Maintenance of protein quality control and turnover is essential for cellular 
homeostasis. In plant organelles this biological process is predominantly 
performed by ATP-dependent proteases. Here, a genetic screen was performed that 
led to the identification of Arabidopsis thaliana Lon1 protease mutants that 
exhibit a post-embryonic growth retardation phenotype. Translational fusion to 
yellow fluorescent protein revealed AtLon1 subcellular localization in plant 
mitochondria, and the AtLon1 gene could complement the respiratory-deficient 
phenotype of the yeast PIM1 gene homolog. AtLon1 is highly expressed in rapidly 
growing plant organs of embryonic origin, including cotyledons and primary 
roots, and in inflorescences, which have increased mitochondria numbers per cell 
to fulfill their high energy requirements. In lon1 mutants, the expression of 
both mitochondrial and nuclear genes encoding respiratory proteins was normal. 
However, mitochondria isolated from lon1 mutants had a lower capacity for 
respiration of succinate and cytochrome c via complexes II and IV, respectively. 
Furthermore, the activity of key enzymes of the tricarboxylic acid (TCA) cycle 
was significantly reduced. Additionally, mitochondria in lon1 mutants had an 
aberrant morphology. These results shed light on the developmental mechanisms of 
selective proteolysis in plant mitochondria and suggest a critical role for 
AtLon1 protease in organelle biogenesis and seedling establishment.