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

Abstract

The previous investigations show that the amount and activity of Rubisco appears the major limitation to effective C₄ photosynthesis at low temperatures. The chilling-tolerant and bioenergy feedstock species Miscanthus×giganteus (M.×giganteus) is exceptionally productive among C₄ grasses in cold climates. It is able to develop photosynthetically active leaves at temperatures 6 °C below the minimum for maize, and achieves a productivity even at 52° N that exceeds that of the most productive C₃ 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 C₄ 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.×giganteus grown at 14 °C and 25 °C, and maize grown at 25 °C. The sequences of M.×giganteus 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.×giganteus 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.×giganteus. In total, these results suggest that the ability of M.×giganteus 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.