Myosin-dependent endoplasmic reticulum motility and F-actin organization in plant cells

Abstract

Plants exhibit an ultimate case of the intracellular motility involving rapid organelle trafficking and continuous streaming of the endoplasmic reticulum (ER). Although it was long assumed that the ER dynamics is actomyosin-driven, the responsible myosins were not identified, and the ER streaming was not characterized quantitatively. Here we developed software to generate a detailed velocity-distribution map for the GFP-labeled ER. This map revealed that the ER in the most peripheral plane was relatively static, whereas the ER in the inner plane was rapidly streaming with the velocities of up to ∼3.5 μm/sec. Similar patterns were observed when the cytosolic GFP was used to evaluate the cytoplasmic streaming. Using gene knockouts, we demonstrate that the ER dynamics is driven primarily by the ER-associated myosin XI-K, a member of a plant-specific myosin class XI. Furthermore, we show that the myosin XI deficiency affects organization of the ER network and orientation of the actin filament bundles. Collectively, our findings suggest a model whereby dynamic three-way interactions between ER, F-actin, and myosins determine the architecture and movement patterns of the ER strands, and cause cytosol hauling traditionally defined as cytoplasmic streaming.myosin XI | actin filament | cytoplasmic streaming | velocity distribution map | Arabidopsis thaliana C ytoplasmic streaming, defined as an extensive intracellular motility in plants, was first described in 1774 (1). It is thought that unidirectional actin filament (AF) bundles and organelleassociated myosin XI, a plant-specific class of myosin motors, cause bulk flow in the cell (reviewed in refs. 2-4). Some myosin XI were indeed reported to slide along AFs in vitro (5, 6). Using immunocytochemical analyses (7-11) and fluorescent proteinlabeled myosins (12, 13), myosin XI have been shown to be associated with the particulate organelles. Recent analyses using gene knockouts and dominant-negative inhibition demonstrated that several class XI myosins have overlapping functions in the rapid movement of organelles (14-18). Among these, myosin XI-K was found to play the most prominent role in the movement of Golgi bodies, peroxisomes, and mitochondria. Studies have also concluded that none of these three organelles fits the paradigm of cytoplasmic streaming, raising the question of what drives this conspicuous process (14, 15).The endoplasmic reticulum (ER), an organelle present in all eukaryotic cells, harbors the largest reservoir of cellular membranes. Cytoskeleton-dependent remodeling of the ER network, which consists of cisternae and interconnected membrane tubes, is well known in both animals and plants (reviewed in refs. 19-22). In addition, dynamic streaming of the plant ER has been observed in subperipheral cytoplasm and transvacuolar strands (23-25). However, the molecular mechanism underlying the ER streaming is not known, although it was reported that the 175 kDa myosin XI was localized on the ER of tobacco cell culture BY-2 (26).Unlike the movement of the part...

Myosins XI modulate host cellular responses and penetration resistance to fungal pathogens

“…These results indicated that each of the four tested myosin genes provides functionally redundant contributions to penetration resistance with myosin XI-K playing a predominant role. Interestingly, myosin XI-K also plays leading roles in transport of various organelles and in cell growth, whereas other highly expressed myosins XI provide redundant contributions to these processes (24,25). The low penetration rates of the nonadapted Bgh in Col-0 were accompanied by abortive infection with development of unilateral haustoria that were completely encased by extended papilla.…”

“…Myosins are molecular motors responsible for AF-based motility (20). Recently, plant class XI myosins were implicated in the organization of actin cytoskeleton, organelle and vesicle transport, cell expansion, and plant growth (21)(22)(23)(24)(25)(26)(27). Although none of the individual myosin gene knockouts produces plant growth defects (22), progressive elimination of two to four highly expressed myosins results in concomitant reduction in cell and plant size (23,24).…”

The Endoplasmic Reticulum Is a Reservoir for WAVE/SCAR Regulatory Complex Signaling in the Arabidopsis Leaf

“…1C). In leaf microsomes, vesicles that are positive for ER marker proteins are relatively dense and are distributed near the bottom of the gradient (Ueda et al, 2010;Zhang et al, 2010). The NAP1-positive membranes overlapped to the greatest degree with the subset of dense membranes that contained known ER marker proteins: the ER-specific GEF SEC12 and the luminal ER protein PDI (Fig.…”

“…in leaf epidermal cells are quite distinct (Qiu et al, 2002;Saint-Jore et al, 2002;Djakovic et al, 2006;Ueda et al, 2010). Therefore, it seemed unlikely that the general ERassociated pool of NAP1 was active in terms of localized ARP2/3 activation.…”

Rice Morphology Determinant-Mediated Actin Filament Organization Contributes to Pollen Tube Growth

“…F-actin is believed to provide tracks for intracellular trafficking events, including cytoplasmic streaming, which also is defined as the coordinated flow of cytosol that carries small organelles and vesicles (Avisar et al, 2008;Ueda et al, 2010;Bloch et al, 2016). Therefore, the disorganization of actin filaments in rmd pollen tubes prompted us to speculate that RMD may affect cytoplasmic streaming.…”