Brassicaceae display variation in efficiency of photorespiratory carbon recapturing mechanisms.

Carbon concentrating mechanisms enhance the carboxylase efficiency of the central photosynthetic enzyme rubisco by providing supra-atmospheric concentrations of CO2 in its surrounding. In the C4 photosynthesis pathway, this feat is realised by combinatory changes to leaf biochemistry and anatomy. In contrast to the C4 pathway, carbon concentration can also be achieved by the photorespiratory glycine shuttle which requires fewer and less complex modifications. Plants displaying CO2 compensation points between 10 to 40 ppm are often considered to utilize such a photorespiratory shuttle and are termed 'C3-C4 intermediates'. In the present study, we perform a physiological, biochemical and anatomical survey of a large number of Brassicaceae species to better understand the C3-C4 intermediate phenotype, including its basic components and its plasticity. Our phylogenetic analysis suggested that C3-C4 metabolism evolved up to five times independently in the Brassicaceae. The efficiency of the pathway showed considerable variation between tested plant species. Centripetal accumulation of organelles in the bundle sheath was consistently observed in all C3-C4 classified taxa indicating a crucial role of anatomical features for CO2 concentrating pathways. Leaf metabolite patterns were strongly influenced by the individual species, but accumulation of photorespiratory shuttle metabolites glycine and serine was generally observed. Analysis of PEPC activities and metabolite composition suggests that C4-like shuttles have not evolved in the investigated Brassicaceae. Convergent evolution of the photorespiratory shuttle indicates that it represents a distinct and fit photosynthesis type.