Abstract
A prominent feature of plant cells is the rapid, incessant movement of the organelles traditionally defined as cytoplasmic streaming and attributed to actomyosin motility. We sequenced six complete Nicotiana benthamiana cDNAs that encode class XI and class VIII myosins. Phylogenetic analysis indicates that these two classes of myosins diverged prior to the radiation of green algae and land plants from a common ancestor and that the common ancestor of land plants likely possessed at least seven myosins. We further report here that movement of Golgi stacks, mitochondria, and peroxisomes in the leaf cells of N. benthamiana is mediated mainly by myosin XI-K. Suppression of myosin XI-K function using dominant negative inhibition or RNA interference dramatically reduced movement of each of these organelles. When similar approaches were used to inhibit functions of myosin XI-2 or XI-F, only moderate to marginal effects were observed. Organelle trafficking was virtually unaffected in response to inhibition of each of the three class VIII myosins. Interestingly, none of the tested six myosins appears to be involved in light-induced movements of chloroplasts. Taken together, these data strongly suggest that myosin XI-K has a major role in trafficking of Golgi stacks, mitochondria, and peroxisomes, whereas myosins XI-2 and XI-F might perform accessory functions in this process. In addition, our analysis of thousands of individual organelles revealed independent movement patterns for Golgi stacks, mitochondria, and peroxisomes, indicating that the notion of coordinated cytoplasmic streaming is not generally applicable to higher plants.
Highlights
A prominent feature of plant cells is the rapid, incessant movement of the organelles traditionally defined as cytoplasmic streaming and attributed to actomyosin motility
Using overexpression of dominant negative myosin mutants and RNA interference (RNAi), we show that the class XI myosin XI-K plays a major role in the movement of Golgi stacks, mitochondria, and peroxisomes in the leaf cells
Two of the three myosins of C. reinhardtii and both myosins of O. lucimarinus belong to class XI, whereas the third myosin of C. reinhardtii belongs to class VIII (Fig. 1A) and, to class VIII myosins of land plants, lacks the dilute domain that is present in all class V, XI, and XIII myosins (Fig. 1B)
Summary
A prominent feature of plant cells is the rapid, incessant movement of the organelles traditionally defined as cytoplasmic streaming and attributed to actomyosin motility. Plant cells are less motile than animal cells, they exhibit extensive intracellular dynamics that involve repositioning of the nucleus, reshaping of the vacuole, and rapid trafficking of the Golgi stacks, mitochondria, and peroxisomes (Shimmen and Yokota, 2004; Wada and Suetsugu, 2004; Matheson et al, 2006) This trafficking was originally characterized in filamentous algae where the continuous directional flow of cytoplasm is believed to carry. Class V myosins are processive molecular motors that function in organelle and vesicle transport and partitioning during cell division, mitotic spindle positioning, localization of mRNA, and establishment of cell polarity (Pruyne et al, 2004; Sellers and Veigel, 2006; Desnos et al, 2007) These diverse functions that require interactions with multiple cargoes can be performed by a single class V myosin, such as yeast (Saccharomyces cerevisiae) Myo2p (Pashkova et al, 2006). Each of these subdomains contains receptors that recognize distinct cargoes
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