Molecular Cell & Developmental Biology
225 Sinsheimer Laboratories
Phone: 831.459.4986
Fax: 831.459.3139
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ROBERT LUDWIG Professor of MCD Biology B.S., University of Michigan Ph.D., Yale University Postdoctoral Research Fellow, Massachusetts Institutes of Technology

225 Sinsheimer Laboratories University of California Santa Cruz, CA 95064 phone 831.459.4084 fax 831.459.3139 ludwig@biology.ucsc.edu office hours

Organelle Targetting of Protein Trafficking in Arabidopsis plants

In Arabidopsis thaliana, thale cress, a model flowering plant, some 5,000 nuclear gene products representing ~25% of the proteome specifically function in organelles, principally chloroplasts and mitochondria. For proper function, these proteins must be specifically and unerringly directed to their organelle targets, a complex trafficking process. To allow this trafficking, precursor proteins carry as ‘zipcode’ specific sequences of amino acids at their proximal ends. Until recently, these ‘zipcodes’ had been thought absolutely specific. However, in Arabidopsis leaves, our research has identified one such precursor protein, ‘plastidic’ glutamine synthetase, encoded by the GLN2 gene, dual-targetted to both leaf mitochondria and chloroplasts (Taira, 2004). Current research in our laboratory is directed towards understanding the mechansims of this dual-targetting.

Nitrogen Recycling during Photorespiration

‘Plastidic’ glutamine synthetase activity is critical to photorespiration, the light dependent uptake of O2 and release of CO2, a process common to all oxygenic photosynthetic organsisms. As it acts contrary to photosynthesis, photorespiration has remained conceptually problematical since its discovery some fifty years ago. During photorespiration, glycine is combusted in leaf mitochondria at high rates yielding free ammonium which must be reassimilated to allow glycine resynthesis. We are also studying whether glutamine synthetase activity, which mediates the first step in ammonium assimilation, need occur in leaf mitochondria and/or chloroplasts by genetically modifying the GLN2 gene to yield variants targetted to a specific organelle and assessing the consequences for photorespiration.

Molecular Biology of Plant-Microbe Interactions

The symbiotic legume root nodule with Rhizobium bacteria as endosymbionts offers a wealth of problems to be analyzed at the molecular level, including those of symbiosis itself. Because nodulated legume plants grow in very nitrogen-stressed environments, this symbiosis is of great agronomic importance. As the symbiotic nodule develops, there follows an elaborate morphogenesis in which rhizobia finally persist as differentiated, specialized, nitrogen-fixing organelles inside plant cells. Our research seeks to understand the molecular regulation of Rhizobium growth and differentiation during symbiosis. We are attempting to reconstruct this process using genetics, biochemistry, physiology, and recombinant DNA technology. With improved understanding of symbiotic nitrogen fixation, we hope to maximize both the yields of the world's legume crops and the efficiencies of cultivation, and to minimize adverse consequences to soil ecosystems, with particular relevance to sustainable agriculture.

Selected Publications

Taira, M., U. Valtersson, B. Burkhardt, and R.A. Ludwig (2004) “Arabidopsis thaliana GLN2-encoded glutamine synthetase is dual-targetted to leaf mitochondria and chloroplasts”, Plant Cell 16: 2048-2058.

Scott, J.D., and R.A. Ludwig (2004) “Azorhizobium caulinodans electron-transferring flavoprotein-N electrochemically couples pyruvate dehydrogenase complex activity to N2 fixation”, Microbiology 150: 117-126.

Ludwig, R.A. (2004) “Microaerophilic bacteria transduce energy via oxidative metabolic gearing”, Research In Microbiology 155: 61-70.

Pauling, D.C., Lapointe, J.P., Paris, C.M. and R.A. Ludwig. "Azorhizobium caulinodans pyruvate dehydrogenase activity is dispensable for aerobic but required for microaerobic growth." Microbiology 147: 2233-2245 (2001).

Kaminski, P.A, Kitts, C.L., Zimmerman, Z. and R.A. Ludwig. "Azorhizobium caulinodans Uses Both Cytbd (Quinol) and Cytcbb3 (Cytc) Terminal Oxidases for Symbiotic N2 Fixation." J. Bacteriology 178: 5989-5994 (1996).

Loroch, A.I., Nguyen, B. and R.A. Ludwig, "FixLJK and NtrBC signals interactively regulate Azorhizobium nifA transcription via overlapping promoters." J. Bacteriology 177: 7210-7221 (1995).

Kitts, C.L. and R.A. Ludwig, "Azorhizobium respires with at least four terminal oxidases." J. Bacteriology 176: 886-895 (1994).

Ludwig, R.A. "Arabidopsis chloroplasts dissimilate L-arginine and L-citrulline for use as N source." Plant Physiology 101: 429-434 (1993).

Kitts, C.L., Lapointe, J.P., Lam, V.T. and R.A. Ludwig, "Elucidation of the complete Azorhizobium nicotinate catabolism pathway." J. Bacteriology 174: 7791-7798 (1992).

Lazo, G.L., Stein, P.A., and R.A. Ludwig. "A transformation competent Arabidopsis genomic library in Agrobacterium." Bio/Technology 9: 963-971 (1991).