Melatonin is required for H2O2‐ and NO‐mediated defense signaling through MAPKKK3 and OXI1 in Arabidopsis thaliana

Abstract 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 MAPKKK 3 and OXI 1 (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 DC 3000 (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 PR 1 and ICS 1 , 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 MAPKKK 3 and OXI 1, followed by triggering of downstream MAPK cascades, such as MAPK 3 and MAPK 6.