Quantitative analysis of organelle distribution and dynamics in Physcomitrella patens protonemal cells

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.

Basally distributed actin array drives embryonic hypocotyl elongation during the seed-to-seedling transition in Arabidopsis

Seed germination is a vital developmental transition for the production of progeny by sexual reproduction in spermatophytes. The seed-to-seedling transition is predominately driven by hypocotyl cell elongation. However, the mechanism that underlies hypocotyl growth remains largely unknown. In this study, we characterized the actin array reorganization in embryonic hypocotyl epidermal cells. Live-cell imaging revealed a basally organized actin array formed during hypocotyl cell elongation. This polarized actin assembly is a barrel-shaped network, which comprises a backbone of longitudinally aligned actin cables and a fine actin cap linking these cables. We provide genetic evidence that the basal actin array formation requires formin-mediated actin polymerization and directional movement of actin filaments powered by myosin XIs. In fh1-1 and xi3ko mutants, actin filaments failed to reorganize into the basal actin array, and the hypocotyl cell elongation was inhibited compared with wild-type plants. Collectively, our work uncovers the molecular mechanisms for basal actin array assembly and demonstrates the connection between actin polarization and hypocotyl elongation during seed-to-seedling transition.

Armadillo repeat-containing kinesin represents the versatile plus-end-directed transporter in Physcomitrella

Kinesin-1, also known as conventional kinesin, is widely used for microtubule plus-end-directed (anterograde) transport of various cargos in animal cells. However, a motor functionally equivalent to the conventional kinesin has not been identified in plants, which lack the kinesin-1 genes. Here we show that plant-specific armadillo repeat-containing kinesin (ARK) is the long sought-after versatile anterograde transporter in plants. In ARK mutants of the moss Physcomitrium patens, the anterograde motility of nuclei, chloroplasts, mitochondria and secretory vesicles was suppressed. Ectopic expression of non-motile or tail-deleted ARK did not restore organelle distribution. Another prominent macroscopic phenotype of ARK mutants was the suppression of cell tip growth. We showed that this defect was attributed to the mislocalization of actin regulators, including RopGEFs; expression and forced apical localization of RopGEF3 partially rescued the growth phenotype of the ARK mutant. The mutant phenotypes were partially rescued by ARK homologues in Arabidopsis thaliana, suggesting the conservation of ARK functions in plants.

Microfluidic Device for High-Resolution Cytoskeleton Imaging and Washout Assays in Physcomitrium (Physcomitrella) patens

Visualizing cytoskeleton dynamics at high spatiotemporal resolution provides valuable insights into the way the dynamics change as well as its interactions with multiple proteins in order to maintain cellular function. Oblique illumination fluorescent microscopy is a popular technique to image cellular events localized near the plasma membrane. In this chapter, we provide detailed protocols for high-resolution cytoskeleton imaging using protonema and gametophore cells of the moss Physcomitrella (Physcomitrium patens) in the microfluidic device. These include preparation of the polydimethylsiloxane (PDMS) device, culture of moss cells, and both short- and long-term oblique illumination fluorescent microscopy. We also describe how to introduce to, and wash out from, the device chemical compounds, such as microtubule-disrupting drugs, during live-cell imaging.

COPII Sec23 proteins form isoform-specific endoplasmic reticulum exit sites with differential effects on polarized growth

Coat Protein complex II (COPII), a coat protein complex that forms vesicles on the endoplasmic reticulum (ER), mediates trafficking to the Golgi. While metazoans have few genes encoding each COPII component, plants have expanded these gene families, leading to the hypothesis that plant COPII has functionally diversified. In the moss Physcomitrium (Physcomitrella) patens, the Sec23/24 gene families are each composed of seven genes. Silencing Sec23/24 revealed isoform-specific contributions to polarized growth, with the closely related Sec23D/E and Sec24C/D essential for protonemal development. Focusing on Sec23, we discovered that Sec23D/E mediate ER-to Golgi transport and are essential for tip growth, with Sec23D localizing to presumptive ER exit sites. In contrast, Sec23A, B, C, F, and G are dispensable and do not quantitatively affect ER-to-Golgi trafficking. However, Δsec23abcfg plants exhibited reduced secretion of plasma membrane cargo. Of the four highly expressed protonemal Sec23 genes, Sec23F/G are members of a divergent Sec23 clade specifically retained in land plants. Notably, Sec23G accumulates on ER-associated foci that are significantly larger, do not overlap with, and are independent of Sec23D. While Sec23D/E form ER exit sites and function as bona fide COPII components essential for tip-growing protonemata, Sec23G and the closely related Sec23F have likely functionally diversified, forming separate and independent ER exit sites and participating in Golgi-independent trafficking pathways.

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.

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.

Inactivation of mitochondrial complex I stimulates chloroplast ATPase in Physcomitrium patens

Light is the ultimate source of energy for photosynthetic organisms, but respiration is fundamental for supporting metabolism during the night or in heterotrophic tissues. In this work, we isolated Physcomitrella (Physcomitrium patens) plants with altered respiration by inactivating Complex I (CI) of the mitochondrial electron transport chain by independently targeting on two essential subunits. Inactivation of CI caused a strong growth impairment even in fully autotrophic conditions in tissues where all cells are photosynthetically active, demonstrating that respiration is essential for photosynthesis. CI mutants showed alterations in the stoichiometry of respiratory complexes while the composition of photosynthetic apparatus was substantially unaffected. CI mutants showed altered photosynthesis with high activity of both Photosystems I and II, likely the result of high chloroplast ATPase activity that led to smaller ΔpH formation across thylakoid membranes, decreasing photosynthetic control on cytochrome b6f in CI mutants. These results demonstrate that alteration of respiratory activity directly impacts photosynthesis in P. patens and that metabolic interaction between organelles is essential in their ability to use light energy for growth.

Fuzzy Recurrence Exponents of Subcellular-Nanostructure Dynamics in Time-lapse Confocal Imaging

Studying the dynamics of nanostructures in the intracellular space is important because it allows gaining insights into the mechanism of complex biological functions of organelles. Understanding such dynamical processes can contribute to the development of nanomedicine for the diagnosis and treatment of many diseases caused by the interaction of multiple genes and environmental factors. Here a quantitative measure of spatial-temporal dynamics of nanostructures within a cell line in the context of nonlinear dynamics is introduced, where early endosomes, late endosomes, and lysosomes recorded by time-lapse confocal imaging are examined. The mathematical derivation of the proposed technique is based on the concept of recurrence dynamics and sequential rate of change over time. The quantification introduced as fuzzy recurrence exponents can be generalized for characterizing the dynamics of experimental evolutions in other nanostructures of living cells captured under the optical microscope.

Quantitative proteomic analysis to capture the role of heat-accumulated proteins in moss plant acquired thermotolerance

At dawn of a scorching summer day, land plants must anticipate upcoming extreme midday temperatures by timely establishing molecular defences that can keep heat-labile membranes and proteins functional. A gradual morning pre-exposure to increasing sub-damaging temperatures induces heat-shock proteins (HSPs) that are central to the onset of plant acquired thermotolerance (AT). To gain knowledge on the mechanisms of AT in the model land plant Physcomitrium patens, we used label-free LC-MS/MS proteomics to quantify the accumulated and depleted proteins before and following a mild heat-priming treatment. High protein crowding is thought to promote protein aggregation, whereas molecular chaperones prevent and actively revert aggregation. Yet, we found that heat priming (HP) did not accumulate HSP chaperones in chloroplasts, although protein crowding was six times higher than in the cytosol. In contrast, several HSP20s strongly accumulated in the cytosol, yet contributing merely 4% of the net mass increase of heat-accumulated proteins. This is in poor concordance with their presumed role at preventing the aggregation of heat-labile proteins. The data suggests that under mild HP unlikely to affect protein stability. Accumulating HSP20s leading to AT, regulate the activity of rare and specific signalling proteins, thereby preventing cell death under noxious heat stress.

SEC6 exocyst subunit contributes to multiple steps of growth and development of Physcomitrella (Physcomitrium patens)

Spatially directed cell division and expansion is important for plant growth and morphogenesis and relies on cooperation between the cytoskeleton and the secretory pathway. The phylogenetically conserved octameric complex exocyst mediates exocytotic vesicle tethering at the plasma membrane. Unlike other exocyst subunits of land plants, the core exocyst subunit SEC6 exists as a single paralog in Physcomitrium patens and Arabidopsis thaliana genomes. Arabidopsis SEC6 (AtSEC6) loss-of-function (LOF) mutation causes male gametophytic lethality. Our attempts to inactivate the P. patens SEC6 gene, PpSEC6, using targeted gene replacement produced two independent partial LOF ('weak allele') mutants via perturbation of the PpSEC6 gene locus. These mutants exhibited the same pleiotropic developmental defects: protonema with dominant chloronema stage; diminished caulonemal filament elongation rate; and failure in post-initiation gametophore development. Mutant gametophore buds, mostly initiated from chloronema cells, exhibited disordered cell file organization and cross-wall perforations, resulting in arrested development at the eight- to 10-cell stage. Complementation of both sec6 moss mutant lines by both PpSEC6 and AtSEC6 cDNA rescued gametophore development, including sexual organ differentiation. However, regular sporophyte formation and viable spore production were recovered only by the expression of PpSEC6, whereas the AtSEC6 complementants were only rarely fertile, indicating moss-specific SEC6 functions.

Actin and Microtubules Differently Contribute to Vacuolar Targeting Specificity during the Export from the ER

Plants rely on both actin and microtubule cytoskeletons to fine-tune sorting and spatial targeting of membranes during cell growth and stress adaptation. Considerable advances have been made in recent years in the comprehension of the relationship between the trans-Golgi network/early endosome (TGN/EE) and cytoskeletons, but studies have mainly focused on the transport to and from the plasma membrane. We address here the relationship of the cytoskeleton with different endoplasmic reticulum (ER) export mechanisms toward vacuoles. These emergent features of the plant endomembrane traffic are explored with an in vivo approach, providing clues on the traffic regulation at different levels beyond known proteins’ functions and interactions. We show how traffic of vacuolar markers, characterized by different vacuolar sorting determinants, diverges at the export from the ER, clearly involving different components of the cytoskeleton.

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.

3D Dissection of Structural Membrane-Wall Contacts in Filamentous Moss Protonemata

In conventional light microscopy, the adjacent cell walls of filamentous moss protonemata are seen from its narrow side thereby obscuring the major area of cell–cell connection. Optical sectioning, segmentation and 3D reconstructions allow the tilting and rotation of intracellular structures thereby greatly improving our understanding of interaction between organelles, membranes and the cell wall. Often, the findings also allow for conclusions on the respective functions. The moss Physcomitrium (Physcomitrella) patens is a model organism for growth, development and morphogenesis. Its filamentous protonemata are ideal objects for microscopy. Here, we investigated the cell wall between two neighboring cells and the connection of membranes towards this wall after plasmolysis in 0.8 M mannitol. An m-green fluorescent protein (GFP)-HDEL cell line was used to visualize the endoplasmatic reticulum (ER), the plasma membrane (PM) was stained with FM4-64. Our studies clearly show the importance of cell–cell contacts in P. patens protonemata. In 86% of the investigated cell pairs, at least one of the protoplasts remained fully attached to the adjacent cell wall. By tilting of z-stacks, volume renderings and 3D reconstructions, we visualized the amount of attached/detached PM and ER components after plasmolysis and membrane piercings through the wall of cell neighbors.

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.

AP2/ERF transcription factors regulate salt-induced chloroplast division in the moss Physcomitrella patens

Chloroplast division is a critical process for the maintenance of appropriate chloroplast number in plant cells. It is known that in some plant species and cell types, environmental stresses can affect chloroplast division, differentiation and morphology, however the significance and regulation of these processes are largely unknown. Here we investigated the regulation of salt stress-induced chloroplast division in protonemal cells of the moss, Physcomitrella patens, and found that, salt stress as one of the major abiotic stresses, induced chloroplast division and resulted in increased chloroplast numbers. We further identified three APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) transcription factors (TFs) that were responsible for this regulation. These AP2/ERF genes were up-regulated under salt stress, and amino acid sequences and phylogenetic analyses indicated that all TFs possess only one conserved AP2 domain and likely belong to the same subgroup of ERF-B3 in the AP2/ERF superfamily. Overexpression of these TFs significantly increased the chloroplast number even in the absence of NaCl stress. On the contrary, inducible overexpression of the dominant repressor form of these TFs suppressed salt stress-induced chloroplast division. Thus, our results suggest that salt stress induced-chloroplast division is regulated through members of the AP2/ERF TF superfamily.

The Moss Physcomitrium (Physcomitrella) patens: A Model Organism for Non-Seed Plants

Since the discovery two decades ago that transgenes are efficiently integrated into the genome of Physcomitrella patens by homologous recombination, this moss has been a premier model system to study evolutionary developmental biology questions, stem cell reprogramming, and the biology of nonvascular plants. P patens was the first non-seed plant to have its genome sequenced. With this level of genomic information, together with increasing molecular genetic tools, a large number of reverse genetic studies have propelled the use of this model system. A number of technological advances have recently opened the door to forward genetics as well as extremely efficient and precise genome editing in P patens Additionally, careful phylogenetic studies with increased resolution have suggested that P patens emerged from within Physcomitrium Thus, rather than Physcomitrella patens, the species should be named Physcomitrium patens Here we review these advances and describe the areas where P patens has had the most impact on plant biology.

Stay in Touch-The Cortical ER of Moss Protonemata in Osmotic Stress Situations

Plasmolysis is usually introduced to cell biology students as a tool to illustrate the plasma membrane: hypertonic solutions cause the living protoplast to shrink by osmotic water loss; hence, it detaches from the surrounding cell wall. What happens, however, with the subcellular structures in the cell cortex during this process of turgor loss? Here, we investigated the cortical endoplasmic reticulum (ER) in moss protonema cells of Physcomitrella patens in a cell line carrying a transgenic ER marker (GFP-HDEL). The plasma membrane was labelled simultaneously with the fluorescent dye FM4-64 to achieve structural separation. By placing the protonemata in a hypertonic mannitol solution (0.8 M), we were able to follow the behaviour of the cortical ER and the protoplast during plasmolysis by confocal laser scanning microscopy (CLSM). The protoplast shape and structural changes of the ER were further examined after depolymerisation of actin microfilaments with latrunculin B (1 µM). In its natural state, the cortical ER is a dynamic network of fine tubes and cisternae underneath the plasma membrane. Under acute and long-term plasmolysis (up to 45 min), changes in the protoplast form and the cortical ER, as well as the formation of Hechtian strands and Hechtian reticula, were observed. The processing of the high-resolution z-scans allowed the creation of 3D models and gave detailed insight into the ER of living protonema cells before, during and after plasmolysis.

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.

Physcomitrella as a model system for plant cell biology and organelle-organelle communication

In multicellular eukaryotic cells, metabolism and growth are sustained by the cooperative functioning of organelles in combination with cell-to-cell communication at the organism level. In land plants, multiple strategies have evolved to adapt to life outside water. As basal land plant, the moss Physcomitrella patens is used for comparative genomics, allowing to study lineage-specific features, as well as to track the evolution of fundamental parameters of plant cell organisation and physiology. P. patens is a versatile model for cell biology research, especially to investigate adaptive growth, stress biology as well as organelle dynamics and interactions. Recent advances include the use of genetically encoded biosensors for in vivo imaging of physiological parameters.

Cytoskeletal discoveries in the plant lineage using the moss Physcomitrella patens

Advances in cell biology have been largely driven by pioneering work in model systems, the majority of which are from one major eukaryotic lineage, the opisthokonts. However, with the explosion of genomic information in many lineages, it has become clear that eukaryotes have incredible diversity in many cellular systems, including the cytoskeleton. By identifying model systems in diverse lineages, it may be possible to begin to understand the evolutionary origins of the eukaryotic cytoskeleton. Within the plant lineage, cell biological studies in the model moss, Physcomitrella patens, have over the past decade provided key insights into how the cytoskeleton drives cell and tissue morphology. Here, we review P. patens attributes that make it such a rich resource for cytoskeletal cell biological inquiry and highlight recent key findings with regard to intracellular transport, microtubule-actin interactions, and gene discovery that promises for many years to provide new cytoskeletal players.

Functional Characterization of a Glycosyltransferase from the Moss Physcomitrella patens Involved in the Biosynthesis of a Novel Cell Wall Arabinoglucan

Mixed-linkage (1,3;1,4)-β-glucan (MLG), an abundant cell wall polysaccharide in the Poaceae, has been detected in ascomycetes, algae, and seedless vascular plants, but not in eudicots. Although MLG has not been reported in bryophytes, a predicted glycosyltransferase from the moss Physcomitrella patens (Pp3c12_24670) is similar to a bona fide ascomycete MLG synthase. We tested whether Pp3c12_24670 encodes an MLG synthase by expressing it in wild tobacco (Nicotiana benthamiana) and testing for release of diagnostic oligosaccharides from the cell walls by either lichenase or (1,4)-β-glucan endohydrolase. Lichenase, an MLG-specific endohydrolase, showed no activity against cell walls from transformed N. benthamiana, but (1,4)-β-glucan endohydrolase released oligosaccharides that were distinct from oligosaccharides released from MLG by this enzyme. Further analysis revealed that these oligosaccharides were derived from a novel unbranched, unsubstituted arabinoglucan (AGlc) polysaccharide. We identified sequences similar to the P. patens AGlc synthase from algae, bryophytes, lycophytes, and monilophytes, raising the possibility that other early divergent plants synthesize AGlc. Similarity of P. patens AGlc synthase to MLG synthases from ascomycetes, but not those from Poaceae, suggests that AGlc and MLG have a common evolutionary history that includes loss in seed plants, followed by a more recent independent origin of MLG within the monocots.

Conditional genetic screen in Physcomitrella patens reveals a novel microtubule depolymerizing-end-tracking protein

Our ability to identify genes that participate in cell growth and division is limited because their loss often leads to lethality. A solution to this is to isolate conditional mutants where the phenotype is visible under restrictive conditions. Here, we capitalize on the haploid growth-phase of the moss Physcomitrella patens to identify conditional loss-of-growth (CLoG) mutants with impaired growth at high temperature. We used whole-genome sequencing of pooled segregants to pinpoint the lesion of one of these mutants (clog1) and validated the identified mutation by rescuing the conditional phenotype by homologous recombination. We found that CLoG1 is a novel and ancient gene conserved in plants. At the restrictive temperature, clog1 plants have smaller cells but can complete cell division, indicating an important role of CLoG1 in cell growth, but not an essential role in cell division. Fluorescent protein fusions of CLoG1 indicate it is localized to microtubules with a bias towards depolymerizing microtubule ends. Silencing CLoG1 decreases microtubule dynamics, suggesting that CLoG1 plays a critical role in regulating microtubule dynamics. By discovering a novel gene critical for plant growth, our work demonstrates that P. patens is an excellent genetic system to study genes with a fundamental role in plant cell growth.

F-Actin Mediated Focusing of Vesicles at the Cell Tip Is Essential for Polarized Growth

F-actin has been shown to be essential for tip growth in an array of plant models, including Physcomitrella patens One hypothesis is that diffusion can transport secretory vesicles, while actin plays a regulatory role during secretion. Alternatively, it is possible that actin-based transport is necessary to overcome vesicle transport limitations to sustain secretion. Therefore, a quantitative analysis of diffusion, secretion kinetics, and cell geometry is necessary to clarify the role of actin in polarized growth. Using fluorescence recovery after photobleaching analysis, we first show that secretory vesicles move toward and accumulate at the tip in an actin-dependent manner. We then depolymerized F-actin to decouple vesicle diffusion from actin-mediated transport and measured the diffusion coefficient and concentration of vesicles. Using these values, we constructed a theoretical diffusion-based model for growth, demonstrating that with fast-enough vesicle fusion kinetics, diffusion could support normal cell growth rates. We further refined our model to explore how experimentally extrapolated vesicle fusion kinetics and the size of the secretion zone limit diffusion-based growth. This model predicts that diffusion-mediated growth is dependent on the size of the region of exocytosis at the tip and that diffusion-based growth would be significantly slower than normal cell growth. To further explore the size of the secretion zone, we used a cell wall degradation enzyme cocktail and determined that the secretion zone is smaller than 6 μm in diameter at the tip. Taken together, our results highlight the requirement for active transport in polarized growth and provide important insight into vesicle secretion during tip growth.

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

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

Long-Term Growth of Moss in Microfluidic Devices Enables Subcellular Studies in Development

Key developmental processes that occur on the subcellular and cellular level or occur in occluded tissues are difficult to access, let alone image and analyze. Recently, culturing living samples within polydimethylsiloxane (PDMS) microfluidic devices has facilitated the study of hard-to-reach developmental events. Here, we show that an early diverging land plant, Physcomitrella patens, can be continuously cultured within PDMS microfluidic chambers. Because the PDMS chambers are bonded to a coverslip, it is possible to image P. patens development at high resolution over long time periods. Using PDMS chambers, we report that wild-type protonemal tissue grows at the same rate as previously reported for growth on solid medium. Using long-term imaging, we highlight key developmental events, demonstrate compatibility with high-resolution confocal microscopy, and obtain growth rates for a slow-growing mutant. By coupling the powerful genetic tools available to P. patens with long-term growth and imaging provided by PDMS microfluidic chambers, we demonstrate the capability to study cellular and subcellular developmental events in plants directly and in real time.

The mitochondrial proteome of the moss Physcomitrella patens

Extant basal land plants are routinely used to trace plant evolution and to track strategies for high abiotic stress resistance. Whereas the structure of mitochondrial genomes and RNA editing are already well studied, mitochondrial proteome research is restricted to a few data sets. While the mitochondrial proteome of the model moss Physcomitrella patens is covered to an estimated 15-25% by proteomic evidence to date, the available data have already provided insights into the evolution of metabolic compartmentation, dual targeting and mitochondrial heterogeneity. This review summarizes the current knowledge about the mitochondrial proteome of P. patens, and gives a perspective on its use as a mitochondrial model system. Its amenability to gene editing, metabolic labelling as well as fluorescence microscopy provides a unique platform to study open questions in mitochondrial biology, such as regulation of protein stability, responses to stress and connectivity to other organelles. Future challenges will include improving the proteomic resources for P. patens, and to link protein inventories and modifications as well as evolutionary differences to the functional level.

Giant peroxisomes in a moss (Physcomitrella patens) peroxisomal biogenesis factor 11 mutant

Peroxisomal biogenesis factor 11 (PEX11) proteins are found in yeasts, mammals and plants, and play a role in peroxisome morphology and regulation of peroxisome division. The moss Physcomitrella patens has six PEX11 isoforms which fall into two subfamilies, similar to those found in monocots and dicots. We carried out targeted gene disruption of the Phypa_PEX11-1 gene and compared the morphological and cellular phenotypes of the wild-type and mutant strains. The mutant grew more slowly and the development of gametophores was retarded. Mutant chloronemal filaments contained large cellular structures which excluded all other cellular organelles. Expression of fluorescent reporter proteins revealed that the mutant strain had greatly enlarged peroxisomes up to 10 μm in diameter. Expression of a vacuolar membrane marker confirmed that the enlarged structures were not vacuoles, or peroxisomes sequestered within vacuoles as a result of pexophagy. Phypa_PEX11 targeted to peroxisome membranes could rescue the knock out phenotype and interacted with Fission1 on the peroxisome membrane. Moss PEX11 functions in peroxisome division similar to PEX11 in other organisms but the mutant phenotype is more extreme and environmentally determined, making P. patens a powerful system in which to address mechanisms of peroxisome proliferation and division.

Imaging Mitosis in the Moss Physcomitrella patens

At first glance, mitosis in plants looks quite different from that in animals. In fact, terrestrial plants have lost the centrosome during evolution, and the mitotic spindle is assembled independently of a strong microtubule organizing center. The phragmoplast is a plant-specific mitotic apparatus formed after anaphase, which expands centrifugally towards the cell cortex. However, the extent to which plant mitosis differs from that of animals at the level of the protein repertoire is uncertain, largely because of the difficulty in the identification and in vivo characterization of mitotic genes of plants. Here, we discuss protocols for mitosis imaging that can be combined with endogenous green fluorescent protein (GFP) tagging or conditional RNA interference (RNAi) in the moss Physcomitrella patens, which is an emergent model plant for cell and developmental biology. This system has potential for use in the high-throughput study of mitosis and other intracellular processes, as is being done with various animal cell lines.

Mitochondrial Dynamics and the ER: The Plant Perspective

Whereas contact sites between mitochondria and the ER have been in the focus of animal and fungal research for several years, the importance of this organellar interface and the molecular effectors are largely unknown for plants. This work gives an introduction into known evolutionary differences of molecular effectors of mitochondrial dynamics and interactions between animals, fungi, and plants. Using the model plant Physcomitrella patens, we provide microscopic evidence for the existence of mitochondria-ER interactions in plants and their correlation with mitochondrial constriction and fission. We further investigate a previously identified protein of unknown function (MELL1), and show that it modulates the amount of mitochondrial association to the ER, as well as mitochondrial shape and number.

Class XI Myosins Move Specific Organelles in Pollen Tubes and Are Required for Normal Fertility and Pollen Tube Growth in Arabidopsis

Pollen tube growth is an essential aspect of plant reproduction because it is the mechanism through which nonmotile sperm cells are delivered to ovules, thus allowing fertilization to occur. A pollen tube is a single cell that only grows at the tip, and this tip growth has been shown to depend on actin filaments. It is generally assumed that myosin-driven movements along these actin filaments are required to sustain the high growth rates of pollen tubes. We tested this conjecture by examining seed set, pollen fitness, and pollen tube growth for knockout mutants of five of the six myosin XI genes expressed in pollen of Arabidopsis (Arabidopsis thaliana). Single mutants had little or no reduction in overall fertility, whereas double mutants of highly similar pollen myosins had greater defects in pollen tube growth. In particular, myo11c1 myo11c2 pollen tubes grew more slowly than wild-type pollen tubes, which resulted in reduced fitness compared with the wild type and a drastic reduction in seed set. Golgi stack and peroxisome movements were also significantly reduced, and actin filaments were less organized in myo11c1 myo11c2 pollen tubes. Interestingly, the movement of yellow fluorescent protein-RabA4d-labeled vesicles and their accumulation at pollen tube tips were not affected in the myo11c1 myo11c2 double mutant, demonstrating functional specialization among myosin isoforms. We conclude that class XI myosins are required for organelle motility, actin organization, and optimal growth of pollen tubes.

The kinesin-like proteins, KAC1/2, regulate actin dynamics underlying chloroplast light-avoidance in Physcomitrella patens

In plants, light determines chloroplast position; these organelles show avoidance and accumulation responses in high and low fluence-rate light, respectively. Chloroplast motility in response to light is driven by cytoskeletal elements. The actin cytoskeleton mediates chloroplast photorelocation responses in Arabidopsis thaliana. In contrast, in the moss Physcomitrella patens, both, actin filaments and microtubules can transport chloroplasts. Because of the surprising evidence that two kinesin-like proteins (called KACs) are important for actin-dependent chloroplast photorelocation in vascular plants, we wanted to determine the cytoskeletal system responsible for the function of these proteins in moss. We performed gene-specific silencing using RNA interference in P. patens. We confirmed existing reports using gene knockouts, that PpKAC1 and PpKAC2 are required for chloroplast dispersion under uniform white light conditions, and that the two proteins are functionally equivalent. To address the specific cytoskeletal elements responsible for motility, this loss-of-function approach was combined with cytoskeleton-targeted drug studies. We found that, in P. patens, these KACs mediate the chloroplast light-avoidance response in an actin filament-dependent, rather than a microtubule-dependent manner. Using correlation-decay analysis of cytoskeletal dynamics, we found that PpKAC stabilizes cortical actin filaments, but has no effect on microtubule dynamics.

Role of plant myosins in motile organelles: is a direct interaction required?

Plant organelles are highly motile, with speed values of 3-7 µm/s in cells of land plants and about 20-60 µm/s in characean algal cells. This movement is believed to be important for rapid distribution of materials around the cell, for the plant's ability to respond to environmental biotic and abiotic signals and for proper growth. The main machinery that propels motility of organelles within plant cells is based on the actin cytoskeleton and its motor proteins the myosins. Most plants express multiple members of two main classes: myosin VIII and myosin XI. While myosin VIII has been characterized as a slow motor protein, myosins from class XI were found to be the fastest motor proteins known in all kingdoms. Paradoxically, while it was found that myosins from class XI regulate most organelle movement, it is not quite clear how or even if these motor proteins attach to the organelles whose movement they regulate.

Quantitative analysis of the mitochondrial and plastid proteomes of the moss Physcomitrella patens reveals protein macrocompartmentation and microcompartmentation

Extant eukaryotes are highly compartmentalized and have integrated endosymbionts as organelles, namely mitochondria and plastids in plants. During evolution, organellar proteomes are modified by gene gain and loss, by gene subfunctionalization and neofunctionalization, and by changes in protein targeting. To date, proteomics data for plastids and mitochondria are available for only a few plant model species, and evolutionary analyses of high-throughput data are scarce. We combined quantitative proteomics, cross-species comparative analysis of metabolic pathways, and localizations by fluorescent proteins in the model plant Physcomitrella patens in order to assess evolutionary changes in mitochondrial and plastid proteomes. This study implements data-mining methodology to classify and reliably reconstruct subcellular proteomes, to map metabolic pathways, and to study the effects of postendosymbiotic evolution on organellar pathway partitioning. Our results indicate that, although plant morphologies changed substantially during plant evolution, metabolic integration of organelles is largely conserved, with exceptions in amino acid and carbon metabolism. Retargeting or regulatory subfunctionalization are common in the studied nucleus-encoded gene families of organelle-targeted proteins. Moreover, complementing the proteomic analysis, fluorescent protein fusions revealed novel proteins at organelle interfaces such as plastid stromules (stroma-filled tubules) and highlight microcompartments as well as intercellular and intracellular heterogeneity of mitochondria and plastids. Thus, we establish a comprehensive data set for mitochondrial and plastid proteomes in moss, present a novel multilevel approach to organelle biology in plants, and place our findings into an evolutionary context.

Polar Expansion Dynamics in the Plant Kingdom: A Diverse and Multifunctional Journey on the Path to Pollen Tubes

Polar expansion is a widespread phenomenon in plants spanning all taxonomic groups from the Charophycean Green Algae to pollen tubes in Angiosperms and Gymnosperms. Current data strongly suggests that many common features are shared amongst cells displaying polar growth mechanics including changes to the structural features of localized regions of the cell wall, mobilization of targeted secretion mechanisms, employment of the actin cytoskeleton for directing secretion and in many cases, endocytosis and coordinated interaction of multiple signal transduction mechanisms prompted by external biotic and abiotic cues. The products of polar expansion perform diverse functions including delivery of male gametes to the egg, absorption, anchorage, adhesion and photo-absorption efficacy. A comparative analysis of polar expansion dynamics is provided with special emphasis on those found in early divergent plants.

Apical myosin XI anticipates F-actin during polarized growth of Physcomitrella patens cells

Tip growth is essential for land colonization by bryophytes, plant sexual reproduction and water and nutrient uptake. Because this specialized form of polarized cell growth requires both a dynamic actin cytoskeleton and active secretion, it has been proposed that the F-actin-associated motor myosin XI is essential for this process. Nevertheless, a spatial and temporal relationship between myosin XI and F-actin during tip growth is not known in any plant cell. Here, we use the highly polarized cells of the moss Physcomitrella patens to show that myosin XI and F-actin localize, in vivo, at the same apical domain and that both signals fluctuate. Surprisingly, phase analysis shows that increase in myosin XI anticipates that of F-actin; in contrast, myosin XI levels at the tip fluctuate in identical phase with a vesicle marker. Pharmacological analysis using a low concentration of the actin polymerization inhibitor latrunculin B showed that the F-actin at the tip can be significantly diminished while myosin XI remains elevated in this region, suggesting that a mechanism exists to cluster myosin XI-associated structures at the cell's apex. In addition, this approach uncovered a mechanism for actin polymerization-dependent motility in the moss cytoplasm, where myosin XI-associated structures seem to anticipate and organize the actin polymerization machinery. From our results, we inferred a model where the interaction between myosin XI-associated vesicular structures and F-actin polymerization-driven motility function at the cell's apex to maintain polarized cell growth. We hypothesize this is a general mechanism for the participation of myosin XI and F-actin in tip growing cells.

Growth mechanisms in tip-growing plant cells

Tip growth is employed throughout the plant kingdom. Our understanding of tip growth has benefited from modern tools in molecular genetics, which have enabled the functional characterization of proteins mediating tip growth. Here we first discuss the evolutionary role of tip growth in land plants and then describe the prominent model tip-growth systems, elaborating on some advantages and disadvantages of each. Next we review the organization of tip-growing cells, the role of the cytoskeleton, and recent developments concerning the physiological basis of tip growth. Finally, we review advances in the understanding of the extracellular signals that are known to guide tip-growing cells.

Physcomitrella patens: a model for tip cell growth and differentiation

The moss Physcomitrella patens has emerged as an excellent model system owing to its amenability to reverse genetics. The moss gametophyte has three filamentous tissues that grow by tip growth: chloronemata, caulonemata, and rhizoids. Because establishment of the moss plant relies on this form of growth, it is particularly suited for dissecting the molecular basis of tip growth. Recent studies demonstrate that a core set of actin cytoskeletal proteins is essential for tip growth. Additional actin cytoskeletal components are required for modulating growth to produce caulonemata and rhizoids. Differentiation into these cell types has previously been linked to auxin, light and nutrients. Recent studies have identified that core auxin signaling components as well as transcription factors that respond to auxin or nutrient levels are required for tip-growing cell differentiation. Future studies may establish a connection between the actin cytoskeleton and auxin or nutrient-induced cell differentiation.