The Evolutionary Origin of C₄ Photosynthesis in the Grass Subtribe Neurachninae

The Australian grass subtribe Neurachninae contains closely related species that use C₃, C₄, and C₂ photosynthesis. To gain insight into the evolution of C₄ photosynthesis in grasses, we examined leaf gas exchange, anatomy and ultrastructure, and tissue localization of Gly decarboxylase subunit P (GLDP) in nine Neurachninae species. We identified previously unrecognized variation in leaf structure and physiology within Neurachne that represents varying degrees of C₃-C₄ intermediacy in the Neurachninae. These include inverse correlations between the apparent photosynthetic carbon dioxide (CO₂) compensation point in the absence of day respiration (C ) and chloroplast and mitochondrial investment in the mestome sheath (MS), where CO₂ is concentrated in C₂ and C₄ Neurachne species; width of the MS cells; frequency of plasmodesmata in the MS cell walls adjoining the parenchymatous bundle sheath; and the proportion of leaf GLDP invested in the MS tissue. Less than 12% of the leaf GLDP was allocated to the MS of completely C₃ Neurachninae species with C values of 56-61 μmol mol^-1, whereas two-thirds of leaf GLDP was in the MS of Neurachne lanigera, which exhibits a newly-identified, partial C₂ phenotype with C of 44 μmol mol^-1 Increased investment of GLDP in MS tissue of the C₂ species was attributed to more MS mitochondria and less GLDP in mesophyll mitochondria. These results are consistent with a model where C₄ evolution in Neurachninae initially occurred via an increase in organelle and GLDP content in MS cells, which generated a sink for photorespired CO₂ in MS tissues.