Insights into the Evolution of C4 Photosynthesis in the Grasses

Most evidence supports the idea that the C₄ photosynthetic pathway arose via a series of innovations that initially enabled plants to refix a large fraction of photorespired CO₂ via the shuttling of Gly from the mesophyll (M) to a sheath of cells surrounding the vascular tissue, where the Gly is decarboxylated. The sheath layer is most commonly the bundle sheath (BS) cells. Associated with the shuttling of Gly is a shift in the expression of the mitochondrial enzyme Gly decarboxylase (GDC) from M cells to the vascular sheath tissue. This also requires that the Gly molecules produced in photorespiration diffuse from M cells to the sheath cells for metabolism. Gly shuttling, also known as C₂ photosynthesis, elevates CO₂ levels in the BS by a factor of two to three times relative to M cells, thereby improving Rubisco efficiency. The C₂ condition is present in more than 40 intermediate species that branch between C₃ and C₄ species on the phylogenetic trees of more than a dozen distinct C₄ lineages. The Australian grass subtribe Neurachninae is interesting to study because it contains closely related species that use C₃, C₄, and C₂ photosynthesis. Since most previous research in this area has examined eudicot species, Khoshravesh et al. (10.1104/pp.19.00925) examined leaf gas exchange, anatomy and ultrastructure, and tissue localization of Gly decarboxylase subunit P (GLDP) in nine Neurachninae species. They identify previously unrecognized variations in leaf structure and physiology within the Neurachne that represents varying degrees of C₃–C₄ intermediacy. Their results are consistent with a model wherein C₄ evolution in the Neurachninae initially occurred via an increase in organelle and GLDP content in MS cells, which generated a sink for photorespired CO₂ in MS tissues. This research indicates the potential of the Neurachninae to provide novel insights into C₄ evolution in grasses, particularly in the initial transition from a C₃ character state.