Stromules are stroma-filled tubules extending from plastids whose rapid extension toward or retraction from other plastids has suggested a role in interplastidic communication and exchange of metabolites. Several studies point to sporadic dilations, kinks, and branches occurring along stromule length but have not elucidated the underlying basis for these occurrences. Similarly, although specific details on interacting partners have been missing, a consensus viewpoint suggests that stromules increase the interactive surface of a plastid with its cytoplasmic surroundings. Here, using live imaging, we show that the behavior of dynamic, pleomorphic stromules strongly coincides with that of cortical endoplasmic reticulum (ER) tubules. Covisualization of fluorescent protein-highlighted stromules and the ER in diverse cell types clearly suggests correlative dynamics of the two membrane-bound compartments. The extension and retraction, as well as directional changes in stromule branches occur in tandem with the behavior of neighboring ER tubules. Three-dimensional and four-dimensional volume rendering reveals that stromules that extend into cortical regions occupy channels between ER tubules possibly through multiple membrane contact sites. Our observations clearly depict coincidental stromule-ER behavior and suggest that either the neighboring ER tubules shape stromules directly or the behavior of both ER and stromules is simultaneously dictated by a shared cytoskeleton-based mechanism. These new observations strongly implicate the ER membrane in interactions with stromules and suggest that their interacting surfaces might serve as major conduits for bidirectional exchange of ions, lipids, and metabolites between the two organelles.

Summary Plant cells harbor two types of endosymbiotic organelle: mitochondria and chloroplasts. As a consequence of endosymbiotic gene transfer, the majority of their proteins are encoded in the nucleus and post‐translationally ‘re’‐imported into the respective target organelle. The corresponding transport signals are usually selective for a single organelle, but several proteins are transported into both the mitochondria and chloroplasts. To estimate the number of proteins with such dual targeting properties in Arabidopsis, we classified the proteins encoded by nuclear genes of endosymbiotic origin according to the respective targeting specificity of their N‐terminal transport signals as predicted by the TargetP software package. Selected examples of the resulting protein classes were subsequently analyzed by transient transformation assays as well as by in organello protein transport experiments. It was found that most proteins with high prediction values for both organelles show dual targeting with both experimental approaches. Unexpectedly, however, dual targeting was even found among those proteins that are predicted to be localized solely in one of the two endosymbiotic organelles. In total, among the 16 candidate proteins analyzed, we identified 10 proteins with dual targeting properties. This unexpectedly high proportion suggests that such transport properties are much more abundant than anticipated.

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Here, we present data showing the directed degradation of target proteins recognized by a specific nanobody in transgenic plants. Green fluorescent protein was depleted by a chimeric nanobody fused to a distinct F-box domain, which enables protein degradation via the ubiquitin proteasome pathway. This technique could thus be used to knock out other proteins of interest in planta using specific, high-affinity binding proteins.

As a result of the endosymbiotic gene transfer, the majority of proteins of mitochondria and chloroplasts is encoded in the nucleus and synthesized in the cytosol as precursor molecules carrying N-terminal transit peptides for the transport into the respective target organelle. In most instances, transport takes place into either mitochondria or chloroplasts, although a few examples of dual targeting into both organelles have been described. Here, we show by a combination of three different experimental strategies that also cytochrome c(1) of potato, a component of the respiratory electron transport chain, is imported not only into mitochondria, but also into plastids. In organello import experiments with isolated mitochondria and chloroplasts, which were analyzed in both single and mixed organelle assays, demonstrate that the processing products accumulating after import within the two endosymbiotic organelles are different in size. Dual targeting of cytochrome c(1) is observed also in vivo, after biolistic transformation of leaf epidermal cells with suitable reporter constructions. Finally, Western analyses employing cytochrome c(1)-specific antiserum provide evidence that the protein accumulates in significant amounts in mitochondria and chloroplasts of both pea and spinach. The possible consequences of our findings on the relevance of the dual targeting phenomenon are discussed.

As a result of the endosymbiotic gene transfer, the majority of proteins of mitochondria and chloroplasts are encoded in the nucleus and synthesized in the cytosol as precursor proteins carrying N-terminal transport signals for the 're-import' into the respective target organelle. Most of these transport signals are monospecific, although some of them have dual targeting properties, that is, they are recognized both by mitochondria and by chloroplasts as target organelles. We have identified alpha-MPP2, one of the two isoforms of the substrate binding subunit of mitochondrial processing peptidase of Arabidopsis thaliana, as a novel member of this class of nuclear-encoded organelle proteins. As demonstrated by in organello transport experiments with isolated organelles and by in vivo localization studies employing fluorescent chimeric reporter proteins, the N-terminal region of the alpha-MPP2 precursor comprises transport signals for the import into mitochondria as well as into chloroplasts. Both signals are found within the N-terminal 79 residues of the precursor protein, where they occupy partly separated and partly overlapping regions. Deletion mapping combined with in organello and in vivo protein transport studies demonstrate an unusual architecture of this transport signal, suggesting a composition of three functionally separated domains.

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