Functions and Remodelling of Plant Peroxisomes

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Functions and Remodelling of Plant Peroxisomes

Rupesh Paudyal, University of Leeds, Leeds, West Yorkshire, UK
Suruchi Roychoudhry, University of Leeds, Leeds, West Yorkshire, UK
James PB Lloyd, University of California, Berkeley, California, USA

Published online: March 2017

DOI: 10.1002/9780470015902.a0001677.pub3

Full Article on Wiley Online Library

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Abstract

Peroxisomes are dynamic eukaryotic organelles that perform a wide range of important metabolic processes. In plants, the peroxisome is the sole organelle to carry out β‐oxidation of fatty acids, break down hydrogen peroxide, and performs several other functions required at different stages of plant development and under different conditions. The ability of this organelle to perform a range of functions depends on the time‐ and process‐dependent import of particular enzymes that enable biochemical reactions to take place inside peroxisomes. While it is important to recruit the right enzymes, it is also important to remove obsolete or damaged enzymes through the turnover of specific proteins. In some cases, degradation of the entire peroxisome is carried out by the process of autophagy, which helps to maintain quality control by removing damaged/dysfunctional/obsolete peroxisomes. Therefore, the diversification of plant peroxisomes for different cellular requirements is achieved through targeted turnover and import of specific enzymes. This article discusses the possible mechanisms and factors involved in functional remodelling of the plant peroxisome from the young seedling peroxisome to the leaf peroxisome. Functions of peroxisomes found in different developmental stages of the plant are also highlighted.

Key Concepts

Peroxisomes are single membrane‐bound eukaryotic organelles that perform a wide range of functions and display remarkable metabolic diversity.
Peroxisomes do not contain any genetic information and therefore all peroxisomal proteins are imported post‐translationally from the cytosol.
Peroxisomes are a major scavenger of hydrogen peroxide and plant peroxisomes are the sole site for β‐oxidation of fatty acids.
Protein content of peroxisomes varies in a developmental and functional manner.
In young seedlings post‐germination, peroxisomes house glyoxylate cycle enzymes that help to generate energy from oil reserves in the seed.
In etiolated mature seedlings, peroxisomes are ‘remodelled’ to perform photorespiration.
Peroxisome remodelling is achieved by removing glyoxylate cycle enzymes and importing photorespiration enzymes.
Glyoxylate cycle enzymes are removed by turnover either inside or outside peroxisomes by proteases and/or by the turnover of obsolete peroxisomes through autophagy process.

Keywords: plant peroxisome; beta‐oxidation; photorespiration; peroxisome remodelling; pexophagy; glyoxysome; glyoxylate cycle; LON2 protease; peroxisome autophagy

	Figure 1. Arabidopsis seedling expressing RFP‐PTS1 and PTS2‐GFP.
	Figure 2. The core β‐oxidation and glyoxylate cycle pathway in young seedling peroxisomes. Triacylglycerols (TAGs) are converted to fatty acids (FAs) by SUGAR DEPENDENT 1 (SDP1) and SUGAR DEPENDENT LIKE 1 (SDPL1) lipases. These fatty acids are then imported into the peroxisome by the CTS1 transporter to enter the β‐oxidation cycle. Imported FAs are re‐esterified by Long Chain Fatty Acids (LACS6/7) and subsequently oxidised by the ACX enzyme family to generate trans‐enoyl‐CoA. This then serves as the substrate for the MULTIFUNCTIONAL PROTEIN FAMILY 2 complex (MFP2) dependent hydration, resulting in the production of 3‐ketoacyl‐CoA, which is then cleaved to generate acetyl‐CoA and a fatty acid molecule which is shortened by two carbon atoms. In young germinating seedlings, the acetyl‐CoA molecule produced as the by‐product of β‐oxidation is taken in the glyoxylate cycle and is utilised to generate organic acids such as malate, succinate, oxaloacetate etc. via the action of peroxisomal enzymes malate synthase (MLS), isocitrate lyase (ICL) and citrate synthase 2 and 3 (CSY2/3).
	Figure 3. The tri‐organelle complex between chloroplasts, peroxisomes and mitochondria during photorespiration. In light exposed tissues with significant photosynthetic activity, the chloroplast and peroxisome physically interact. This facilitates the exchange of the toxic by‐products of photosynthesis to be transported to the peroxisome for detoxification. The peroxisome also physically interacts with the mitochondrion to exchange further metabolites during photorespiration. These molecules are transferred to different organelles by porin, porin‐like channels and yet unidentified transporters. Chloroplast transport of glycolate and glycerate is thought to be mediated by Plastidal glycolate glycerate translocator 1 (PLGG1).
	Figure 4. Re‐modelling of the plant peroxisome. (a) Young‐seedling peroxisome (0–3 days post‐germination) housing glyoxylate cycle enzymes that help break down lipids into sugars in early seedlings are subjected to degradation in etiolated seedlings. Four‐ to six‐day‐old seedlings post germination, start to degrade glyoxylate cycle enzymes inside the peroxisome through LON2 protease (b), or they are exported to the cytosol by the receptor export motif (REM) where they are degraded (c), and at the same time photorespiratory enzymes are imported into the peroxisome. Alternatively, peroxisome containing obsolete enzymes is engulfed by the autophagosome (d) to be turned over by autophagy (e). The transition from early seedling peroxisome to leaf‐type peroxisome is completed after 7 days (e). Peroxisomes degraded by pexophagy are replaced by newly de novo formed peroxisomes or through the division of existing peroxisomes (e). Pexophagy can be induced by different factors such as oxidative stress and nutrient deficiency. It also serves as a quality control for peroxisomes by removing obsolete/damaged/dysfunctional peroxisomes even under optimal conditions (e). Glyoxylate cycle enzymes ( ); photorespiratory enzymes ( ); enzymes present in both early‐seedling peroxisome and leaf peroxisome ( ); autophagosome ( ); peroxisome ( ); docking and translocation motif ( ); receptor export motif ( ).

Article Title:	Functions and Remodelling of Plant Peroxisomes
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