A model depicting the dual-level regulation of ACS activity by MPK3/MPK6-dependent and independent pathways during pathogen-induced ethylene production.

<p>Members of all three types of ACS isoforms are involved in pathogen-induced ethylene production. In <i>B. cinerea</i>-infected plants, Type I (ACS2/ACS6) isoforms contribute the most (∼85%). ACS2 and ACS6 are regulated by the MPK3/MPK6 cascade at both transcriptional and protein stability levels. The transcriptional up-regulation is mediated by WRKY33, a MPK3/MPK6 substrate. Type II (ACS8 and ACS11) and Type III (ACS7) isoforms are activated at the transcriptional level although the regulatory pathway(s) involved is not clear at present. Increase in total cellular ACS activity drives the elevated ethylene production, which triggers downstream responses.