Chloroplasts extend stromules independently and in response to internal redox signals

Abstract

A fundamental mystery of plant cell biology is the occurrence of "stromules," stroma-filled tubular extensions from plastids (such as chloroplasts) that are universally observed in plants but whose functions are, in effect, completely unknown. One prevalent hypothesis is that stromules exchange signals or metabolites between plastids and other subcellular compartments, and that stromules are induced during stress. Until now, no signaling mechanisms originating within the plastid have been identified that regulate stromule activity, a critical missing link in this hypothesis. Using confocal and superresolution 3D microscopy, we have shown that stromules form in response to light-sensitive redox signals within the chloroplast. Stromule frequency increased during the day or after treatment with chemicals that produce reactive oxygen species specifically in the chloroplast. Silencing expression of the chloroplast NADPH-dependent thioredoxin reductase, a central hub in chloroplast redox signaling pathways, increased chloroplast stromule frequency, whereas silencing expression of nuclear genes related to plastid genome expression and tetrapyrrole biosynthesis had no impact on stromules. Leucoplasts, which are not photosynthetic, also made more stromules in the daytime. Leucoplasts did not respond to the same redox signaling pathway but instead increased stromule formation when exposed to sucrose, a major product of photosynthesis, although sucrose has no impact on chloroplast stromule frequency. Thus, different types of plastids make stromules in response to distinct signals. Finally, isolated chloroplasts could make stromules independently after extraction from the cytoplasm, suggesting that chloroplast-associated factors are sufficient to generate stromules. These discoveries demonstrate that chloroplasts are remarkably autonomous organelles that alter their stromule frequency in reaction to internal signal transduction pathways.chloroplasts | stromules | redox signaling | light signaling | leucoplasts

Polyprenols Are Synthesized by a Plastidial cis-Prenyltransferase and Influence Photosynthetic Performance

“…Considering that a significant proportion of plastidial polyprenols also reside in the chloroplast envelope, it is tempting to speculate that these compounds also influence envelope dynamics, in much the same way as their thylakoid counterparts. Processes that are critically altered by any change in envelope fluidity, such as plastid division (Osteryoung and Nunnari, 2003), metabolite exchange (Mehrshahi et al, 2013), or the sporadic extension and retraction of stroma filled tubules known as stromules (Schattat et al, 2012;Mathur et al, 2013;Brunkard et al, 2015), would conceivably be affected by the abundance of polyprenols. The latter process is particularly relevant in this context as it has long been speculated that stromule formation, for which the mechanism is unknown, requires modulation of plastid envelope membrane architecture (Thomson and Whatley, 1980;Kwok and Hanson, 2004).…”

Redox- and Reactive Oxygen Species-Dependent Signaling into and out of the Photosynthesizing Chloroplast

“…Interestingly, stromule formation was shown to depend on NADPH-dependent TRX reductase C, and silencing its expression doubled the rate of stromule formation. Isolated chloroplasts were able to form stromules, suggesting that stromules intrinsically emerge independent of cytosolic factors (Brunkard et al, 2015), but their emergence might be supported by interaction with microtubules (Kwok and Hanson, 2003). In addition to light and direct changes in redox state, which mimic abiotic stress conditions, stromule formation responds to temperature (Holzinger et al, 2007) and abscisic acid .…”

“…Their exact function remains elusive (Natesan et al, 2005). Recently, Brunkard et al (2015) demonstrated that in chloroplasts and leucoplasts, stromules form more frequently in the light than in darkness. Moreover, inhibitors of PET rapidly increase the frequency of stromule formation in chloroplasts, suggesting that redox plays a role in controlling stromule formation.…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…For instance, it has been suggested that chloroplasts extend stroma filled tubules called stromules in response to internal ROS accumulation following inhibition of the electron transport chain (pETC; Brunkard et al, 2015) and other plastids respond to increased sugar levels in a cell . Similarly, the rapid fission of tubular mitochondria increases their population in a cell in response to high cytosolic sugar content, high ROS levels and light stimuli (Yoshinaga et al, 2005;Yu et al, 2006Yu et al, , 2008Jhun et al, 2013;Jaipargas et al, 2015).…”