Either soluble or plastidic expression of recombinant protoporphyrinogen oxidase modulates tetrapyrrole biosynthesis and photosynthetic efficiency in transgenic rice

Protoporphyrinogen oxidase (Protox) is the last shared enzyme of the porphyrin pathway. As a continuation of our previous work in which the transgenic rice plants expressing the Bacillus subtilis Protox in the cytoplasm or the plastid showed resistance to diphenyl ether herbicide, this study was undertaken to identify the effects of tertapyrrole biosynthesis in these transgenic rice plants. The transgenic plants either targeted into plastids or expressed in cytoplasm showed higher Protox activity than wild-type plants did. Photosynthetic activity, measured as a quantum yield of photosystem 2, was slightly higher in transgenic plants than in wildtype plants, but chloropbyll contents were not significantly different between transgenic and wild-type plants. As for porphyrin biosynthesis, both cytoplasm-expressed and plastid-targeted transgenic plants showed increased synthesis of aminolevulinic acid, Mg-Proto 9, and protoheme in comparison to wild-type plants whereas synthesis of protoporphyrin 9 was similar for wild-type and transgenic plants. These results indicate that either cytoplasm or plastid expression of B. subtilis Protox in rice can upregulate the porphyrin pathway leading to increase in photosynthetic efficiency in plants.