Research Overview

Rubisco is an enzyme that initiates photosynthetic carbon metabolism by combining atmospheric carbon dioxide with a sugar phosphate -- ribulose bisphosphate -- in a reaction called carboxylation. The activity of rubisco can limit the photosynthetic potential and efficiency of plants in many instances. For example, plants use about 25 percent of the nitrogen present in their leaves for the synthesis of rubisco because the carboxylation rate is slow compared to most other metabolic processes. Also, rubisco cannot prevent oxygen from reacting with some of the ribulose bisphosphate, allowing oxygenation to occur instead of carboxylation. Plants recycle the product of this reaction in a process called photorespiration, but nevertheless must release one carbon dioxide molecule for every two oxygen molecules that react. The oxygenase reaction further reduces photosynthetic potential under limiting light conditions because photorespiration, like photosynthesis, requires energy.

When environmental conditions are not favorable, it is often advantageous to reduce, or regulate, rubisco activity. Rubisco activase is the protein that accomplishes this regulation. In most natural systems, there is a trade-off between regulation and maximal performance. It is not yet clear whether this holds true for rubisco regulation because we don't have an adequate understanding of the regulatory process. We are working to develop a detailed knowledge of the properties of regulation in order to know what genetic changes would benefit the plant. We are also developing a means to introduce those changes into the plant.

In particular, research in our lab focuses on the following three areas:

1. We are developing methods to replace the rubisco protein present in soybeans with a genetically engineered version. We want to replace the natural enzyme with one that may be more suitable for the higher atmospheric carbon dioxide concentrations that will exist in the near future. Genetic engineering of rubisco is difficult at the whole-plant level because it involves chloroplast transformation -- a process only recently developed in tobacco. We are attempting to extend these methods to soybeans.
2. As a result of our research into the structure, activity, and regulation of rubisco activase, we have created a protein that is not regulated like the normal protein. We are studying Arabidopsis plants transformed with this altered protein to learn more about the role of rubisco regulation in determining photosynthesis and growth.
3. We are continuing to characterize the regulation of rubisco activase activity and how this protein alters the activity of rubisco. Currently, we are investigating the role of thylakoids and light in the regulation of rubisco activase and the extent to which moderate heat stress inhibits photosynthesis by reducing the activation of rubisco. We are also studying the interaction between rubisco and rubisco activase using site-directed mutagenesis.

The goal of research in the Portis laboratory is to improve the photosynthetic potential and efficiency of plants by altering the properties and regulation of rubisco.

Recent Advances

We recently completed a study in which we determined photosynthesis and growth-to-maturity using Arabidopsis thaliana with only 30 percent of the normal (wild type) levels of rubisco activase (via plant transformation with antisense rubisco activase DNA). Rubisco activase is normally present at fairly high levels in plants and regulates the activity of rubisco. Due to reduced activation of rubisco in the plants in our study, photosynthesis was reduced by 13 percent. However, this small reduction resulted in a 35 percent reduction in growth as the plants reached maturity because growth is the cumulative result of photosynthesis throughout the life of the plants. This study supports the expectation that even small increases in photosynthesis due to improved catalysis by rubisco or other photosynthetic processes could have considerable impact on plant growth.

Lab Personnel

Select Publications

1. Eckardt, N.A., & Portis, A.R., Jr. Heat denaturation profiles of rubisco and rubisco activase and the inability of rubisco activase to restore activity of heat denatured rubisco. Plant Physiol. In press.
2. Eckardt, N.A., Snyder, G.W., Portis, A.R., Jr., & Ogren W.L. Growth and photosynthesis under high and low irradiance of Arabidopsis thaliana antisense mutants with reduced rubisco activase content. Plant Physiol. In press.
3. Esau, B.D., Snyder, G.W., & Portis, A.R., Jr. 1996. Differential effects of N- and C-terminal deletions on the two activities of rubisco activase. Arch. Biochem. Biophys. 326:100-105.
4. Portis, A.R., Jr. 1992. Regulation of ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Annu. Rev. Plant Physiol. Plant Mol. Biol. 43:415-437.
5. Portis, A.R., Jr. 1995. The regulation of rubisco by rubisco activase. J. Exp. Bot. 46:1285-1291.
6. Portis, A.R., Jr., Lilley, R.M., & Andrews, T.J. 1995. Subsaturating ribulose-1,5-bisphosphate concentration promotes inactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco): Studies using continuous substrate addition in the presence and absence of rubisco activase. Plant Physiol. 109:1441-1451.
7. Wang, Z-Y., & Portis, A.R., Jr. 1992. Dissociation of ribulose-1,5-bisphosphate bound to ribulose-1,5-bisphosphate carboxylase/oxygenase and its enhancement by ribulose-1,5-bisphosphate carboxylase/oxygenase activase-mediated hydrolysis of ATP. Plant Physiol. 99:1348-1353.
8. Wang, Z-Y., Ramage, R.T., & Portis, A.R., Jr. 1993. Mg2+ and ATP or adenosine-5'-[g-thio]-triphosphate (ATP-g-S) enhances intrinsic fluorescence and induces aggregation which increases the activity of spinach rubisco activase. Biochim Biophys. Acta. 1202:47-55.
9. Wang, Z-Y., Snyder, G.W., Esau, B.D., Portis, A.R., Jr., & Ogren, W.L. 1992. Species-dependent variation in the interaction of substrate-bound rubisco and rubisco activase. Plant Physiol. 100:1858-1862.