The Evolutionary Origin of C₄ Photosynthesis in the Grass Subtribe Neurachninae

The Australian grass subtribe Neurachninae contains closely related species that use C-3, C-4, and C-2 photosynthesis. To gain insight into the evolution of C-4 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 C-3-C-4 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 C-2 and C-4 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 C-3 Neurachninae species with C-* values of 56-61 mu mol mol(-1), whereas two-thirds of leaf GLDP was in the MS of Neurachne lanigera, which exhibits a newly-identified, partial C-2 phenotype with C-* of 44 mu mol mol(-1). Increased investment of GLDP in MS tissue of the C-2 species was attributed to more MS mitochondria and less GLDP in mesophyll mitochondria. These results are consistent with a model where C-4 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.