Mitogen-activated protein kinase 3 and 6 regulate Botrytis cinerea-induced ethylene production in Arabidopsis

Plants challenged by pathogens, especially necrotrophic fungi such as Botrytis cinerea, produce high levels of ethylene. At present, the signaling pathways underlying the induction of ethylene after pathogen infection are largely unknown. MPK6, an Arabidopsis stress-responsive mitogen-activated protein kinase (MAPK) was previously shown to regulate the stability of ACS2 and ACS6, two type I ACS isozymes (1-amino-cyclopropane-1-carboxylic acid synthase). Phosphorylation of ACS2 and ACS6 by MPK6 prevents rapid degradation of ACS2/ACS6 by the 26S proteasome pathway, resulting in an increase in cellular ACS activity and ethylene biosynthesis. Here, we show that MPK3, which shares high homology and common upstream MAPK kinases with MPK6, is also capable of phosphorylating ACS2 and ACS6. In the mpk3 mutant background, ethylene production in gain-of-function GVG-NtMEK2(DD) transgenic plants was compromised, suggesting that MPK6 and MPK3 function together to stabilize ACS2 and ACS6. Using a liquid-cultured seedling system, we found that B. cinerea-induced ethylene biosynthesis was greatly compromised in mpk3/mpk6 double mutant seedlings. In contrast, ethylene production decreased only slightly in the mpk6 single mutant and not at all in the mpk3 single mutant, demonstrating overlapping roles for these two highly homologous MAPKs in pathogen-induced ethylene induction. Consistent with the role of MPK3/MPK6 in the process, mutation of ACS2 and ACS6, two genes encoding downstream substrates of MPK3/MPK6, also reduced B. cinerea-induced ethylene production. The residual levels of ethylene induction in the acs2/acs6 double mutant suggest the involvement of additional ACS isoforms, possibly regulated by MAPK-independent pathway(s).