Research

Research

 

My laboratory is interested in the molecular basis for organelle biogenesis and inter-organellar networks in plants.

 

Chloroplast biogenesis

We have a longstanding program focused on chloroplast biogenesis. The plastids are a highly divergent group of organelles that provide essential metabolic and signaling functions within all plant cells.  The archetypical plastid is the chloroplast, the organelle that provides the capacity for all plants to perform photosynthesis.  Plastids, and consequently the advent of the plant kingdom, arose by the endosymbiosis of a photosynthetic cyanobacterium-like prokaryote by a nucleated host cell.  As the evolution of complex, multi-cellular plants progressed to give rise to specialized cell types and tissues, the endosymbiont also evolved into distinct plastid types to provide specialized metabolic functions for specific cells and tissues.  As a result, modern land plants contain at least a half-dozen plastid types with distinct morphologies and functions.  Despite this diversity, two unifying principles apply to all plastids.  First, plastid differentiation is reversible, allowing plastids to interconvert and differentiate in concert with their cellular host.  Second, gene transfer to the nucleus has reduced the plastid genome to ~120 genes in land plants.  As a result, plastid biogenesis is reliant on the import of thousands of nucleus-encoded proteins (~3500 proteins in Arabidopsis thaliana) after completion of their synthesis on cytoplasmic ribosomes.  My laboratory is interested in understanding the molecular basis and regulation of the protein import pathways into plastids.  This work has significant implications for understanding how plants adapt the functions of their plastids in response to growth, development, and physiological and environmental changes.

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The majority of plastid proteins are encoded in the nucleus and translated on cytosolic ribosomes. Plastid preproteins contain an N-terminal transit peptide that is necessary and sufficient to target proteins to the organelle. The transit peptide is recognized at the surface of the plastid by two GTPase receptors of the TOC complex (brown), Toc159 (159) and Toc33 (33), at the outer envelope membrane (OM). The receptors initiate membrane transport via a GTP-dependent switch, and the preprotein translocates through an associated beta-barrel channel, Toc75 (75) of the TOC complex. Import occurs simultaneously across TOC and TIC (blue) and is driven by an ATP-dependent import associated chaperone network, which constitutes the import motor (orange). The transit peptide is removed by the stromal processing peptidase upon import, and the chaperone network assists in folding and assembly of the newly imported proteins. Proteins destined for the inner envelope or thylakoid membranes are subsequently recognized by conserved sub-organellar targeting machineries.

 

Inter-organellar metabolic networks to optimize photosynthesis and yield

Our second area of interest aims to use our expertise in organelle biology to enhance crop productivity.  In particular, we are interested in how the metabolic networks in chloroplasts and mitochondria can be optimized to increase the efficiency of photosynthesis and improve crop yields.  Photosynthesis is highly sensitive to varying environmental conditions, including temperature, light, water and nutrient availability. Under typical field conditions, the oxygenation reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)  accounts for a >40% decrease in the efficiency of carbon fixation in C3 photosynthesis, the pathway of carbon fixation used by the majority of crops and about 85% of all terrestrial plant species.  The products of oxygenation also inhibit photosynthesis and must be recycled via photorespiration via a complex inter-organellar metabolic network involving chloroplasts, mitochondria and peroxisomes.  Although increasing atmospheric CO2 concentrations are predicted to result in some increased productivity in crop species, the positive impact of increased CO2 is offset by increased temperatures and decreased rainfall in many highly productive agricultural regions.  Thus, strategies to balance the flux through photosynthetic and photorespiratory metabolism, while maintaining resilience to changing environmental conditions have the potential to broadly impact crop yields.  Our lab is interested in understanding the role of organellar metabolic networks in photosynthetic metabolism and how these networks can be optimized to maintain photosynthetic efficiency and yield in the face of environmental stress.  Our work in this area is part of a collaborative project, GROE Camelina: Genomic Research for Oil Enhancement in Camelina, which employs a tissue-specific and whole-plant systems approach to identify the major regulatory mechanisms that limit oil-seed yields in Camelina sativa.   

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A model of the systems-level control of carbon assimilation transport and allocation to triacylglycerol (TAG) synthesis in Camelina sativa. Our research project elements employ a whole-plant systems approach to identify and engineer the major regulatory mechanisms that limit 1) carbon fixation in photosynthetically active source tissues (leaves), 2) the transport of fixed carbon from source to sink tissues (seeds), and 3) the allocation of fixed carbon to TAG production in seeds