A transcription factor module mediating C(2) photosynthesis in the Brassicaceae.

C(4) photosynthesis has arisen from the ancestral C(3) state in over sixty lineages of angiosperms. It is widely accepted that an early step in C(4) evolution is associated with the appearance of so-called C(2) photosynthesis caused by loss of glycine decarboxylase activity from mesophyll cells followed by activation in the bundle sheath. Although changes in cis to a distal enhancer upstream of the P-subunit of GLYCINE DECARBOXYLASE (GLDP) from C(2) Moricandia enable loss of expression from mesophyll cells, the mechanism then allowing GLDP expression in the bundle sheath is not known. Here we identify a MYC-MYB transcription factor module previously associated with the control of glucosinolate biosynthesis as the basis of this foundational event in the evolution of C(2) photosynthesis. Specifically, we find that in the C(3) state this MYC-MYB module already patterns GLDP expression to bundle sheath cells. As a consequence, when GLDP expression is lost from the mesophyll, the MYC-MYB dependent expression in the bundle sheath is revealed. Evolution of C(2) photosynthesis is thus associated with a MYC-MYB based transcriptional network already present in the C(3) state. This work identifies a molecular genetic mechanism underlying the bundle sheath accumulation of glycine decarboxylase required for C(2) photosynthesis and thus a fundamental step in the evolution of C(4) photosynthesis.