Melatonin is required for H 2 O 2 - and NO-mediated defense signaling through MAPKKK3 and OXI1 in Arabidopsis thaliana.

Melatonin influences plant innate immunity through the mitogen-activated protein kinase (MAPK) pathway. However, the most upstream MAPK component in melatonin signaling and the dependence of generation of a reactive oxygen species (ROS) burst on melatonin synthesis and signaling remain unclear. In this study, treatment of several mekk (alias mapkkk)-knockout Arabidopsis mutants with melatonin revealed that the MAPKKK3 and OXI1 (oxidative signal-inducible1) kinases are responsible for triggering melatonin-induced defense signaling pathways. In addition, melatonin induction upon infection with the avirulent pathogen Pseudomonas syringae DC3000 (avrRpt2) was independent of H ₂ O ₂ and NO individually, but dependent on the combination of H ₂ O ₂ and NO. Moreover, melatonin-mediated induction of the expression of defense-related genes, such as PR1 and ICS1, was not altered in the H ₂ O ₂ -deficient rbohD/F-knockout mutant cotreated with an NO scavenger, indicating that melatonin functions downstream of the ROS and NO burst. Collectively, the data indicate that melatonin-mediated induction of an innate immune response requires multiple signaling molecules and activation of MAPKKK3 and OXI1, followed by triggering of downstream MAPK cascades, such as MAPK3 and MAPK6.
(© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)