C ₄ anatomy can evolve via a single developmental change

Abstract

C ₄ photosynthesis is a complex trait that boosts productivity in warm environments. Paradoxically, it evolved independently in numerous plant lineages, despite requiring specialised leaf anatomy. The anatomical modifications underlying C ₄ evolution have previously been evaluated through interspecific comparisons, which capture numerous changes besides those needed for C ₄ functionality. Here, we quantify the anatomical changes accompanying the transition between non‐C ₄ and C ₄ phenotypes by sampling widely across the continuum of leaf anatomical traits in the grass Alloteropsis semialata. Within this species, the only trait that is shared among and specific to C ₄ individuals is an increase in vein density, driven specifically by minor vein development that yields multiple secondary effects facilitating C ₄ function. For species with the necessary anatomical preconditions, developmental proliferation of veins can therefore be sufficient to produce a functional C ₄ leaf anatomy, creating an evolutionary entry point to complex C ₄ syndromes that can become more specialised.