Ancient duons may underpin spatial patterning of gene expression in C₄ leaves

If the highly efficient C₄ photosynthesis pathway could be transferred to crops with the C₃ pathway there could be yield gains of up to 50%. It has been proposed that the multiple metabolic and developmental modifications associated with C₄ photosynthesis are underpinned by relatively few master regulators that have allowed the evolution of C₄ photosynthesis more than 60 times in flowering plants. Here we identify a component of one such regulator that consists of a pair of cis-elements located in coding sequence of multiple genes that are preferentially expressed in bundle sheath cells of C₄ leaves. These motifs represent duons as they play a dual role in coding for amino acids as well as controlling the spatial patterning of gene expression associated with the C₄ leaf. They act to repress transcription of C₄ photosynthesis genes in mesophyll cells. These duons are also present in the C₃ model Arabidopsis thaliana, and, in fact, are conserved in all land plants and even some algae that use C₃ photosynthesis. C₄ photosynthesis therefore appears to have coopted an ancient regulatory code to generate the spatial patterning of gene expression that is a hallmark of C₄ photosynthesis. This intragenic transcriptional regulatory sequence could be exploited in the engineering of efficient photosynthesis of crops.