Abstract
C4 photosynthesis results from a set of anatomical features and biochemical components that act together to concentrate CO2 within the leaf and boost productivity. This complex trait evolved independently many times, resulting in various realizations of the phenotype, but in all C4 plants the primary fixation of atmospheric carbon is catalyzed by phosphoenolpyruvate carboxylase. Comparisons of C4 and non-C4 PEPC from a few closely related species suggested that the enzyme was modified to meet the demands of the C4 cycle. However, very few C4 groups have been investigated, hampering general conclusions. To test the hypothesis that distant C4 lineages underwent convergent biochemical changes, we compare the kinetic variation between C4 and non-C4 PEPC 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-C4 and C4 PEPC 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 lineage Panicum, which might indicate that optimization of PEPC for the C4 context increases with evolutionary time.
Highlights
C4 photosynthesis is a CO2-concentrating mechanism that boosts productivity in tropical conditions (Atkinson et al, 2016)
In Flaveria, the C4 and non-C4 ppc-1E2 genes corresponding to the C4 F. trinervia and the C3 species F. pringlei were analyzed [ppcA as described in Svensson et al (1997)]
The two nonC4 isoforms differ in their KmPEP which is three-fold lower in the F. pringlei enzyme (Table 1)
Summary
C4 photosynthesis is a CO2-concentrating mechanism that boosts productivity in tropical conditions (Atkinson et al, 2016). PEPC produces the four-carbon acid oxaloacetate, which is rapidly converted into the more stable four-carbon acids malate and/or aspartate (Bräutigam et al, 2014) These acids are shuttled to a cell isolated from the atmosphere in which Rubisco is localized, and CO2 is released. This biochemical pumping of CO2 leads to an increase of the relative concentration of CO2 by a factor of 10 when compared to a non-C4 cell, and a dramatic increase of photosynthetic efficiency at high temperature (Ehleringer and Bjorkman, 1977; von Caemmerer and Furbank, 2003; Sage, 2004; Sage et al, 2012)
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