Nitrate reductase, a nitric oxide-producing enzyme: induction by pathogen signals

Nitric oxide (NO) is believed to act as an effector for defense signaling in plant cells. Nitrate reductase (NR) is one of the NO-producing enzymes in plants. Here, we report that infection of Phytophthora infestans, the fungal pathogen of potato late blight, into potato tubers caused a transient increase in the NR transcript in an incompatible, but not a compatible, interaction. Treatment of potato tubers with the fungal elicitor hyphal wall components (HWC) from the fungus induced NR gene at the transcriptional and protein level. Soluble proteins from HWC-treated tubers exhibited enhanced nitrite-dependent NO production. The inhibitor experiments for protein kinase and extracellular Ca2+ on the HWC-induced accumulation of the NR transcript suggested involvement of a calcium-independent protein kinase in regulation of the NR gene. Additionally, we isolated cDNA clones encoding NR from a potato tuber cDNA library; two isogenes were designated StNR5 and StNR6, respectively. Reverse transcription-polymerase chain reaction analyses using gene-specific primers showed that transcripts for both StNR5 and StNR6 were induced by HWC in potato tubers. This is the first report that pathogen signals induce expression of the NR gene.