Project Summary Telomeres are evolutionarily conserved protein-DNA complexes at the physical ends of linear eukaryotic chromosomes. Telomeres shorten with age in most human somatic cells, and their initial length pre-determines cellular lifespan. Mutations in telomere maintenance genes lead to cancer, premature aging and a number of age-related disorders. While mean telomere length in humans shows considerable inter-individual variation and appears to be under strong genetic control, the exact nature of factors establishing telomere length set point remains elusive. Using the model plant Arabidopsis thaliana, we previously identified several candidate genes underlying natural telomere length variation in this species. In this proposal, we will utilize genetic, genomic, biochemical and epigenetic approaches to explore their functions in telomere length control and other major cellular processes, and to gain an evolutionary perspective on functional gene pleiotropy. In Aim 1, we will explore several hypotheses for direct and indirect roles of Arabidopsis TERT gene, which encodes the catalytic subunit of telomerase, in establishing natural telomere length polymorphism across Arabidopsis genotypes. We will also employ powerful Arabidopsis genomic and transcriptomic tools to identify and characterize TERT trans-regulators and uncover novel factors underlying natural variation in telomerase enzyme activity levels across multiple Arabidopsis genotypes. Through a series of genetic complementation experiments with Arabidopsis telomere length mutants, experiments in Aim 2 will address the nature and extent of functional redundancies between telomere biology, ribosome biogenesis and chromatin assembly, and establish the blueprint for dissecting telomeric versus non-telomeric roles of identified genes. Additionally, in Aim 2 we will utilize several innovative genomic and epigenetic approaches to identify and validate additional candidate genes involved in telomere length control. Overall, the results of this study are expected to significantly increase our understanding of genetic differences underlying telomere length polymorphism in natural Arabidopsis populations. Because modes of telomere regulation are highly conserved, our data may also provide novel insight into the molecular basis for different rates of aging and predisposition to human diseases associated with telomere length abnormalities.