Way back in 1982 in USA  I had a chance to discuss with the old botanist Laetsch  who published scholarly research paper detailing structure of chloroplasts in C4 plant using electron microscope and most of his pictures find way in most of the text books. He clearly demonstrated spatial separation of enzymes in mesophyll and bundle sheath cells . The mesophyll cells have PEPcase enzyme in their cytoplasm and initially fix the Carbon di oxide from HCO3  into Oxaloacetate which is rapidly converted into malate . I further recall a discussion reportedly between Melvin Calvin from California USA and Hatch and Slack from Australia who reported C4 cycle for the first time some decades ago. Melvin Calvin who was awarded Noble Prize for his discovery of C3 cycle suggested that there is only one cycle which fixes the Carbon di oxide as the  carbon di oxide initially fixed by PEPcase in bundle sheath cells into oxaloacetate and  malate due to the activity of PEPcase is decarboxylated in the Bundle sheath cells and its the enzyme RuBISCo by the pathway : the so called C3 cycle of Melvin Calvin. Thus Melvin Calvin was right in my opinion that its only cycle propounded by him truely fixes Carbon dio oxide and PEPcase plays a role to bring the Carbon di oxide to the vicinity of Bundle sheath chloroplasts.
In our experiments with plant tissue culture using Arachis hypogaea, and Daucus carota
( see our book Neumann , Kumar and Imani 2010 Springer) and published papers by us Kumar et al  1977, 1982, Kumar et al 1992) and also by a group of contemporary scientists ( Barz and Husemann during 70,s and 80,s it was established that cultured tissues of C3 plants have activity of PEPcase enzyme. We refrained in calling it a "C4" callus. The callus tissue from Maize in dedifferentiated stage also showed activity of both the enzymes RuBISCO and PEPcase depending on differention status  ( Kumar et al 1986) .
PEPcase has different isoforms and some of them have role anaplerotic C02 fixation , and some PEPcase fixed C02 in dark ( CAM cycle) . Recently scores of papers published in reputed journals claim to work in the direction of inserting genes of PEPcase into rice a step in making it a C4 plant.
I recall the statement of Laetsch that it requires spatial separation of enzymes to be called as C4 plant and how many genes are involved in controlling structure of " bundle sheath " AND WITHOUT CREATING THAT IN RICE HOW WE CAN CLAIM IT A STEP IN THE DIRECTION  OF C4
is a matter of debate.