Disrupting Autophagy Restores Peroxisome Function to an Arabidopsis lon2 Mutant and Reveals a Role for the LON2 Protease in Peroxisomal Matrix Protein Degradation

Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005.
The Plant Cell (Impact Factor: 9.34). 10/2013; 25(10). DOI: 10.1105/tpc.113.113407
Source: PubMed


Peroxisomes house critical metabolic reactions that are essential for seedling development. As seedlings mature, metabolic requirements change, and peroxisomal contents are remodeled. The resident peroxisomal protease LON2 is positioned to degrade obsolete or damaged peroxisomal proteins, but data supporting such a role in plants have remained elusive. Arabidopsis thaliana lon2 mutants display defects in peroxisomal metabolism and matrix protein import but appear to degrade matrix proteins normally. To elucidate LON2 functions, we executed a forward-genetic screen for lon2 suppressors, which revealed multiple mutations in key autophagy genes. Disabling core autophagy-related gene (ATG) products prevents autophagy, a process through which cytosolic constituents, including organelles, can be targeted for vacuolar degradation. We found that atg2, atg3, and atg7 mutations suppressed lon2 defects in auxin metabolism and matrix protein processing and rescued the abnormally large size and small number of lon2 peroxisomes. Moreover, analysis of lon2 atg mutants uncovered an apparent role for LON2 in matrix protein turnover. Our data suggest that LON2 facilitates matrix protein degradation during peroxisome content remodeling, provide evidence for the existence of pexophagy in plants, and indicate that peroxisome destruction via autophagy is enhanced when LON2 is absent.

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    • "In peroxisomes from yeast, fungi and mammal cells, LON proteases have been shown to be capable of selectively degrading oxidatively damaged peroxisomal matrix proteins and may act as a key player in the coordination of ROS metabolism in peroxisomes (Kumar et al., 2014; Nordgren and Fransen, 2014). A genetic screen for suppressors of the lon2 mutant has demonstrated that the degradation of proteins from the glyoxylate cycle during the functional transition of glyoxysomes to leaf peroxisomes, requiring not only LON2 but also autophagy, is involved in this process (Farmer et al., 2013; Shibata et al., 2014). "
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    ABSTRACT: Peroxisomes are highly dynamic, metabolically active organelles that used to be regarded as a sink for H2O2 generated in different organelles. However, peroxisomes are now considered to have a more complex function, containing different metabolic pathways, and they are an important source of reactive oxygen species (ROS), nitric oxide (NO) and reactive nitrogen species (RNS). Over-accumulation of ROS and RNS can give rise oxidative and nitrosative stress, but when produced at low concentrations they can act as signalling molecules. This review focuses on the production of ROS and RNS in peroxisomes and their regulation by antioxidants. ROS production is associated with metabolic pathways such as photorespiration and fatty acid β-oxidation, and disturbances in any of these processes can be perceived by the cell as an alarm that triggers defence responses. Genetic and pharmacological studies have shown that photorespiratory H2O2 can affect nuclear gene expression, regulating the response to pathogen infection and light intensity. Proteomic studies have shown that peroxisomal proteins are targets for oxidative modification, S-nitrosylation and nitration and have highlighted the importance of these modifications in regulating peroxisomal metabolism and signalling networks. The morphology, size, number and speed of movement of peroxisomes can also change in response to oxidative stress, meaning that an ROS/redox receptor is required. Information available on the production and detection of NO/RNS in peroxisomes is more limited. Peroxisomal homeostasis is critical for maintaining the cellular redox balance and is regulated by ROS, peroxisomal proteases and autophagic processes. Peroxisomes play a key role in many aspects of plant development and acclimation to stress conditions. These organelles can sense ROS/redox changes in the cell and thus trigger rapid and specific responses to environmental cues involving changes in peroxisomal dynamics as well as ROS- and NO-dependent signalling networks, although the mechanisms involved have not yet been established. Peroxisomes can therefore be regarded as a highly important decision-making platform in the cell, where ROS and RNS play a determining role. © The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email:
    Annals of Botany 06/2015; DOI:10.1093/aob/mcv074 · 3.65 Impact Factor
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    • "Autophagy in plants has also been shown to function during abiotic stresses such as salt, drought, and oxidative stress (Xiong et al., 2007; Liu et al., 2009) as well as in response to pathogens (Liu and Bassham, 2012). In addition, selective autophagic degradation of chloroplasts has been demonstrated (Ishida et al., 2014), and evidence of peroxisome and starch degradation by autophagy has been recently provided (Farmer et al., 2013; Kim et al., 2013; Wang et al., 2013; Avin-Wittenberg and Fernie, 2014). Although hypersensitivity to nutrient starvation is a major phenotype of atg mutants, only very few recent studies have attempted to uncover the metabolic implications underlying it (Guiboileau et al., 2012; Izumi et al., 2013; Masclaux-Daubresse et al., 2014), and although we are making great strides in our understanding of the metabolic impact of autophagy in plants, many questions still remain open. "
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    ABSTRACT: Germination and early seedling establishment are developmental stages in which plants face limited nutrient supply as their photosynthesis mechanism is not yet active. For this reason, the plant must mobilize the nutrient reserves provided by the mother plant in order to facilitate growth. Autophagy is a catabolic process enabling the bulk degradation of cellular constituents in the vacuole. The autophagy mechanism is conserved among eukaryotes, and homologs of many autophagy-related (ATG) genes have been found in Arabidopsis thaliana. T-DNA insertion mutants (atg mutants) of these genes display higher sensitivity to various stresses, particularly nutrient starvation. However, the direct impact of autophagy on cellular metabolism has not been well studied. In this work, we used etiolated Arabidopsis seedlings as a model system for carbon starvation. atg mutant seedlings display delayed growth in response to carbon starvation compared with wild-type seedlings. High-throughput metabolomic, lipidomic, and proteomic analyses were performed, as well as extensive flux analyses, in order to decipher the underlying causes of the phenotype. Significant differences between atg mutants and wild-type plants have been demonstrated, suggesting global effects of autophagy on central metabolism during carbon starvation as well as severe energy deprivation, resulting in a morphological phenotype. © 2015 American Society of Plant Biologists. All rights reserved.
    The Plant Cell 02/2015; 27(2). DOI:10.1105/tpc.114.134205 · 9.34 Impact Factor
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    • "Glyoxysomes can transform into leaf glyoxysomes by specifically removing malate synthase and isocitrate lyase, two peroxisomal enzymes of the glyoxylate cycle, through PexAD, concomitant with the import of enzymes required for photorespiration [40]. Furthermore, several groups have recently shown that this organelle remodeling process can be accelerated by selectively degrading oxidized glyoxysomes when the LON2 protease is disabled [39] [57]. However, it is still unknown whether LON2 regulates pexophagy directly or indirectly . "
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    ABSTRACT: Peroxisomes are ubiquitous organelles present in nearly all eukaryotic cells. Conserved functions of peroxisomes encompass beta-oxidation of fatty acids and scavenging of reactive oxygen species generated from diverse peroxisomal metabolic pathways. Peroxisome content, number, and size can change quickly in response to environmental and/or developmental cues. To achieve efficient peroxisome homeostasis, peroxisome biogenesis and degradation must be orchestrated. We review the current knowledge on redox regulated peroxisome biogenesis and degradation with an emphasis on yeasts and plants.
    12/2014; 24. DOI:10.1016/j.redox.2014.12.006
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