Cold tolerance of C-4 photosynthesis in Miscanthus x giganteus: Adaptation in amounts and sequence of C-4 photosynthetic enzymes
Field-grown Miscanthus X giganteus maintains high photosynthetic quantum yields and biomass productivity in cool temperate climates. It is related to maize (Zea mays) and uses the same NADP-malic enzyme C-4 pathway. This study tests the hypothesis that M. X giganteus, in contrast to maize, forms photosynthetically competent leaves at low temperatures with altered amounts of pyruvate orthophosphate dikinase (PPDK) and Rubisco or altered properties of PPDK. Both species were grown at 25degreesC/20degreesC or 14degreesC/11degreesC (day/night), and leaf photosynthesis was measured from 5degreesC to 38degreesC. Protein and steady-state transcript levels for Rubisco, PPDK, and phosphoenolpyruvate carboxylase were assessed and the sequence of C-4-PPDK from M. X giganteus was compared with other C-4 species. Low temperature growth had no effect on photosynthesis in M. X giganteus, but decreased rates by 80% at all measurement temperatures in maize. Amounts and expression of phosphoenolpyruvate carboxylase were affected little by growth temperature in either species. However, PPDK and Rubisco large subunit decreased >50% and >30%, respectively, in cold-grown maize, whereas these levels remained unaffected by temperature in M. X giganteus. Differences in protein content in maize were not explained by differences in steady-state transcript levels. Several different M. X giganteus C-4-PPDK cDNA sequences were found, but putative translated protein sequences did not show conservation of amino acids contributing to cold stability in Flaveria brownii C-4-PPDK. The maintenance of PPDK and Rubisco large subunit amounts in M. X giganteus is consistent with the hypothesis that these proteins are critical to maintaining high rates of C-4 photosynthesis at low temperature.