Abstract
Plants possess two myosin classes, VIII and XI. The myosins XI are implicated in organelle transport, filamentous actin organization, and cell and plant growth. Due to the large size of myosin gene families, knowledge of these molecular motors remains patchy. Using deep transcriptome sequencing and bioinformatics, we systematically investigated myosin genes in two model plants, Arabidopsis (Arabidopsis thaliana) and Brachypodium (Brachypodium distachyon). We improved myosin gene models and found that myosin genes undergo alternative splicing. We experimentally validated the gene models for Arabidopsis myosin XI-K, which plays the principal role in cell interior dynamics, as well as for its Brachypodium ortholog. We showed that the Arabidopsis gene dubbed HDK (for headless derivative of myosin XI-K), which emerged through a partial duplication of the XI-K gene, is developmentally regulated. A gene with similar architecture was also found in Brachypodium. Our analyses revealed two predominant patterns of myosin gene expression, namely pollen/stamen-specific and ubiquitous expression throughout the plant. We also found that several myosins XI can be rhythmically expressed. Phylogenetic reconstructions indicate that the last common ancestor of the angiosperms possessed two myosins VIII and five myosins XI, many of which underwent additional lineage-specific duplications.
Highlights
Plants possess two myosin classes, VIII and XI
With several new plant genomes available, we are in a position to validate the classification of myosins proposed previously (Bezanilla et al, 2003; Avisar et al, 2008b) and potentially gain new insights into the evolution of myosins among diverse land plants and algae
The two algal myosins from Acetabularia peniculus that cluster with class XI myosins were classified as a separate class XIII (Foth et al, 2006), but our phylogenetic analysis shows that there is no need for this additional class
Summary
Plants possess two myosin classes, VIII and XI. The myosins XI are implicated in organelle transport, filamentous actin organization, and cell and plant growth. The most recent work on the triple and quadruple myosin XI gene knockouts highlighted critical contributions of the highly expressed myosins XI-K, XI-1, XI-2, and XI-I to both diffuse and polarized cell expansion as well as to plant growth and development (Peremyslov et al, 2010). Inactivation of these myosins resulted in stunted plants, delayed flowering, and dramatic reductions in cell sizes, up to 10-fold in the case of root hairs. A recent study of the two myosins XI in the moss P. patens revealed functions in polarized cell growth and F-actin organization that are analogous to those of Arabidopsis myosins XI (Vidali et al, 2010)
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