WOULD TRANSFORMATION C3 CROP PLANTS WITH FOREIGN RUBISCO INCREASE PRODUCTIVITY? A COMPUTATIONAL ANALYSIS EXTRAPOLATING FROM KINETIC PROPERTIES TO CANOPY PHOTOSYNTHESIS

Authors: ZHU, XIN-GUANG - UNIV OF ILLINOIS; PORTIS JR, ARCHIE; LONG, STEPHEN - UNIV OF ILLINOIS

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2003
Publication Date: 2/18/2004
Citation: Zhu, X-G., Portis Jr., A.R., Long, S.P. 2004. Would transformation of C3 crop plants with foreign Rubisco increase productivity? A computational analysis extrapolating from kinetic properties to canopy photosynthesis. Plant Cell and Environment. 27:155-165.

Interpretive Summary: The activity of Rubisco, the enzyme that captures carbon dioxide, often limits photosynthesis, the process by which plants use light energy from the sun to make carbohydrates for growth from carbon dioxide and water. Genetic modification of Rubisco to increase the specificity for carbon dioxide relative to oxygen would decrease photorespiration and in principle should increase crop productivity. In this study, a biochemical model for leaf photosynthesis was coupled to a canopy biophysical microclimate model to quantify how theoretical changes in the kinetic parameters of Rubisco or replacing the typical crop Rubisco with naturally occurring Rubiscos will increase total crop carbon gain. This information will benefit scientists attempting to modify the properties Rubisco in ways beneficial for increased photosynthesis by crop plants.

Technical Abstract: Genetic modification of Rubisco to increase the specificity for CO2 relative to O2(t) would decrease photorespiration and in principle should increase crop productivity. When the kinetic properties of Rubisco from different photosynthetic organisms are compared, it appears that forms with high t have low maximum catalytic rates of carboxylation per active site (Kcc). If we assume that an inverse relationship between Kcc and t exists, as implied from measurements, and that an increased concentration of Rubisco per unit leaf area is not possible, will increasing t result in increased leaf and canopy photosynthesis? A steady-state biochemical model for leaf photosynthesis was coupled to a canopy biophysical microclimate model and used to explore this question. C3 photosynthetic CO2 uptake rate (A) is either limited by the maximum rate of Rubisco activity (Vcmax) or by the rate of regeneration of ribulose-1, 5-bisphosphate, in turn determined by the rate of whole chain electron transport (J). Thus, if J is limiting, an increase in t will increase net CO2 uptake because more products of the electron transport chain will be partitioned away from photorespiration into photosynthesis. The effect of an increase in t on Rubisco-limited photosynthesis depends on both Kcc and the concentration of CO2, ([CO2]). Assuming a strict inverse relationship between Kcc and t, the simulations showed that a decrease not increase in t increases Rubisco-limited photosynthesis at the current atmospheric [CO2], but the increase is observed only in high light. In crop canopies, significant amounts of both light-limited and light-saturated photosynthesis contribute to total crop carbon gain. For canopies, the present average t found in C3 terrestrial plants is supra-optimal for the present atmospheric [CO2] of 370 umol mol-1, but would be optimal for 210 umol mol-1, a value close to the average of the last 400,000 y. Replacing the average Rubisco of terrestrial C3 plants with one having a lower and optimal t would increase canopy carbon gain by 3%. Because there are significant deviations from the strict inverse relationship between Kcc and t, we also used the canopy model to compare the rates of canopy photosynthesis for several Rubiscos with well-defined kinetic constants. These simulations suggest that very substantial increases (>25%) in crop carbon gain could result if specific Rubiscos having either a higher t or higher Kcc can be successfully expressed in C3 plants.