Suppressing photorespiration in rice leaves using artificial microRNAs to silence glycine decarboxylase activity

Photorespiration is an energy wasteful process in plants that serves as a way to recover fixed carbon as a result of the oxygenase activity of Rubisco. The observations on plants having C3, C3-C4 intermediate, C4-like, and C4 photosynthesis shows that the gradual relocation of the photorespiratory cycle from the mesophyll cells to the bundle sheath cells triggered the evolution of C4 plants. Engineering C3 crops, such as rice, to perform C4 photosynthesis would provide the necessary increase in their yield potential needed to cope with growing human population. One of the core enzymes of the photorespiratory cycle is the Glycine Decarboxylase Complex (GDC). GDC along with serine-hydroxymethyl transferase converts two glycine molecules into one serine molecule with the concomitant release of carbon dioxide and ammonia. To trigger C4 phenomena in rice, an artificial microRNA (amiRNA) was designed to silence the GDCH expression in mesophyll cells using maize PEP-carboxylase promoter. Stopping the activity of the GDC in mesophyll cells would lead in an increase in glycine concentration that can be transported to bundle sheath cells where it can be decarboxylated, and act as a secondary CO₂ concentrating mechanism. GDCH is encoded by a small gene family composed of Os02g07410, Os06g45670, and Os10g37180. The amiRNA was designed to target Os10g37180 isoform which is highly expressed in leaves and photosynthetic tissues. Analysis of homozygous transgenic plants showed no expression of the Os10g37180 gene in the leaves using RT-PCR and western blot. Reduction of Os10g37180 expression affected photosynthetic activity and growth of transgenic plants however the plants can still tolerate ambient CO₂ condition. Among the other isoforms only Os02g07410 were still expressed in the leaves of the transgenic plants which presents the possibility that it could complement the function of Os10g37180 in leaves. Current experiment is being done to target Os02g07410 using the same amiRNA technology into the Os10g37180 knockdown plant.