Peroxisomes are cellular compartments that house a variety of processes. In plants, peroxisomal enzymes catalyze numerous critical metabolic reactions, including fatty acid breakdown, certain photosynthesis-related reactions, and metabolism of several hormones. A distinguishing and remarkable feature of peroxisomes is their ability to import folded and even multimeric cargo proteins. Most proteins necessary for building peroxisomes (peroxins) are dedicated to importing peroxisomal enzymes from the cytoplasm, where the proteins are synthesized. A broad outline has emerged that describes how proteins are imported into animal, fungal, and plant peroxisomes. Whereas some elements of peroxisome biogenesis appear to be universal, other components display surprising variability. It is therefore critical to examine these import proteins in greater detail to define their roles in plants. This project will elucidate the molecular mechanisms of peroxisome protein import in plants by analyzing peroxin mutants in the reference plant Arabidopsis thaliana for peroxisome defective phenotypes, peroxisomal protein import, and receptor levels. Genetic and physical interactions among the peroxins will provide insight into how the peroxins work together. A genetic screen will provide a means to discover peroxins unique to plants or not yet discovered in other systems by taking advantage of, without being limited by, what is known in other systems. Characterizing plant peroxisomal protein import will reveal the roles of genes critical for the function and assembly of plant peroxisomes, and will reveal the divergent and conserved facets of this process among plants, animals, and fungi.
In addition to investigating peroxisome functions, this research will provide undergraduate, graduate, and postdoctoral researchers with the broad interdisciplinary training bridging genomics, molecular genetics, cell biology, and biochemistry that will be essential to the emerging generation of scientists. Rice University and the PI are committed to training and educating both graduate and undergraduate students while continuing to foster the participation of individuals from underrepresented groups in research.
Peroxisomes are subcellular compartments (organelles) that sequester various oxidative reactions, thereby protecting other cellular components from oxidative damage. In plants, peroxisomes are essential for development and environmental responses from embryogenesis to senescence. All peroxisomes contain hydrogen peroxide-producing oxidases as well as enzymes to decompose hydrogen peroxide; other metabolism harbored within the organelle is diverse and varies by cell type, developmental stage, and species. For example, plant peroxisomes break down fatty acids to provide energy to seedlings prior to the onset of photosynthesis. T he necessity of seedling peroxisomes can be bypassed by providing seedlings with sugar supplementation; sugar-dependence during seedling development is thus a hallmark of a peroxisome deficiency. We discovered that peroxisomes also convert a proto-hormone into an active hormone, which enabled us to easily screen for peroxisome-defective mutants and provided us with quantitative measures of the severity of mutant defects. With support from the NSF, we have investigated the relationships of proteins needed to deliver cargo proteins to the interior of the peroxisome and isolated novel peroxisome-defective mutants. Our analysis of these mutants revealed new peroxisomal protein import components, unanticipated interdependencies among peroxisome biogenesis factors, and important differences and similarities between peroxisome biogenesis in plants and more widely studied systems (yeast and mammals). The plant mutants isolated and characterized through this research comprise a molecular toolkit for further studies of peroxisome biogenesis, maintenance, and dynamics that may facilitate future peroxisome-based improvements in plant metabolism and stress adaptations. The results from this funding were disseminated in 16 peer-reviewed publications. This funding also supported development of the scientific workforce through the training of eight graduate and postdoctoral scientists. Two former postdoctal scientists are now university faculty members or industry scientists. Project graduate students are currently completing their doctoral research or have obtained postdoctoral or scientific editing positions. In addition, more than 20 undergraduates from Rice University and several primarily undergraduate institutions conducted mentored research projects (from one summer to three years in duration) related to the grant objectives; these undergraduates have gone on to PhD, MD/PhD, MD, and physician assistant training programs.
