Archie Portis

 

Emeritus Professor of Plant Biology and USDA/ARS, Department of Crop Science,
Program in Physiological & Molecular Plant Biology

193 ERML MC-051
(217) 333-3083

Education

PhD, 1976, Cornell University

Research

Photosynthetic carbon metabolism is initiated by the enzyme, Rubisco, which captures atmospheric carbon dioxide by combining it with a sugar phosphate, ribulose bisphosphate, in a reaction called carboxylation. However Rubisco is not able to prevent oxygen from reacting with some of the ribulose bisphosphate, allowing oxgenation to occur instead of carboxylation. Previous research spanning many years has consistently shown that the photosynthetic potential and efficiency of plants can be limited by the activity of Rubisco over a wide variety of conditions. Therefore research in my laboratory is directed at the goal of improving the photosynthetic potential and efficiency of plants by altering the properties and regulation of Rubisco. A fairly detailed knowledge of these processes is required in order to know what changes to make and a means to introduce these changes into the plant is required. Currently, we have projects that are directed at both these problems and they are summarized below:


a) We are developing methods to replace the Rubisco protein present in tobacco with another to demonstrate proof of principle that photosynthesis can be increase by genetic engineering of Rubisco. This is difficult at the whole plant level because it involves both nuclear and chloroplast transformation. We are attempting to replace the natural enzyme in tobacco with one predicted to be more suitable for the higher carbon dioxide levels that will exist in the near future. This approach will exploit our current knowledge that natural variation exists in both maximal activity and relative ratio of carboxylation to oxygenation even though the exact structural differences responsible for these kinetic differences are not understood.


b) We are determining the effects of altered regulation of Rubisco on photosynthetic potential. Our pioneering studies of the structure, activity and regulation of Rubisco activase have recently been complemented by the discovery that Rubisco activity is regulated by redox sensing through one of the Rubisco activase isoforms. We also have been able to alter the ADP/ATP regulation of activase using site-directed mutagenesis. Mutant Arabidopsis plants lacking Rubisco activase have been transformed to express each isoform separately or one of the mutant proteins. They are being characterized to learn more about the role of Rubisco regulation in determining photosynthesis and growth.


c) We are continuing to characterize Rubisco activase and how this protein regulates the activity of Rubisco. Major areas of current investigation are the relationships between light intensity and redox in regulation of Rubisco activase, studies of the interaction between Rubisco and Rubisco activase using site-directed mutagenesis, and a detailed investigation of the effects of moderate heat stress on plants, which reduces the activation of Rubisco.


Publications

Wang D, Portis AR (2007) A novel nucleu-encoded chloroplast protein, PIFI, is involved in NAD(P)H dehydrogenase complex-mediated chlororespiratory electron transport in Arabidopsis. Plant Physiology 144, 1742-1752.

 

Wang D, Portis AR (2006) Increased sensitivity of oxidized large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase to ADP inhibition is due to an interaction between its carboxyl extension and nucleotide-binding pocket. Journal of Biological Chemistry 281, 25241-25249.

 

Kim KM, Portis AR (2006) Kinetic analysis of the slow activation of Rubisco during catalysis: effects of temperature, O2 and Mg++. Photosynthesis Research 87, 195-204.