Studies On The Antibacterial Function Of Melatonin In Pseudomonas Syringae Infested Arabidopsis Thaliana

Melatonin(MT)is a pleiotropic molecule in plants.Two synthetic enzymes,N-acetylserotonin methyltransferase(ASMT)and Serotonin N-acetyltransferase(SNAT),are involved in the last two steps of melatonin synthesis.Melatonin has been shown to be involved in plant responses to biotic stresses,but few studies have addressed the signalling pathways and molecular mechanisms of endogenous melatonin in plant disease resistance.The transgenic lines S46×A27-1 and S46×A27-4,which double overexpress the ASMT and SNAT genes,and the plant mutant lines s1×a2-12 and s1×a2-54,which double knockout the ASMT and SNAT genes,were screened by crossing single mutant or single overexpression lines already obtained,combined with the Pipecolic acid mutant ald1 and the salicylic acid mutant sid2,to systematically investigate the role of endogenous and exogenous melatonin in local resistance to bacterial infection in Arabidopsis thaliana.During bacterial infection,the calmodulin SARD1 binds to CBP60 g and activates the expression of ALD1 and SID2 genes,leading to increased levels of Pip and SA;elevated SA levels induce the expression of downstream PR genes(PR1 and PR5),ultimately increasing the local defence capacity of the plant.It was found that melatonin could regulate this signalling pathway by promoting the expression of ALD1,SID2,SARD1 and CBP60 g defence genes,further amplifying the disease resistance of plants.ALD1 was also found to promote SID2,but probably through MT.In this process,increased expression levels of SID2 genes may positively feedback to promote the expression of the transcription factor CBP60 g.Meanwhile,based on our laboratory’s previous research on the effects of melatonin in plant systemic resistance,we also exogenously applied 100 μM melatonin to ald1,fmo1 and sid2 mutants to investigate the role of exogenous melatonin signalling in the induction of systemic acquired resistance in Arabidopsis.Upon infestation by pathogenic bacteria,plants locally initiate the expression of a range of defence genes.Accumulated Pip is transported to uninfected distal tissues,where it leads to increased levels of MT.MT promotes the expression of SARD1 and CBP60 g,which in turn causes increased expression of ALD1,FMO1 and SID2.Pip regulates EDS1 expression,SA synthesis and NHP synthesis in a manner that acts on MT.NHP most likely regulates SA and PAD4 synthesis.Ultimately,this increases the expression of PR1 and PR5 genes and induces the production of SAR in Arabidopsis.Thus,our study could demonstrate that melatonin is a potent regulator of plant stress resistance,that it can function as an immune response signalling molecule in Arabidopsis local and systemic defence,and that it is involved in multiple pathways of regulation.Our findings help to complement related plant resistance signaling pathways as well as lay the foundation for further studies on the interactions between melatonin and other defense signaling molecules.