A protoporphyrinogen oxidase gene expression influences responses of transgenic rice to oxyfluorfen

We compared the defense against photoinhibitions and oxidative stress in transgenic rice expressing a modified Myxococcus xanthus protoporphyrinogen oxidase (MxProtox) gene and in wild-type (WT) rice. Although the MxProtox transgenic lines had higher content of protoporphyrin IX (Proto IX) than the untreated controls, they did not exhibit a drastic accumulation of Proto IX as in the WT after 2 d of 50 mu M oxyfluorfen (OF) treatment. In the transgenic lines S4 and S11, the transcriptions of OsProtox and modified MxProtox genes were almost sustained in response to OF, although transcription of OsProtox was greatly down-regulated in the WT. The excess Proto IX in the WT plants treated with OF generated a severe stress mediated by singlet oxygen (O-1(2)), leading to a prominent increases in electrolyte leakage and malondialdehyde production. This stress in the WT necessitated not only a substantial accumulation of zeaxanthin and antheraxanthin, but also strong increases in activities of superoxide dismutase, catalase, and peroxidase as well as transcriptions of CatalaseB, Ascorbate Peroxidase, and Heme Oxygenase2 genes. By contrast, the transgenic plants did not result in any noticeable increase in these parameters. Our results demonstrate that the transgenic rice expressing modified MxProtox efficiently prevented accumulation of photosensitizing Proto IX through sustaining higher transcriptions of porphyrin biosynthetic genes, thereby reducing the stress imposed by OF.