Transorganellar complementation redefines the biochemical continuity of endoplasmic reticulum and chloroplasts

Abstract

Tocopherols are nonpolar compounds synthesized and localized in plastids but whose genetic elimination specifically impacts fatty acid desaturation in the endoplasmic reticulum (ER), suggesting a direct interaction with ER-resident enzymes. To functionally probe for such interactions, we developed transorganellar complementation, where mutated pathway activities in one organelle are experimentally tested for substrate accessibility and complementation by active enzymes retargeted to a companion organelle. Mutations disrupting three plastid-resident activities in tocopherol and carotenoid synthesis were complemented from the ER in this fashion, demonstrating transorganellar access to at least seven nonpolar, plastid envelope-localized substrates from the lumen of the ER, likely through plastid:ER membrane interaction domains. The ability of enzymes in either organelle to access shared, nonpolar plastid metabolite pools redefines our understanding of the biochemical continuity of the ER and chloroplast with profound implications for the integration and regulation of organelle-spanning pathways that synthesize nonpolar metabolites in plants.n addition to meeting cellular energy needs through photosynthesis, chloroplasts are centers of anabolic metabolism that contain complete biosynthetic pathways (e.g., for de novo synthesis of fatty acids, amino acids, tocopherols, and carotenoids) and participate in numerous pathways that span multiple subcellular compartments (e.g., for synthesis of membrane lipids, monoterpenes, diterpenes, and photorespiration). Such metabolism requires exchange of a multitude of polar and nonpolar metabolites with the extraplastidic environment and consistent with this, proteomic and bioinformatic analysis of the chloroplast envelope identified 102 transporter candidates (Dataset S1). Sixty-six have recognized functions as ion or metabolite transporters, but only one transports nonpolar metabolites. This apparent paucity of nonpolar metabolite transporters in the envelope, despite the large numbers of nonpolar metabolites synthesized by plastids, highlights a significant gap in our understanding of plant metabolism.Tocochromanols are one well-studied group of nonpolar compounds synthesized and localized in plastids that include the biosynthetically related tocopherols, tocotrienols, and plastochromanol-8 (PC8) (Fig. 1). Tocochromanol biosynthesis has been fully elucidated, null mutants with well-defined biochemical phenotypes are available for each reaction, and with the exception of p-hydroxyphenylpyruvate dioxygenase, all biosynthetic activities localize to the plastid inner envelope where synthesis occurs (1, 2). Because tocochromanols are only present in chloroplast membranes (1, 3), it was assumed that their functions would be restricted to this organelle; however, many tocopheroldeficient mutant phenotypes are, instead, consistent with impacts on extraplastidic processes. These include alterations in membrane lipids, formation of secretory pathway-derived vesicles, cell-wall developmen...

Synthesis and transfer of galactolipids in the chloroplast envelope membranes of Arabidopsis thaliana

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The endoplasmic reticulum exerts control over organelle streaming during cell expansion

“…Furthermore, attachment of ER to chloroplasts and Golgi stacks have been demonstrated through experiments based on laser optical tweezers showing that ER tubules follow chloroplasts or Golgi stacks that are pulled by the tweezers (Andersson et al, 2007;Hawes et al, 2010;Sparkes et al, 2009a). Although the nature of the proteins that enable the connections of the ER with other organelles is yet unknown, it has been hypothesized that the ER can contact chloroplasts through hemifused membrane domains (Mehrshahi et al, 2013). Therefore, it is possible that the ER contributes to the movement of the other organelles through heterotypic membrane connections that depend on contacts between proteins or lipid bilayers.…”

Flavonoids in plant rhizospheres: secretion, fate and their effects on biological communication

“…These different localizations of successive biosynthetic enzymes suggest possible intracellular movement of intermediates between the ER and plastids (Figure 3), a movement that may also occur during the synthesis of sophoraflavanone G in S. flavescens (Sasaki et al 2008;Yamamoto et al 2000Yamamoto et al , 2001. Recently, using mutants in tocopherol biosynthesis and the transorganellar complementation approach, in which plastid-localized enzymes were retargeted to the ER, nonpolar substrates in plastids were found to be accessed by ER-localized enzymes (Mehrshahi et al 2013). These observations suggest interorganellar metabolic pathways, with a variety of compounds including pterocarpan and prenylated flavonoids present in one organelle modifiable by enzymes localized to another organelle.…”