The actin cytoskeleton plays a role in mobility of many different organelles in plant cells, including chloroplasts, mitochondria, Golgi, and peroxisomes. While progress has been made in identifying the myosin motors involved in trafficking of various plant organelles, not all of the cargoes mobilized by different members of the myosin XI family have yet been identified. The involvement of myosins in chloroplast positioning and mitochondrial movement was demonstrated by expression of a virus-induced gene silencing (VIGS) construct in tobacco. When VIGS with two different conserved sequences from a myosin XI motor was performed in plants with either GFP-labeled plastids or mitochondria, chloroplast positioning in the dark was abnormal, and mitochondrial movement ceased. Because these and prior observations have implicated a role for myosins and the actin cytoskeleton in plastid and stromule movement, we searched for myosin tail domains that could associate with plastids and stromules. While a yellow fluorescent protein (YFP) fusion with the entire tail region of myosin XI-F was usually found only in the cytoplasm, we observed that an Arabidopsis or Nicotiana benthamiana YFP::myosin XI-F tail domain homologous to the yeast myo2p vacuole-binding domain associated with plastids and stromules after transient expression in N. benthamiana. Taken together, these observations implicate myosin motor proteins in dynamics of plastids and stromules.

SummaryPlastid number and morphology vary dramatically between cell types and at different developmental stages. Furthermore, in C4 plants such as maize, chloroplast ultrastructure and biochemical functions are specialized in mesophyll and bundle sheath cells, which differentiate acropetally from the proplastid form in the leaf base. To develop visible markers for maize plastids, we have created a series of stable transgenics expressing fluorescent proteins fused to either the maize ubiquitin promoter, the mesophyll-specific phosphoenolpyruvate carboxylase (PepC) promoter, or the bundle sheath-specific Rubisco small subunit 1 (RbcS) promoter. Multiple independent events were examined and revealed that maize codon-optimized versions of YFP and GFP were particularly well expressed, and that expression was stably inherited. Plants carrying PepC promoter constructs exhibit YFP expression in mesophyll plastids and the RbcS promoter mediated expression in bundle sheath plastids. The PepC and RbcS promoter fusions also proved useful for identifying plastids in organs such as epidermis, silks, roots and trichomes. These tools will inform future plastid-related studies of wild-type and mutant maize plants and provide material from which different plastid types may be isolated.

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