On the mechanism of C4 photosynthesis intermediate exchange between Kranz mesophyll and bundle sheath cells in grasses

C-4 photosynthesis involves cell-to-cell exchange of photosynthetic intermediates between the Kranz mesophyll (KMS) and bundle sheath (BS) cells. This was believed to occur by simple diffusion through plentiful plasmodesmatal (PD) connections between these cell types. The model of C-4 intermediates' transport was elaborated over 30 years ago and was based on experimental data derived from measurements at the time. The model assumed that plasmodesmata occupied about 3% of the interface between the KMS and BS cells and that the plasmodesmata structure did not restrict metabolite movement. Recent advances in the knowledge of plasmodesmatal structure put these assumptions into doubt, so a new model is presented here taking the new anatomical details into account. If one assumes simple diffusion as the sole driving force, then calculations based on the experimental data obtained for C-4 grasses show that the gradients expected of C-4 intermediates between KMS and BS cells are about three orders of magnitude higher than experimentally estimated. In addition, if one takes into account that the plasmodesmata microchannel diameter might constrict the movement of C-4 intermediates of comparable Stokes' radii, the differences in concentration of photosynthetic intermediates between KMS and BS cells should be further increased. We believe that simple diffusion-driven transport of C-4 intermediates between KMS and BS cells through the plasmodesmatal microchannels is not adequate to explain the C-4 metabolite exchange during C-4 photosynthesis. Alternative mechanisms are proposed, involving the participation of desmotubule and/or active mechanisms as either apoplasmic or vesicular transport.