Speaker: Dan Kliebenstein, professor in the Department of Plant Sciences, College of Agricultural and Environmental Sciences, and Center for Population Biology Genome Center at University of California, Davis

Title: Complex mechanism and selection; glucosinolates from lab to continent

Abstract: A critical limitation of biology from quantitative genetics to evolution to mechanistic systems biology is that we do not have any understanding of the true level of complexity within any single trait. This includes simple basics from how many genes affect a single trait to higher-level topics like how do genes within a network interact with each other and the environment. There are key small model networks studied but the evidence suggests that 1000s of genes control most traits. As a collaborative effort, we are pursuing the complexity plant metabolism.

As a beginning, we have been working on the natural variation of the glucosinolate defense metabolites within Arabidopsis thaliana. We develop the use of metabolite-to-transcript correlations to cloning the entire biosynthetic pathway and genes underlying the major quantitative trait loci. Moving these genetic loci to the field showed that they control fitness within the field and that their genetic variation within the species is maintained by fluctuating selection. Extending this work, we were able to use Genome Wide Association mapping with co-transcriptional networks to develop a method to identify causal genes with a >70% validation rate. This allowed us to estimate that > 1,000 genes control glucosinolate and primary metabolite natural variation. A meta-analysis showed that all of our populations were vastly underpowered to estimate the true complexity of the system.  Thus, without the development of new populations, it will not be possible to answer questions about the true molecular complexity of natural variation for secondary metabolism in any plant.

To test if regulatory complexity was equally vast, we shifted approaches to conduct global Yeast-1-hybrid analysis with all of the promoters from the glucosinolate biosynthetic pathway. We have currently identified several hundred transcription factors that interact with the promoters and that have glucosinolate phenotypes when mutated. These transcription factors display extensive quantitative epistasis and tissue/ environment conditionality. Studying the link between the transcription factors and glucosinolate phenotype showed that measuring the phenotype in any individual environment had a low validation rate. However, by screening across multiple environments and multiple plant tissues, it was possible to increase this rate to over 70%.  This suggests that the transcriptional regulation of a simple secondary metabolite pathway is complex, precise and specific. Further, regulatory connections to the metabolic were not co-linear suggesting that a pathways regulatory logic is not determined by the order of enzymes but instead by either flux connectivity or biological roles of the intermediates. Using field-trials, we have extended this mechanistic complexity to show that there is extensive fluctuating selection on the glucosinolate pathway that is predominantly linked to epistasis within the pathway.

Host: Dior Kelley, GDCB assistant professor

Please join us for refreshments before the seminar outside Room 1414 of the Molecular Biology Building.