PROJECT SUMMARY: Peroxisome Biogenesis, Dynamics, and Degradation Peroxisomes are eukaryotic organelles that are essential for life in plants and metazoans. Peroxisomes sequester various oxidative reactions, thereby enhancing metabolic efficiency while protecting cytosolic constituents from oxidative damage. Although our knowledge about peroxisome function and dysfunction is increasing, mechanistic understanding of how this critical organelle is formed, maintained, and recycled remains incomplete. The proposed studies aim to fill these knowledge gaps by tackling the following questions: How and where do peroxisomes originate? How and why do intralumenal vesicles form within peroxisomes? How are proteins imported into the organelle? How do peroxisomes interact with other organelles? How is peroxisomal quality control enforced? These questions will be addressed using Arabidopsis thaliana; the unique peroxisomal functions, small size, and facile genetics of this model allow straightforward modulation of peroxisomal processes in an intact multicellular organism. Moreover, the relatively large size of plant peroxisomes (compared to yeast and mammalian peroxisomes) offers exceptional opportunities to decipher peroxisome biogenesis and membrane intricacies using live-cell imaging. The proposed studies will train the next generation of scientists in cutting-edge genetic, biochemical, and cell biological approaches and equip them to address fundamental cell biological questions. Peroxisomal defects underlie peroxisome biogenesis disorders, a group of inherited recessive syndromes that are generally fatal in infancy or childhood and are characterized by wide-ranging symptoms including poor growth, multi-organ dysfunctions, hearing and vision loss, and psychomotor retardation. Peroxisome dysfunction also contributes to common age-related diseases (e.g., neurodegeneration, type 2 diabetes, hearing loss) that are exacerbated by oxidative stress. The proposed studies will exploit unique aspects of plant peroxisomes while taking advantage of knowledge from fungal and mammalian systems to provide insights that will likely apply to many eukaryotes. Continued development of evolutionarily disparate systems with unique advantages for elucidating peroxisome biology will advance hypotheses and mechanistic models to expand and refine our understanding of this essential organelle.