Plasmodesmal connectivity in C₄ Gynandropsis gynandra is induced by light and dependent on photosynthesis

In leaves of C₄ plants, the reactions of photosynthesis become restricted between two compartments. Typically, this allows accumulation of C₄ acids in mesophyll (M) cells and subsequent decarboxylation in the bundle sheath (BS). In C₄ grasses, proliferation of plasmodesmata between these cell types is thought to increase cell-to-cell connectivity to allow efficient metabolite movement. However, it is not known whether C₄ dicotyledons also show this enhanced plasmodesmal connectivity and so whether this is a general requirement for C₄ photosynthesis is not clear. How M and BS cells in C₄ leaves become highly connected is also not known. We investigated these questions using 3D- and 2D-electron microscopy on the C₄ dicotyledon Gynandropsis gynandra as well as phylogenetically close C₃ relatives. The M-BS interface of C₄ G. gynandra showed higher plasmodesmal frequency compared with closely related C₃ species. Formation of these plasmodesmata was induced by light. Pharmacological agents that perturbed photosynthesis reduced the number of plasmodesmata, but this inhibitory effect could be reversed by the provision of exogenous sucrose. We conclude that enhanced formation of plasmodesmata between M and BS cells is wired to the induction of photosynthesis in C₄ G. gynandra.