Nitrate reductase-mediated nitric oxide generation is essential for fungal elicitor-induced camptothecin accumulation of Camptotheca acuminata suspension cell cultures

Secondary metabolite accumulation and nitric oxide (NO) generation are two common responses of plant cells to fungal elicitors, and NO has been reported to play important roles in elicitor-induced secondary metabolite production. However, the source of elicitor-triggered NO generation in plant cells remains largely unknown. To investigate the origin of elicitor-triggered NO, we examined nitrate reductase (NR) activities and the expression levels of NIA1 and NIA2 genes of Camptotheca acuminata cells treated with PB90, a protein elicitor from Phytophthora boehmeriae. The data show that PB90 treatment stimulates NR activity and induces upregulation of NIA1 but does not affect NIA2 expression in the cells. Pretreatment of the cells with NR inhibitors tungstate and Gln abolishes not only the fungal elicitor-triggered NR activities but also the PB90-induced NO generation. Treatment of PB90 enhances camptothecin contents of the cells, suggesting that the fungal elicitor might stimulate camptothecin biosynthesis. Furthermore, application of tungstate and Gln suppresses the fungal elicitor-induced camptothecin accumulation of the cells and the suppression of NR inhibitors on PB90-induced camptothecin production can be reversed by NO via its donor sodium nitroprusside. Together, the results suggest that NIA1 is sensitive to PB90 and the fungal elicitor-induced upregulation of NIA1 may lead to higher NR activity. Furthermore, our data demonstrate that NR is involved in the fungal elicitor-triggered NO generation and the fungal elicitor induces camptothecin production of C. acuminata cells dependently on NR-mediated NO generation.