Disrupting Autophagy Restores Peroxisome Function to anArabidopsis lon2Mutant and Reveals a Role for the LON2 Protease in Peroxisomal Matrix Protein Degradation  

Abstract

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.

Autophagy-Related Proteins Are Required for Degradation of Peroxisomes inArabidopsisHypocotyls during Seedling Growth

“…While this work was being revised, Farmer et al (2013) reported that atg7, atg3, and atg2 mutations were isolated from a forwardgenetic screen for lon2 suppressors, providing independent evidence for pexophagy in young Arabidopsis seedlings. Our observation of pexophagy in Arabidopsis seedlings supports their model that defective autophagy in the absence of LON2 proteases leads to the persistence of small peroxisomes, which enables the suppression of various lon2 phenotypes.…”

Peroxisome Function, Biogenesis, and Dynamics in Plants

“…Interestingly, lon2 and PEX1/pex1-3 peroxisomes appear enlarged, and preventing autophagy restores peroxisome size in both mutants (Farmer et al, 2013;Goto-Yamada et al, 2014;Rinaldi et al, 2017), suggesting that these enlarged peroxisomes are pexophagy intermediates. PEX1 dysfunction in yeast (Nuttall et al, 2014) and mammalian cells (Law et al, 2017) also triggers pexophagy.…”

“…Arabidopsis pexophagy was recently demonstrated (Farmer et al, 2013;Kim et al, 2013;Shibata et al, 2013). During seedling development, peroxisome functions shift from fatty acid utilization to photorespiration (Titus and Becker, 1985;Nishimura et al, 1986;Sautter, 1986;.…”

“…The autophagy machinery coordinates peroxisome abundance together with the peroxisomal matrix protease LON2 (Farmer et al, 2013). The chaperone activity of LON2 normally inhibits pexophagy (Goto-Yamada et al, 2014), and as cells age, lon2 mutants develop b-oxidation defects and low peroxisomal protein levels due to heightened pexophagy (Farmer et al, 2013).…”

“…The chaperone activity of LON2 normally inhibits pexophagy (Goto-Yamada et al, 2014), and as cells age, lon2 mutants develop b-oxidation defects and low peroxisomal protein levels due to heightened pexophagy (Farmer et al, 2013).…”