Kinetic Modifications of C 4 PEPC Are Qualitatively Convergent, but Larger in Panicum Than in Flaveria .

C(4)photosynthesis results from a set of anatomical features and biochemical components that act together to concentrate CO(2)within the leaf and boost productivity. This complex trait evolved independently many times, resulting in various realizations of the phenotype, but in all C(4)plants the primary fixation of atmospheric carbon is catalyzed by phosphoenolpyruvate carboxylase. Comparisons of C(4)and non-C4PEPC from a few closely related species suggested that the enzyme was modified to meet the demands of the C(4)cycle. However, very few C(4)groups have been investigated, hampering general conclusions. To test the hypothesis that distant C(4)lineages underwent convergent biochemical changes, we compare the kinetic variation between C(4)and non-C4PEPC from a previously assessed young lineage (Flaveria, Asteraceae) with those from an older lineage found within the distantly related grass family (Panicum). Despite the evolutionary distance, the kinetic changes between the non-C(4)and C4PEPC are qualitatively similar, with a decrease in sensitivity for inhibitors, an increased specificity (k(cat)/K-m) for bicarbonate, and a decreased specificity (k(cat)/K-m) for PEP. The differences are more pronounced in the older lineagePanicum, which might indicate that optimization of PEPC for the C(4)context increases with evolutionary time.