Myosin XI Is Required for Actin-Associated Movement of Plastid Stromules

Abstract

Stromules are highly dynamic stroma-filled tubules extending from the surface of plastids and occasionally interconnecting individual plastids, allowing the movement of complex biological molecules between the interconnected plastids. Experiments with inhibitors of cytoskeleton assembly have indicated the involvement of an actin-based system in stromule movement. However, the motor protein associated with the system had not been identified. Here, we present direct evidence that myosin XI is involved in the formation and movement of stromules in tobacco leaves. Application of 2,3-butanedione 2-monoxime, an inhibitor of myosin ATPase activity, resulted in the loss of stromules from tobacco leaf epidermal cells. Transient RNA interference of myosin XI in leaves of Nicotiana benthamiana also resulted in the loss of stromules from epidermal cells, without any effect on transcripts for actin or myosin VIII. Transient expression of a GFP-tagged myosin XI tail domain in tobacco leaf epidermal cells showed that the fusion protein localized to the chloroplast envelope, as well as to mitochondria and other organelles. Our findings identify myosin XI as a key protein involved in the formation and movement of stromules.

Chloroplasts extend stromules independently and in response to internal redox signals

“…Previous reports have investigated stromules using a variety of plastid types in a broad range of species and tissues, generally assuming that stromules act similarly in all cells (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). In light of the clear differences in signals that influence leucoplast and chloroplast stromule formation (Figs.…”

“…With the finding that signals originating within the chloroplast can trigger stromule formation, we then explored whether stromule formation is dependent on cytosolic structures (e.g., the cytoskeleton), as has been suggested previously (10,15), or if chloroplasts can make stromules on their own. Previous studies have argued that stromule formation is guided and supported by the cytoskeleton and endoplasmic reticulum, but whether stromule formation requires these external factors is unknown (10,30).…”

“…Several studies have identified conditions that can induce or decrease stromule formation (14)(15)(16)(17)(18), concluding that stromule frequency can change in response to abiotic stress, phytohormone signaling, and massive disruption of cellular function (e.g., strong inhibition of cytosolic translation or of actin microfilament dynamics). Almost nothing is known about the genetics of stromules; some mutants with strong morphological defects in plastids, such as mutants with improper plastid division or lacking plastid mechanosensitive channels, cannot form stromules at normal frequencies, but these plastids are so severely misshapen that their stromule frequencies cannot be directly compared with wild-type plastids (19,20).…”

Class XI Myosins Are Required for Development, Cell Expansion, and F-Actin Organization in Arabidopsis

“…Important corollaries of myosin activity in structuring F-actin are concomitant contributions to the overall ER architecture (Figure 6), motility (Ueda et al, 2010), and intricate arrangement of the ER components (Sparkes et al, 2009a). It will be intriguing to see if the newly discovered role of myosins XI in plastid dynamics (Natesan et al, 2009;Sattarzadeh et al, 2009) involves myosin-driven F-actin reorganization, transport of the plastid stromules, or both.…”

“…These myosins were also implicated in endoplasmic reticulum (ER) streaming (Ueda et al, 2010) and remodeling (Sparkes et al, 2009a) as well as in plastid dynamics (Natesan et al, 2009;Sattarzadeh et al, 2009). In addition, some class XI myosins play a role in root hair elongation (Ojangu et al, 2007;Peremyslov et al, 2008;Prokhnevsky et al, 2008).…”