Activation of a stress-responsive mitogen-activated protein kinase cascade induces the biosynthesis of ethylene in plants

Plants under stress from both biotic and abiotic sources produce increased levels of ethylene, which is perceived by ethylene receptors and triggers cellular responses further downstream. Protein phosphorylation and dephosphorylation were implicated in the regulation of ethylene induction by stresses based on studies using protein kinase and phosphatase inhibitors. However, the kinase(s) involved remains to be determined. Using a conditional gain-of-function transgenic system, we demonstrate that the activation of SIPK, a tobacco mitogen-activated protein kinase (MAPK), by NtMEK2(DD), an active mutant of the upstream kinase of SIPK, resulted in a dramatic increase in ethylene production. The increase in ethylene after the activation of SIPK coincided with a dramatic increase in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) activity, which was followed by the activation of a subgroup of ACS and ACC oxidase (ACO) genes, suggesting that either the activation of unidentified ACS(s) or post-transcriptional regulation is involved. Infection with Tobacco mosaic virus (TMV), which is known to activate the SIPK cascade and induce ethylene biosynthesis, also induced the same ACSs and ACOs. After ethylene production in NtMEK2(DD) plants, strong activation of ETHYLENE-RESPONSE FACTOR (ERF) genes was observed, similar to the effect in NN tobacco plants infected with TMV. In contrast to previous reports, no major increase in jasmonic acid (JA) and methyl jasmonate (MJ) was detected after the activation of SIPK/WIPK in NtMEK2(DD) transgenic plants. These results suggest that the induction of ethylene but not JA/MJ is involved in plant defense responses mediated by the NtMEK2-SIPK/WIPK pathway.