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In leaves of C 4 plants, the reactions of photosynthesis become restricted between two compartments. Typically, this allows accumulation of C 4 acids in mesophyll (M) cells and subsequent decarboxylation in the bundle sheath (BS). In C 4 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 C 4 dicotyledons also show this enhanced plasmodesmal connectivity and so whether this is a general requirement for C 4 photosynthesis is not clear. How M and BS cells in C 4 leaves become highly connected is also not known. We investigated these questions using 3D- and 2D-electron microscopy on the C 4 dicotyledon Gynandropsis gynandra as well as phylogenetically close C 3 relatives. The M-BS interface of C 4 G. gynandra showed higher plasmodesmal frequency compared with closely related C 3 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 C 4 G. gynandra., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

Arabidopsis thaliana mutants lacking the SS4 isoform of starch synthase have strongly reduced numbers of starch granules per chloroplast, suggesting that SS4 is necessary for the normal generation of starch granules. To establish whether it plays a direct role in this process, we investigated the circumstances in which granules are formed in ss4 mutants. Starch granule numbers and distribution and the accumulation of starch synthase substrates and products were investigated during ss4 leaf development, and in ss4 mutants carrying mutations or transgenes that affect starch turnover or chloroplast volume. We found that immature ss4 leaves have no starch granules, but accumulate high concentrations of the starch synthase substrate ADPglucose. Granule numbers are partially restored by elevating the capacity for glucan synthesis (via expression of bacterial glycogen synthase) or by increasing the volumes of individual chloroplasts (via introduction of arc mutations). However, these granules are abnormal in distribution, size and shape. SS4 is an essential component of a mechanism that coordinates granule formation with chloroplast division during leaf expansion and determines the abundance and the flattened, discoid shape of leaf starch granules., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)