| Summary: | Background Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the key reaction in the photosynthetic assimilation of CO2. In C4 plants CO2 is supplied to Rubisco by an auxiliary CO2-concentrating pathway that helps to maximize the carboxylase activity of the enzyme while suppressing its oxygenase activity. As a consequence, C4 Rubisco exhibits a higher maximum velocity but lower substrate specificity compared with the C3 enzyme. Specific amino-acids in Rubisco are associated with C4 photosynthesis in monocots, but it is not known whether selection has acted on Rubisco in a similar way in eudicots. Methodology/Principal Findings We investigated Rubisco evolution in Amaranthaceae sensu lato (including Chenopodiaceae), the third-largest family of C4 plants, using phylogeny-based maximum likelihood and Bayesian methods to detect Darwinian selection on the chloroplast rbcL gene in a sample of 179 species. Two Rubisco residues, 281 and 309, were found to be under positive selection in C4 Amaranthaceae with multiple parallel replacements of alanine by serine at position 281 and methionine by isoleucine at position 309. Remarkably, both amino-acids have been detected in other C4 plant groups, such as C4 monocots, illustrating a striking parallelism in molecular evolution. Conclusions/Significance Our findings illustrate how simple genetic changes can contribute to the evolution of photosynthesis and strengthen the hypothesis that parallel amino-acid replacements are associated with adaptive changes in Rubisco.
|
|---|