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Autophagy Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/kaup20
Plant cell remodeling by autophagy a
a
a
Jimi Kim , Han Nim Lee & Taijoon Chung a
Department of Biological Sciences; Pusan National University; Busan, Korea Published online: 31 Jan 2014.
Click for updates To cite this article: Jimi Kim, Han Nim Lee & Taijoon Chung (2014) Plant cell remodeling by autophagy, Autophagy, 10:4, 702-703, DOI: 10.4161/auto.27953 To link to this article: http://dx.doi.org/10.4161/auto.27953
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Autophagic Punctum Autophagic Punctum
Autophagy 10:4, 702–703; April 2014; © 2014 Landes Bioscience
Plant cell remodeling by autophagy Jimi Kim, Han Nim Lee, and Taijoon Chung*
Downloaded by [University of Auckland Library] at 02:28 13 February 2015
Department of Biological Sciences; Pusan National University; Busan, Korea
P
lant seedlings are not photoautotrophs until they are equipped with photosynthetic machinery. Some plant cells are remodeled after being exposed to light, and a group of peroxisomal proteins are degraded during the remodeling. Autophagy was proposed as one of the mechanisms for the degradation of peroxisomal proteins. We recently showed that ATG7-dependent autophagy is partially responsible for the degradation of obsolete peroxisomal proteins during Arabidopsis seedling growth.
Keywords: pexophagy, degradation, hypocotyl, autophagosome, ATG, autophagy-related Abbreviations: ATG, autophagy-related; GFP, green fluorescent protein; ICL, isocitrate lyase; LP, leaf peroxisome; MLS, malate synthase; SP, seedling peroxisome; TP, transitional peroxisome Submitted: 01/14/2014 Revised: 01/21/2014 Accepted: 01/22/2014 http://dx.doi.org/10.4161/auto.27953 *Correspondence to: Taijoon Chung; Email: [email protected] Punctum to: Kim J, Lee H, Lee HN, Kim SH, Shin KD, Chung T. Autophagy-related proteins are required for degradation of peroxisomes in Arabidopsis hypocotyls during seedling growth. Plant Cell 2013; 25:4956–66; PMID:24368791; http://dx.doi.org/10.1105/tpc.113.117960
Young plant seedlings rely on a stored source of fixed carbon until they are able to carry out photosynthesis upon exposure to light. In the plant model species Arabidopsis thaliana, the main food storage is in the form of oil bodies containing triacylglycerol, which is broken down into fatty acids during seed germination. β-oxidation of fatty acids in seedling peroxisomes (SPs) produces acetyl-CoA units that are subsequently used to form organic acids by the glyoxylate cycle. In the cytosol, gluconeogenesis converts the organic acids into sugars, which are needed for biosynthesis of various macromolecules to promote rapid cell division and growth. Thus, the glyoxylate cycle plays a pivotal role for early seedling growth, and SPs are distinguished by the presence of isocitrate lyase (ICL) and malate synthase (MLS), 2 matrix enzymes participating in the glyoxylate cycle. An Arabidopsis seedling consists of root, hypocotyl, 2 cotyledons, and true leaves hidden between the cotyledons (Fig. 1). When a seedling is grown in the dark or underground, its hypocotyl
702 Autophagy
rapidly elongates, which helps the seedling encounter sunlight. When the seedling is exposed to light, the hypocotyl stops elongating, the upper part of the seedling turns green and performs photosynthesis, and SPs are transformed to leaf peroxisomes (LPs). During the SP-LP transition, ICL and MLS are rapidly degraded while peroxisomes import enzymes involved in photorespiration, a process where plant cells recycle a photosynthetic byproduct from chloroplasts. Previous studies using immunoelectron microscopy identified transitional peroxisomes (TPs) containing both obsolete ICL and new photorespiratory enzymes. Three mechanisms for the degradation of peroxisomal matrix proteins such as ICL and MLS were proposed: i) degradation in the peroxisomes by resident proteases, ii) degradation in the cytosol by proteasomes after the proteins are retranslocated from peroxisomes, and iii) pexophagy, or selective autophagic degradation of the peroxisomes in the vacuole. However, the molecules responsible for the first 2 mechanisms are not clear, and the presence of pexophagy in plant cells has not been demonstrated. By contrast, pexophagy in methylotropic yeast has been described in greater detail. We investigated a potential role of autophagy-related (ATG) proteins in peroxisome homeostasis, and observed a reduction in the number of fluorescent peroxisomal puncta in Arabidopsis hypocotyls incubated in a light/dark cycle for 5 d. The reduction was negligible in the hypocotyls of atg7 and atg5 mutants, which were previously shown to be defective in ATG8 lipidation and autophagic
Volume 10 Issue 4
©2014 Landes Bioscience. Do not distribute.
Switching peroxisomes for green life
delivery of GFP-ATG8 autophagic markers. In cotyledons, the difference between wild type and mutants was not obvious for a week, although 20-d-old mutant cotyledons accumulated more peroxisomes than the wild type did. Degradation of endogenous ICL and MLS in hypocotyls was delayed by atg5 and atg7 mutations but not completely inhibited, suggesting alternative degradation mechanism for ICL and MLS. This delay was less conspicuous at the whole-seedling level, indicating that autophagy preferably acts in hypocotyls. Indeed, our time-course RNA analysis revealed that the ATG7 transcript
level was transiently increased in 5-d-old hypocotyls but not in cotyledons. We further investigated the autophagic targeting of peroxisomes to the central vacuole in hypocotyl cells. A punctate fluorescent signal of the peroxisomal marker was occasionally detected in the vacuole when wild-type seedlings were incubated with concanamycin A, an inhibitor of vacuolar proton pumps. In contrast, no peroxisomal signal was seen in the vacuole of atg7 seedlings treated with the inhibitor. We also observed a fraction of GFP-ATG8apositive autophagic vesicles that overlapped with puncta of the peroxisomal marker.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed. Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (2011–0010683) and by a grant from the Next-Generation BioGreen 21 Program (No. PJ009004), Rural Development Administration, Korea.
www.landesbioscience.com Autophagy 703
©2014 Landes Bioscience. Do not distribute.
Downloaded by [University of Auckland Library] at 02:28 13 February 2015
Figure 1. Diagram showing light-dependent development of plant seedlings (upper part) and a hypothetical change in the organelles of hypocotyl cells (lower part). Oil bodies (OB) supply fatty acids to seedling peroxisomes (SP). When a seedling is exposed to light, some of the SPs are sequestered within an autophagosome (A) and rapidly degraded in the vacuole (not shown). Photorespiratory enzymes are imported to a remaining pool of SPs, which will form transitional peroxisomes (TP). Some TPs may be targeted by autophagy, as shown here by the presence of a phagophore (PG), which will form another autophagosome. Leaf peroxisomes (LP) may be developed from remaining TPs, and will constitute the photorespiration cycle together with chloroplasts (C) and mitochondria (not shown).
Taken together, our data indicated that peroxisomes in Arabidopsis hypocotyl cells are degraded in the vacuole in an ATG7-dependent manner. Our model based on the data (Fig. 1, bottom panels) explains how autophagy contributes to the transition of SPs to LPs. When seedlings need an internal source of fixed carbon, oil bodies provide SPs with fatty acids. When the seedlings are capable of photosynthesis, SPs containing obsolete ICL and MLS are sequestered by a phagophore and degraded in the vacuole. A fraction of TPs may also be targeted to the vacuole by autophagy, since we observed overaccumulation of a photorespiratory enzyme in atg7 and atg5 hypocotyls. TPs that are not degraded by autophagy may become LPs after any residual ICL and MLS are selectively degraded by resident proteases and/ or by the ubiquitin-proteasome system. Of note, our reverse genetics study on plant pexophagy is also supported by forward genetics data obtained by 2 research groups, whose work was either cited in our paper or published as an accompanying paper. Future research will be focused on the mechanisms of pexophagy in hypocotyls. It will be interesting to determine whether target recognition is conserved between animal and plant kingdoms. In addition, we hope to identify upstream signals inducing autophagy in Arabidopsis hypocotyls.
Autophagy Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/kaup20
Plant cell remodeling by autophagy a
a
a
Jimi Kim , Han Nim Lee & Taijoon Chung a
Department of Biological Sciences; Pusan National University; Busan, Korea Published online: 31 Jan 2014.
Click for updates To cite this article: Jimi Kim, Han Nim Lee & Taijoon Chung (2014) Plant cell remodeling by autophagy, Autophagy, 10:4, 702-703, DOI: 10.4161/auto.27953 To link to this article: http://dx.doi.org/10.4161/auto.27953
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Autophagic Punctum Autophagic Punctum
Autophagy 10:4, 702–703; April 2014; © 2014 Landes Bioscience
Plant cell remodeling by autophagy Jimi Kim, Han Nim Lee, and Taijoon Chung*
Downloaded by [University of Auckland Library] at 02:28 13 February 2015
Department of Biological Sciences; Pusan National University; Busan, Korea
P
lant seedlings are not photoautotrophs until they are equipped with photosynthetic machinery. Some plant cells are remodeled after being exposed to light, and a group of peroxisomal proteins are degraded during the remodeling. Autophagy was proposed as one of the mechanisms for the degradation of peroxisomal proteins. We recently showed that ATG7-dependent autophagy is partially responsible for the degradation of obsolete peroxisomal proteins during Arabidopsis seedling growth.
Keywords: pexophagy, degradation, hypocotyl, autophagosome, ATG, autophagy-related Abbreviations: ATG, autophagy-related; GFP, green fluorescent protein; ICL, isocitrate lyase; LP, leaf peroxisome; MLS, malate synthase; SP, seedling peroxisome; TP, transitional peroxisome Submitted: 01/14/2014 Revised: 01/21/2014 Accepted: 01/22/2014 http://dx.doi.org/10.4161/auto.27953 *Correspondence to: Taijoon Chung; Email: [email protected] Punctum to: Kim J, Lee H, Lee HN, Kim SH, Shin KD, Chung T. Autophagy-related proteins are required for degradation of peroxisomes in Arabidopsis hypocotyls during seedling growth. Plant Cell 2013; 25:4956–66; PMID:24368791; http://dx.doi.org/10.1105/tpc.113.117960
Young plant seedlings rely on a stored source of fixed carbon until they are able to carry out photosynthesis upon exposure to light. In the plant model species Arabidopsis thaliana, the main food storage is in the form of oil bodies containing triacylglycerol, which is broken down into fatty acids during seed germination. β-oxidation of fatty acids in seedling peroxisomes (SPs) produces acetyl-CoA units that are subsequently used to form organic acids by the glyoxylate cycle. In the cytosol, gluconeogenesis converts the organic acids into sugars, which are needed for biosynthesis of various macromolecules to promote rapid cell division and growth. Thus, the glyoxylate cycle plays a pivotal role for early seedling growth, and SPs are distinguished by the presence of isocitrate lyase (ICL) and malate synthase (MLS), 2 matrix enzymes participating in the glyoxylate cycle. An Arabidopsis seedling consists of root, hypocotyl, 2 cotyledons, and true leaves hidden between the cotyledons (Fig. 1). When a seedling is grown in the dark or underground, its hypocotyl
702 Autophagy
rapidly elongates, which helps the seedling encounter sunlight. When the seedling is exposed to light, the hypocotyl stops elongating, the upper part of the seedling turns green and performs photosynthesis, and SPs are transformed to leaf peroxisomes (LPs). During the SP-LP transition, ICL and MLS are rapidly degraded while peroxisomes import enzymes involved in photorespiration, a process where plant cells recycle a photosynthetic byproduct from chloroplasts. Previous studies using immunoelectron microscopy identified transitional peroxisomes (TPs) containing both obsolete ICL and new photorespiratory enzymes. Three mechanisms for the degradation of peroxisomal matrix proteins such as ICL and MLS were proposed: i) degradation in the peroxisomes by resident proteases, ii) degradation in the cytosol by proteasomes after the proteins are retranslocated from peroxisomes, and iii) pexophagy, or selective autophagic degradation of the peroxisomes in the vacuole. However, the molecules responsible for the first 2 mechanisms are not clear, and the presence of pexophagy in plant cells has not been demonstrated. By contrast, pexophagy in methylotropic yeast has been described in greater detail. We investigated a potential role of autophagy-related (ATG) proteins in peroxisome homeostasis, and observed a reduction in the number of fluorescent peroxisomal puncta in Arabidopsis hypocotyls incubated in a light/dark cycle for 5 d. The reduction was negligible in the hypocotyls of atg7 and atg5 mutants, which were previously shown to be defective in ATG8 lipidation and autophagic
Volume 10 Issue 4
©2014 Landes Bioscience. Do not distribute.
Switching peroxisomes for green life
delivery of GFP-ATG8 autophagic markers. In cotyledons, the difference between wild type and mutants was not obvious for a week, although 20-d-old mutant cotyledons accumulated more peroxisomes than the wild type did. Degradation of endogenous ICL and MLS in hypocotyls was delayed by atg5 and atg7 mutations but not completely inhibited, suggesting alternative degradation mechanism for ICL and MLS. This delay was less conspicuous at the whole-seedling level, indicating that autophagy preferably acts in hypocotyls. Indeed, our time-course RNA analysis revealed that the ATG7 transcript
level was transiently increased in 5-d-old hypocotyls but not in cotyledons. We further investigated the autophagic targeting of peroxisomes to the central vacuole in hypocotyl cells. A punctate fluorescent signal of the peroxisomal marker was occasionally detected in the vacuole when wild-type seedlings were incubated with concanamycin A, an inhibitor of vacuolar proton pumps. In contrast, no peroxisomal signal was seen in the vacuole of atg7 seedlings treated with the inhibitor. We also observed a fraction of GFP-ATG8apositive autophagic vesicles that overlapped with puncta of the peroxisomal marker.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed. Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (2011–0010683) and by a grant from the Next-Generation BioGreen 21 Program (No. PJ009004), Rural Development Administration, Korea.
www.landesbioscience.com Autophagy 703
©2014 Landes Bioscience. Do not distribute.
Downloaded by [University of Auckland Library] at 02:28 13 February 2015
Figure 1. Diagram showing light-dependent development of plant seedlings (upper part) and a hypothetical change in the organelles of hypocotyl cells (lower part). Oil bodies (OB) supply fatty acids to seedling peroxisomes (SP). When a seedling is exposed to light, some of the SPs are sequestered within an autophagosome (A) and rapidly degraded in the vacuole (not shown). Photorespiratory enzymes are imported to a remaining pool of SPs, which will form transitional peroxisomes (TP). Some TPs may be targeted by autophagy, as shown here by the presence of a phagophore (PG), which will form another autophagosome. Leaf peroxisomes (LP) may be developed from remaining TPs, and will constitute the photorespiration cycle together with chloroplasts (C) and mitochondria (not shown).
Taken together, our data indicated that peroxisomes in Arabidopsis hypocotyl cells are degraded in the vacuole in an ATG7-dependent manner. Our model based on the data (Fig. 1, bottom panels) explains how autophagy contributes to the transition of SPs to LPs. When seedlings need an internal source of fixed carbon, oil bodies provide SPs with fatty acids. When the seedlings are capable of photosynthesis, SPs containing obsolete ICL and MLS are sequestered by a phagophore and degraded in the vacuole. A fraction of TPs may also be targeted to the vacuole by autophagy, since we observed overaccumulation of a photorespiratory enzyme in atg7 and atg5 hypocotyls. TPs that are not degraded by autophagy may become LPs after any residual ICL and MLS are selectively degraded by resident proteases and/ or by the ubiquitin-proteasome system. Of note, our reverse genetics study on plant pexophagy is also supported by forward genetics data obtained by 2 research groups, whose work was either cited in our paper or published as an accompanying paper. Future research will be focused on the mechanisms of pexophagy in hypocotyls. It will be interesting to determine whether target recognition is conserved between animal and plant kingdoms. In addition, we hope to identify upstream signals inducing autophagy in Arabidopsis hypocotyls.
