The existence of C₄-bundle-sheath-like photosynthesis in the mid-vein of C₃ rice

Background: Recent studies have shown that C₄-like photosynthetic pathways partly reside in photosynthetic cells surrounding the vascular system of C₃ dicots. However, it is still unclear whether this is the case in C₃ monocots, especially at the molecular level. Results: In order to fill this gap, we investigated several characteristics required for C₄ photosynthesis, including C₄ pathway enzymes, cyclic/non-cyclic photophosphorylation rates, the levels and assembly state of photosynthetic machineries, in the mid-veins of C₃ monocots rice with leaf laminae used as controls. The signature of photosystem photochemistry was also recorded via non-invasive chlorophyll a fluorescence and reflectance changes at 820 nm in vivo. Our results showed that rice mid-veins were photosynthetically active with higher levels of three C₄ decarboxylases. Meanwhile, the linear electron transport chain was blocked in mid-veins due to the selective loss of dysfunctional photosystem II subunits. However, photosystem I was sufficient to support cyclic electron flow in mid-veins, reminiscent of the bundle sheath in C₄ plants. Conclusions: The photosynthetic attributes required for C₄ photosynthesis were identified for the first time in the monocotyledon model crop rice, suggesting that this is likely a general innate characteristic of C₃ plants which might be preconditioned for the C₄ pathway evolution. Understanding these attributes would provide a base for improved strategies for engineering C₄ photosynthetic pathways into rice.