XI-E (Myo11C2)-green fluorescent protein (GFP) restored the slow pollen tube growth and seed deficiency phenotypes of the myo11c1 myo11c2 double mutant and Myo11C2-GFP partially colocalized with mitochondria, peroxisomes and Golgi stacks. Furthermore, decreased mitochondrial movement and subapical accumulation were detected in myo11c1 myo11c2 double mutant pollen tubes. Fluorescence recovery after photobleaching experiments showed that the fluorescence recoveries of GFP-RabA4d and AtPRK1-GFP at the pollen tube tip of the myo11c1 myo11c2 double mutant were lower than those of the wild type were after photobleaching. These results suggest that Myo11C2 may be associated with mitochondria, peroxisomes and Golgi stacks, and play a crucial role in organelle movement and apical accumulation of secretory vesicles in pollen tubes of Arabidopsis thaliana.

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.

Opposing, Polarity-Driven Nuclear Migrations Underpin Asymmetric Divisions to Pattern Arabidopsis Stomata

Multicellular development depends on generating and precisely positioning distinct cell types within tissues. During leaf development, pores in the epidermis called stomata are spaced at least one cell apart for optimal gas exchange. This pattern is primarily driven by iterative asymmetric cell divisions (ACDs) in stomatal progenitors, which generate most of the cells in the tissue. A plasma membrane-associated polarity crescent defined by BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) and BREVIS RADIX family (BRXf) proteins is required for asymmetric divisions and proper stomatal pattern, but the cellular mechanisms that orient ACDs remain unclear. Here, utilizing long-term, quantitative time-lapse microscopy, we identified two oppositely oriented nuclear migrations that precede and succeed ACD during epidermal patterning. The pre- and post-division migrations are dependent on microtubules and actin, respectively, and the polarity crescent is the unifying landmark that is both necessary and sufficient to orient both nuclear migrations. We identified a specific and essential role for MYOXI-I in controlling post-ACD nuclear migration. Loss of MYOXI-I decreases stomatal density, owing to an inability to accurately orient a specific subset of ACDs. Taken together, our analyses revealed successive and polarity-driven nuclear migrations that regulate ACD orientation in the Arabidopsis stomatal lineage.

Plant Lipid Bodies Traffic on Actin to Plasmodesmata Motorized by Myosin XIs

Late 19th-century cytologists observed tiny oil drops in shoot parenchyma and seeds, but it was discovered only in 1972 that they were bound by a half unit-membrane. Later, it was found that lipid bodies (LBs) arise from the endoplasmic reticulum. Seeds are known to be packed with static LBs, coated with the LB-specific protein OLEOSIN. As shown here, apices of Populustremula x P. tremuloides also express OLEOSIN genes and produce potentially mobile LBs. In developing buds, PtOLEOSIN (PtOLE) genes were upregulated, especially PtOLE6, concomitant with LB accumulation. To investigate LB mobility and destinations, we transformed Arabidopsis with PtOLE6-eGFP. We found that PtOLE6-eGFP fusion protein co-localized with Nile Red-stained LBs in all cell types. Moreover, PtOLE6-eGFP-tagged LBs targeted plasmodesmata, identified by the callose marker aniline blue. Pharmacological experiments with brefeldin, cytochalasin D, and oryzalin showed that LB-trafficking requires F-actin, implying involvement of myosin motors. In a triple myosin-XI knockout (xi-k/1/2), transformed with PtOLE6-eGFP, trafficking of PtOLE6-eGFP-tagged LBs was severely impaired, confirming that they move on F-actin, motorized by myosin XIs. The data reveal that LBs and OLEOSINs both function in proliferating apices and buds, and that directional trafficking of LBs to plasmodesmata requires the actomyosin system.

Update on plant cytokinesis: rule and divide

Many decisions made during plant development depend on the placement of the cytokinetic wall. Cytokinesis involves the biogenesis of the cell plate that progresses centrifugally and until the fusion of the cell plate with the parental cell wall. The phragmoplast facilitates the growth of the cell plate and directs it's insertion at the cell cortex by a mechanism known as phragmoplast guidance. Communication between the phragmoplast and its destination, the cortical division zone, however, is not well understood. The preprophase band predicts the site of cell plate fusion, seemingly controlling the site of the cortical division zone establishment, but recent results suggest the role of this cytoskeletal array to be rather subtle. This is indirectly supported by certain types of phragmoplast-driven cell division in mosses and algae, which lack preprophase bands. In this review article, we summarize recent insight concerning phragmoplast expansion and guidance.

Plant Kinesin-12: Localization Heterogeneity and Functional Implications

Kinesin-12 family members are characterized by an N-terminal motor domain and the extensive presence of coiled-coil domains. Animal orthologs display microtubule plus-end directed motility, bundling of parallel and antiparallel microtubules, plus-end stabilization, and they play a crucial role in spindle assembly. In plants, kinesin-12 members mediate a number of developmental processes including male gametophyte, embryo, seedling, and seed development. At the cellular level, they participate in critical events during cell division. Several kinesin-12 members localize to the phragmoplast midzone, interact with isoforms of the conserved microtubule cross-linker MICROTUBULE-ASSOCIATED PROTEIN 65 (MAP65) family, and are required for phragmoplast stability and expansion, as well as for proper cell plate development. Throughout cell division, a subset of kinesin-12 reside, in addition or exclusively, at the cortical division zone and mediate the accurate guidance of the phragmoplast. This review aims to summarize the current knowledge on kinesin-12 in plants and shed some light onto the heterogeneous localization and domain architecture, which potentially conceals functional diversification.

Molecular and physiological control of adventitious rooting in cuttings: phytohormone action meets resource allocation

BACKGROUND

Adventitious root (AR) formation in excised plant parts is a bottleneck for survival of isolated plant fragments. AR formation plays an important ecological role and is a critical process in cuttings for the clonal propagation of horticultural and forestry crops. Therefore, understanding the regulation of excision-induced AR formation is essential for sustainable and efficient utilization of plant genetic resources.

SCOPE

Recent studies of plant transcriptomes, proteomes and metabolomes, and the use of mutants and transgenic lines have significantly expanded our knowledge concerning excision-induced AR formation. Here, we integrate new findings regarding AR formation in the cuttings of diverse plant species. These findings support a new system-oriented concept that the phytohormone-controlled reprogramming and differentiation of particular responsive cells in the cutting base interacts with a co-ordinated reallocation of plant resources within the whole cutting to initiate and drive excision-induced AR formation. Master control by auxin involves diverse transcription factors and mechanically sensitive microtubules, and is further linked to ethylene, jasmonates, cytokinins and strigolactones. Hormone functions seem to involve epigenetic factors and cross-talk with metabolic signals, reflecting the nutrient status of the cutting. By affecting distinct physiological units in the cutting, environmental factors such as light, nitrogen and iron modify the implementation of the genetically controlled root developmental programme.

CONCLUSION

Despite advanced research in the last decade, important questions remain open for future investigations on excision-induced AR formation. These concern the distinct roles and interactions of certain molecular, hormonal and metabolic factors, as well as the functional equilibrium of the whole cutting in a complex environment. Starting from model plants, cell type- and phase-specific monitoring of controlling processes and modification of gene expression are promising methodologies that, however, need to be integrated into a coherent model of the whole system, before research findings can be translated to other crops.

Genetic screen for factors mediating PIN polarization in gravistimulated Arabidopsis thaliana hypocotyls

Gravitropism is an adaptive response that orients plant growth parallel to the gravity vector. Asymmetric distribution of the phytohormone auxin is a necessary prerequisite to the tropic bending both in roots and shoots. During hypocotyl gravitropic response, the PIN3 auxin transporter polarizes within gravity-sensing cells to redirect intercellular auxin fluxes. First gravity-induced PIN3 polarization to the bottom cell membranes leads to the auxin accumulation at the lower side of the organ, initiating bending and, later, auxin feedback-mediated repolarization restores symmetric auxin distribution to terminate bending. Here, we performed a forward genetic screen to identify regulators of both PIN3 polarization events during gravitropic response. We searched for mutants with defective PIN3 polarizations based on easy-to-score morphological outputs of decreased or increased gravity-induced hypocotyl bending. We identified the number of hypocotyl reduced bending (hrb) and hypocotyl hyperbending (hhb) mutants, revealing that reduced bending correlated typically with defective gravity-induced PIN3 relocation whereas all analyzed hhb mutants showed defects in the second, auxin-mediated PIN3 relocation. Next-generation sequencing-aided mutation mapping identified several candidate genes, including SCARECROW and ACTIN2, revealing roles of endodermis specification and actin cytoskeleton in the respective gravity- and auxin-induced PIN polarization events. The hypocotyl gravitropism screen thus promises to provide novel insights into mechanisms underlying cell polarity and plant adaptive development.

Division Plane Establishment and Cytokinesis

Plant cells divide their cytoplasmic content by forming a new membrane compartment, the cell plate, via a rerouting of the secretory pathway toward the division plane aided by a dynamic cytoskeletal apparatus known as the phragmoplast. The phragmoplast expands centrifugally and directs the cell plate to the preselected division site at the plasma membrane to fuse with the parental wall. The division site is transiently decorated by the cytoskeletal preprophase band in preprophase and prophase, whereas a number of proteins discovered over the last decade reside continuously at the division site and provide a lasting spatial reference for phragmoplast guidance. Recent studies of membrane fusion at the cell plate have revealed the contribution of functionally conserved eukaryotic proteins to distinct stages of cell plate biogenesis and emphasize the coupling of cell plate formation with phragmoplast expansion. Together with novel findings concerning preprophase band function and the setup of the division site, cytokinesis and its spatial control remain an open-ended field with outstanding and challenging questions to resolve.

Plant cell division - defining and finding the sweet spot for cell plate insertion

The plant microtubules form unique arrays using acentrosomal microtubule nucleation pathways, yet utilizing evolutionary conserved centrosomal proteins. In cytokinesis, a multi-component cytoskeletal apparatus, the phragmoplast mediates the biosynthesis of the new cell plate by dynamic centrifugal expansion, a process that demands exquisite coordination of microtubule turnover and endomembrane trafficking. At the same time, the phragmoplast is guided to meet with the parental wall at a cortical site that is predefined before mitotic entry and transiently marked by the preprophase band of microtubules. The cortical division zone maintains positional information of the selected division plane for the entire duration of cell division and for the guidance of the phragmoplast during cytokinesis. Its establishment is an essential requirement for normal plant organogenesis, due to the confinement of cells by rigid cell walls.

Myosins XI Are Involved in Exocytosis of Cellulose Synthase Complexes

In plants, cellulose is synthesized at the cell surface by plasma membrane (PM)-localized cellulose synthase (CESA) complexes (CSCs). The molecular and cellular mechanisms that underpin delivery of CSCs to the PM, however, are poorly understood. Cortical microtubules have been shown to interact with CESA-containing compartments and mark the site for CSC delivery, but are not required for the delivery itself. Here, we demonstrate that myosin XI and the actin cytoskeleton mediate CSC delivery to the PM by coordinating the exocytosis of CESA-containing compartments. Measurement of cellulose content indicated that cellulose biosynthesis was significantly reduced in a myosin xik xi1 xi2 triple-knockout mutant. By combining genetic and pharmacological disruption of myosin activity with quantitative live-cell imaging, we observed decreased abundance of PM-localized CSCs and reduced delivery rate of CSCs in myosin-deficient cells. These phenotypes correlated with a significant increase in failed vesicle secretion events at the PM as well as an abnormal accumulation of CESA-containing compartments at the cell cortex. Through high-resolution spatiotemporal assays of cortical vesicle behavior, we identified defects in CSC vesicle tethering and fusion at the PM. Furthermore, disruption of myosin activity reduced the delivery of several other secretory markers to the PM and reduced constitutive and receptor-mediated endocytosis. These findings reveal a previously undescribed role for myosin in vesicle secretion and cellulose production at the cytoskeleton-PM-cell wall nexus.

Class XI Myosins Contribute to Auxin Response and Senescence-Induced Cell Death in Arabidopsis

The integrity and dynamics of actin cytoskeleton is necessary not only for plant cell architecture but also for membrane trafficking-mediated processes such as polar auxin transport, senescence, and cell death. In Arabidopsis, the inactivation of actin-based molecular motors, class XI myosins, affects the membrane trafficking and integrity of actin cytoskeleton, and thus causes defective plant growth and morphology, altered lifespan and reduced fertility. To evaluate the potential contribution of class XI myosins to the auxin response, senescence and cell death, we followed the flower and leaf development in the triple gene knockout mutant xi1 xi2 xik (3KO) and in rescued line stably expressing myosin XI-K:YFP (3KOR). Assessing the development of primary inflorescence shoots we found that the 3KO plants produced more axillary branches. Exploiting the auxin-dependent reporters DR5::GUS and IAA2::GUS, a significant reduction in auxin responsiveness was found throughout the development of the 3KO plants. Examination of the flower development of the plants stably expressing the auxin transporter PIN1::PIN1-GFP revealed partial loss of PIN1 polarization in developing 3KO pistils. Surprisingly, the stable expression of PIN1::PIN1-GFP significantly enhanced the semi-sterile phenotype of the 3KO plants. Further we investigated the localization of myosin XI-K:YFP in the 3KOR floral organs and revealed its expression pattern in floral primordia, developing pistils, and anther filaments. Interestingly, the XI-K:YFP and PIN1::PIN1-GFP shared partially overlapping but distinct expression patterns throughout floral development. Assessing the foliar development of the 3KO plants revealed increased rosette leaf production with signs of premature yellowing. Symptoms of the premature senescence correlated with massive loss of chlorophyll, increased cell death, early plasmolysis of epidermal cells, and strong up-regulation of the stress-inducible senescence-associated gene SAG13 in 3KO plants. Simultaneously, the reduced auxin responsiveness and premature leaf senescence were accompanied by significant anthocyanin accumulation in 3KO tissues. Collectively, our results provide genetic evidences that Arabidopsis class XI myosins arrange the flower morphogenesis and leaf longevity via contributing to auxin responses, leaf senescence, and cell death.

Myosin XI motors: back on the scene at the division machine

This article comments on: Abu-Abied M, Belausov E, Hagay S, Peremyslov V, Dolja V, Sadot E. 2018. Myosin XI-K is involved in root organogenesis, polar auxin transport and cell division. Journal of Experimental Botany 69, 2869–2881.