Can the cold tolerance of C4 photosynthesis in Miscanthusxgiganteus relative to Zea mays be explained by differences in activities and thermal properties of Rubisco?

The previous investigations show that the amount and activity of Rubisco appears the major limitation to effective C-4 photosynthesis at low temperatures. The chilling-tolerant and bioenergy feedstock species Miscanthusxgiganteus (M.xgiganteus) is exceptionally productive among C-4 grasses in cold climates. It is able to develop photosynthetically active leaves at temperatures 6 degrees C below the minimum for maize, and achieves a productivity even at 52 degrees N that exceeds that of the most productive C-3 crops at this latitude. This study investigates whether this unusual low temperature tolerance can be attributed to differences in the amount or kinetic properties of Rubisco relative to maize. An efficient protocol was developed to purify large amounts of functional Rubisco from C-4 leaves. The maximum carboxylation activities (V-max), activation states, catalytic rates per active site (K-cat) and activation energies (E-a) of purified Rubisco and Rubisco in crude leaf extracts were determined for M.xgiganteus grown at 14 degrees C and 25 degrees C, and maize grown at 25 degrees C. The sequences of M.xgiganteus Rubisco small subunit mRNA are highly conserved, and 91% identical to those of maize. Although there were a few differences between the species in the translated protein sequences, there were no significant differences in the catalytic properties (V-max, K-cat, and E-a) for purified Rubisco, nor was there any effect of growth temperature in M.xgiganteus on these kinetic properties. Extracted activities were close to the observed rates of CO2 assimilation by the leaves in vivo. On a leaf area basis the extracted activities and activation state of Rubisco did not differ significantly, either between the two species or between growth temperatures. The activation state of Rubisco in leaf extracts showed no significant difference between warm and cold-grown M.xgiganteus. In total, these results suggest that the ability of M.xgiganteus to be productive and maintain photosynthetically competent leaves at low temperature does not result from low temperature acclimation or adaptation of the catalytic properties of Rubisco.