AANAT and infection: Using the knockout mutant of serotonin N-acetyltransferase (SNAT) which is the penultimate enzyme in melatonin biosynthesis pathway, Lee et al. found that snat knockout mutants exhibited susceptibility to pathogen Pst DC3000 (avrRpt2) infection that coincided with decreased endogenous melatonin and SA levels as well as reduced induction of defense genes, this indicated that melatonin-elicited pathogen resistance is SA-dependent in Arabidopsis33.