My laboratory is interested in the molecular basis for organelle biogenesis and function in plants.
Chloroplast Protein Import
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.
Increasing Photosynthetic Carbon Capture for Crop Improvement
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. The relative inefficiency of photochemical conversion of light energy to fixed carbon during photosynthesis is a major factor limiting crop productivity. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme catalyzing the first phase of carbon fixation via the carboxylation of ribulose-1,5-bisphosphate (RuBP), is a foremost contributor to this inefficiency. O2 effectively competes for CO2 at the active site of Rubisco resulting in oxygenation of RuBP and non-productive photorespiration at the expense of the productive carboxylation reaction. To date, efforts to increase the specificity and catalytic activity of Rubisco directly have met with limited success. As an alternative, considerable attention has been given to the engineering of carbon concentrating mechanisms (CCMs) of cyanobacteria and algae as potential solutions for increased carboxylation vs. oxygenation activity at Rubisco. These aquatic photosynthetic organisms evolved under CO2-limiting conditions, and their CCMs sequester Rubisco and concentrate CO2 at the enzyme, thereby increasing carbon assimilation by several orders of magnitude. We have focused on investigating the impact of expressing components of the algal CCM in plants. These studies illustrate both direct and indirect positive effects on photosynthesis and crop productivity of increased CO2 availability at Rubisco by engineering components of microbial CCMs into plants.