PUB - Publikationen an der Universität Bielefeld

C-4 photosynthesis is a complex trait that boosts productivity in tropical conditions. Compared with C-3 species, the C-4 state seems to require numerous novelties, but species comparisons can be confounded by long divergence times. Here, we exploit the photosynthetic diversity that exists within a single species, the grass Alloteropsis semialata, to detect changes in gene expression associated with different photosynthetic phenotypes. Phylogenetically informed comparative transcriptomics show that intermediates with a weak C-4 cycle are separated from the C-3 phenotype by increases in the expression of 58 genes (0.22% of genes expressed in the leaves), including those encoding just three core C-4 enzymes: aspartate aminotransferase, phosphoenolpyruvate carboxykinase, and phosphoenolpyruvate carboxylase. The subsequent transition to full C-4 physiology was accompanied by increases in another 15 genes (0.06%), including only the core C-4 enzyme pyruvate orthophosphate dikinase. These changes probably created a rudimentary C-4 physiology, and isolated populations subsequently improved this emerging C-4 physiology, resulting in a patchwork of expression for some C-4 accessory genes. Our work shows how C-4 assembly in A. semialata happened in incremental steps, each requiring few alterations over the previous step. These create short bridges across adaptive landscapes that probably facilitated the recurrent origins of C-4 photosynthesis through a gradual process of evolution.