On the cover: Most plants exhibit C3 photosynthesis, which is less efficient than the C4 photosynthetic pathway. The C4 pathway arose multiple independent times from the default C3 pathway, but the processes by which this occurred remains elusive and is the current focus of intensive studies ultimately aimed at synthesising the C4 pathway in C3 crops. This special focus issue organised by Taniguchi et al. (2016; pp.879-880) presents the latest research into the evolution, biochemistry and physiology of C4 biology and future prospective for C4 research.