Photosynthesis in C3-C4 intermediate Moricandia species

Evolution of C₄ photosynthesis is not distributed evenly in the plant kingdom. Particularly interesting is the situation in the Brassicaceae, because the family contains no C₄ species, but several C₃-C₄ intermediates, mainly in the genus Moricandia Investigation of leaf anatomy, gas exchange parameters, the metabolome, and the transcriptome of two C₃-C₄ intermediate Moricandia species, M. arvensis and M. suffruticosa, and their close C₃ relative M. moricandioides enabled us to unravel the specific C₃-C₄ characteristics in these Moricandia lines. Reduced CO₂ compensation points in these lines were accompanied by anatomical adjustments, such as centripetal concentration of organelles in the bundle sheath, and metabolic adjustments, such as the balancing of C and N metabolism between mesophyll and bundle sheath cells by multiple pathways. Evolution from C₃ to C₃-C₄ intermediacy was probably facilitated first by loss of one copy of the glycine decarboxylase P-protein, followed by dominant activity of a bundle sheath-specific element in its promoter. In contrast to recent models, installation of the C₃-C₄ pathway was not accompanied by enhanced activity of the C₄ cycle. Our results indicate that metabolic limitations connected to N metabolism or anatomical limitations connected to vein density could have constrained evolution of C₄ in Moricandia.