Physiologia Plantarum 2011 Copyright  Physiologia Plantarum 2011, ISSN 0031-9317 REVIEW The multifaceted role of Lon proteolysis in seedling establishment and maintenance of plant organelle function: living from protein destruction Stamatis Rigas, Gerasimos Daras, Dikran Tsitsekian and Polydefkis Hatzopoulos * Department of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece Correspondence *Corresponding author, e-mail: phat@aua.gr Received 23 May 2011; revised 10 August 2011 doi:10.1111/j.1399-3054.2011.01537.x Intracellular selective proteolysis is an important post-translational regulatory mechanism maintaining protein quality control by removing defective, damaged or even deleterious protein aggregates. The ATP-dependent Lon protease is a key component of protein quality control that is highly conserved across the kingdoms of living organisms. Major advancements have been made in bacteria and in non-plant organisms to understand the role of Lon in protection against protein oxidation, ageing and neurodegenerative diseases. This review presents the progress currently made in plants. The Lon gene family in Arabidopsis consists of four members that produce distinct protein isoforms localized in several organelles. Lon1 and Lon4 that potentially originate from a recent gene duplication event are dual-targeted to mitochondria and chloroplasts through distinct mechanisms revealing divergent evolution. Arabidopsis mutant analysis showed that mitochondria and peroxisomes biogenesis or maintenance of function is modulated by Lon1 and Lon2, respectively. Consequently, the lack of Lon selective proteolysis leading to growth retardation and impaired seedling establishment can be attributed to defects in the oil reserve mobilization pathway. The current progress in Arabidopsis research uncovers the role of Lon in the proteome homeostasis of plant organelles and stimulates biotechnology scenarios of plant tolerance against harsh abiotic conditions because of climate instability. The AAA + molecular machines in the cell: protein saviors or destructors Maintenance of cellular homeostasis relies on the tight regulation between protein repair and removal of damaged enzymes or obsolete regulatory proteins. In bacteria and eukaryotic organelles, protein quality control is performed by members of the AAA + protein superfamily (ATPases associated with diverse cellular activities) including chaperones and proteases that represent two sides of the same coin (Neuwald Abbreviations – AAA + , ATPases associated with diverse cellular activities; CAT, catalase; COX, cytochrome c oxidase; ER, endoplasmic reticulum; FFAs, free fatty acids; HPR, hydroxypyruvate reductase; PMDH, peroxisomal malate dehydrogenase; PTS, peroxisome-targeting signal; SSD, sensor and substrate discrimination; TCA, tricarboxylic acid cycle; TAG, triacylglycerols; TSS, transcription start sites. et al. 1999, Iyer et al. 2004, Sauer et al. 2004). The molecular chaperones facilitate native conformation of nascent polypeptides, prevent aggregation and assist in assembly of multiprotein complexes. Conversely, the ATP-dependent proteases that belong to Lon, Clp and FtsH families degrade irreparably damaged or improperly synthesized proteins into peptides and amino acids, which are transported out of the subcellular compartments (Adam et al. 2001, Sinvany-Villalobo et al. 2004, Janska et al. 2010). This recycle contributes to raw materials for de novo protein synthesis. Physiol. Plant. 2011