Bundle Sheath Patterning in Maize for Engineering C4 Photosynthesis

A worldwide effort is underway to engineer the C4 photosynthetic pathway in rice and other C3 crops. The major obstacle at present is that the genetic basis for development of specialized C4 leaf anatomy is a mystery. The key to C4 specialization is that two independent carbon fixation reactions must be compartmentalized for C4 photosynthesis to work. In maize, and many other C4 plants, compartmentation is accomplished by a unique leaf architecture, known as Kranz anatomy, in which bundle sheath (BS) and mesophyll cells are arranged in concentric rings around the veins. Investigations into anatomy and development of this tissue has lead to a new hypothesis that would explain the repeated convergent evolution of Kranz-type C4 photosynthesis in plants. In brief, Kranz anatomy and C4 physiology is a result of the endodermal tissue, usually only present in roots and stems, projecting into the leaf blade. Using reverse genetics to identify mutants in maize, we were able to show the Scarecrow gene, which regulates endodermal identity in roots, also regulates Kranz anatomy. Mutations in Scarecrow result in proliferation of BS cells, abnormal differentiation of BS cells and BS chloroplasts, loss of minor veins, and reduction of vein density, which are all key features of C4 structure. This finding, that C4 may arise from an established and highly conserved repertoire of signals that governs endodermal differentiation in roots and stems, casts a more optimistic light on the task of engineering Kranz-type C4 into C3 crops.