Evolution of C4 phosphoenolpyruvate carboxykinase in grasses, from genotype to phenotype

Abstract

C4 photosynthesis is an adaptation over the classical C 3 pathway that has evolved multiple times independently. These convergences are accompanied by strong variations among the independent C 4 lineages. The decarboxylating enzyme used to release CO2 around Rubisco particularly differs between C4 species, a criterion used to distinguish three distinct biochemical C4 subtypes. The phosphoenolpyruvate carboxykinase (PCK) serves as a primary decarboxylase in a minority of C4 species. This enzyme is also present in C3 plants, where it is responsible for nonphotosynthetic functions. The genetic changes responsible for the evolution of C4-specific PCK are still unidentified. Using phylogenetic analyses on PCK sequences isolated from C 3 and C4 grasses, this study aimed at resolving the evolutionary history of C4-specific PCK enzymes. Four independent evolutions of C4-PCK were shown to be driven by positive selection, and nine C4-adaptive sites underwent parallel genetic changes in different C4 lineages. C4-adaptive residues were also observed in C4 species from the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) subtype and particularly in all taxa where a PCK shuttle was previously suggested to complement the NADP-ME pathway. Acquisitions of C4-specific PCKs were mapped on a species tree, which revealed that the PCK subtype probably appeared at the base of the Chloridoideae subfamily and was then recurrently lost and secondarily reacquired at least three times. Linking the genotype to subtype phenotype shed new lights on the evolutionary transitions between the different C4 subtypes. © The Author 2008. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved.