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

Update on Myosin Motors: Molecular Mechanisms and Physiological Functions

“…In the case of myosin XI, it became clear from mutant analysis in Arabidopsis that the activity of at least some isoforms is necessary for cell expansion. This was most evident in root hairs and pollen tubes that depend heavily on the delivery of secretory vesicles to the growing tip of these cells (Ojangu et al, 2007;Peremyslov et al, 2008;Park and Nebenführ, 2013;Madison et al, 2015). Combinations of several myosin XI mutants resulted in broader effects and progressively smaller plants that were made up of smaller cells in a wide range of tissues Peremyslov et al, 2010;Ojangu et al, 2012), suggesting the largely redundant function of these myosins in cell expansion.…”

“…Two candidates for this mechanism are the active delivery of secretory vesicles to the cell surface to directly support growth (Crowell et al, 2010) and the general mixing of cytoplasm, which is assumed to stimulate metabolism (Shimmen and Yokota, 1994). In addition, it is possible that the effect of myosin on growth is more indirect, for example by affecting actin filament dynamics (Park and Nebenführ, 2013;Cai et al, 2014) and organization (Peremyslov et al, 2010;Ueda et al, 2010;Madison et al, 2015). While there seems to be some experimental support for the first model (Cramer and Mitchison, 1995;Gutierrez et al, 2009;Sampathkumar et al, 2013), the second remains largely unexplored.…”

“…While the plastid movements are much slower in the myosin mutant (Okamoto et al, 2015), we do not know whether the organization of actin filaments also is affected. This latter point is of particular importance, since we know that myosins can affect actin organization (Peremyslov et al, 2010;Ueda et al, 2010;Madison et al, 2015) and since fiz1 mutants that are defective in ACTIN8 also show similar defects in straightening behavior (Okamoto et al, 2015). More fundamentally, it is not clear at this time whether these two myosins function in sensing the growth curvature, possibly via the actin cytoskeleton, or whether the mutant phenotype results from a defect in the response pathway after detection of the growth curvature.…”

“…Fungal pathogens are known to trigger reorganization of the actin cytoskeleton as well as the accumulation of several organelles in the area of the penetration site (Takemoto et al, 2003). Since myosins can increase actin dynamics (Park and Nebenführ, 2013;Cai et al, 2014) and affect actin organization (Peremyslov et al, 2010;Ueda et al, 2010;Madison et al, 2015), it is reasonable to assume that this actin rearrangement upon fungal attack is compromised in the myosin mutants. Taken together, these observations suggest that similar mechanisms may be involved in fungal pathogen resistance to those in cellular expansion (see above).…”

“…Myosin XI is similar to myosin V in animals (Kinkema and Schiefelbein, 1994) due to the length of the neck region, which allows for 35-nm steps along actin filaments (Tominaga et al, 2003), and the DIL domain-containing globular tail, which functions in cargo binding (Li and Nebenführ, 2007). Myosin XI has been identified as the motor that propels organelles during cytoplasmic streaming that can reach speeds of over 10 mm s 21 (Peremyslov et al, , 2010Prokhnevsky et al, 2008;Ueda et al, 2010;Madison et al, 2015). In angiosperms, the myosin XI motor family has evolved into up to eight subtypes (Myo11A-Myo11H; see Text Box 1) with a variable number of isoforms per group (Mühlhausen and Kollmar, 2013).…”