Isolation and functional characterization of the mitogen-activated protein kinase kinase gene MAPKK1 from Panax notoginseng

Mitogen-activated protein kinase (MAPK) cascades are important signal transduction mechanisms that connect plant cellular and nuclear responses, which play key roles in plant development and stress responses. Here, the novel MAPK kinase (MAPKK) geneMAPKK1was isolated fromPanax notoginseng(Burk) F.H. Chen. Exogenous methyl jasmonate (MeJA), salicylic acid (SA), ethylene, and hydrogen peroxide treatments induced the transcription level ofPnMAPKK1inP. notoginsengroots. Additionally,PnMAPKK1expression actively responded toFusarium solaniinfection, a major causal agent ofP. notoginsengroot rot disease. ThePnMAPKK1gene was further fused with the green fluorescent protein gene in a plant expression vector and transformed into onion (Allium cepa) epidermal cells. Laser scanning confocal microscopy confirmed that the PnMAPKK1 protein localized in the cytoplasm. In addition, the plant overexpression vector pCAMBIA2300s-PnMAPKK1was constructed and transformed into tobacco (Nicotiana tabacumL. cv. Xanthi). ThePnMAPKK1transgenic tobacco lines showed much stronger resistance levels toF. solaniinfection than the wild type. Moreover, the JA and SA contents in thePnMAPKK1transgenic tobacco lines were significantly higher than those in wild type duringF. solaniinfection. The up-regulation of several marker genes (PAL4,PR1c,WRKY1,Defensin, andLOC4) involved in JA and SA signaling was correlated with the overexpression ofPnMAPKK1in tobacco. Thus, thePnMAPKK1gene is a positive regulator of defense responses toF. solaniinP.notoginseng.