Myosin XI Is Essential for Tip Growth inPhyscomitrella patens 

Abstract

Class XI myosins are plant specific and responsible for cytoplasmic streaming. Because of the large number of myosin XI genes in angiosperms, it has been difficult to determine their precise role, particularly with respect to tip growth. The moss Physcomitrella patens provides an ideal system to study myosin XI function. P. patens has only two myosin XI genes, and these genes encode proteins that are 94% identical to each other. To determine their role in tip growth, we used RNA interference to specifically silence each myosin XI gene using 59 untranslated region sequences. We discovered that the two myosin XI genes are functionally redundant, since silencing of either gene does not affect growth or polarity. However, simultaneous silencing of both myosin XIs results in severely stunted plants composed of small rounded cells. Although similar to the phenotype resulting from silencing of other actin-associated proteins, we show that this phenotype is not due to altered actin dynamics. Consistent with a role in tip growth, we show that a functional, full-length fusion of monomeric enhanced green fluorescent protein (mEGFP) to myosin XI accumulates at a subcortical, apical region of actively growing protonemal cells.

Actin Interacting Protein1 and Actin Depolymerizing Factor Drive Rapid Actin Dynamics inPhyscomitrella patens

“…The NLS4 LA line transformed with ADF-RNAi had the highest correlation coefficient, and these values were similar to those observed when treating control plants with the F-actin stabilizing drug jasplakinolide (Vidali et al, 2010). The NLS4 LA Daip1 line showed intermediate correlation coefficient values with respect to the control and the ADF-RNAi transformed plants.…”

“…The apical enrichment is dynamic both in intensity and position (Vidali et al, 2009a). Unlike other actin binding proteins, such as class II formins (Vidali et al, 2009c) and myosin XI (Vidali et al, 2010), AIP1-mCherry does not become enriched at the cell apex ( Figure 4A). Instead, AIP1-mCherry remains diffusely cytosolic.…”

“…We quantified the changes in F-actin organization in these time series using a previously described algorithm (Vidali et al, 2010) that calculates the correlation of the intensity of the LA signal at all pixel locations between time points. Dynamic F-actin has a low correlation coefficient with subsequent time points.…”

“…It is tempting to suggest that these actin binding proteins are important for tip growth by promoting the reorganization of F-actin. However, a recent loss-of-function study demonstrated that myosin XI is critical for tip growth but does not alter actin dynamics (Vidali et al, 2010). Therefore, it is imperative to quantify the effects of actin binding protein loss of function on actin dynamics to understand the mechanism by which these proteins function.…”

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

“…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).…”