Marssonin spots in the apple are a serious fungal disease propagated by Diplocarpon mali . This condition causes an early loss of leaves in the apple trees which compromises the resistance of the trees and reduces the yield of the fruit . The fungus initially attacks the leaves, but can spread to the fruit . Yin and colleagues  used the leaves of Malus prunifolia to examine the effectiveness of melatonin as an inhibitor of Marssonine phytopathology. The authors have shown that the pretreatment of apple trees with melatonin has improved their resistance to the fungus, while without melatonin the leaves become yellowed and the leaf spots expand (Figure 14). Other measures, including the total number of lesions, photosystem II efficiency, chlorophyll content, H levels2OR2 intracellular and high levels of enzymes for the defense of plants all support the actions of protection of melatonin against these biotic stresses. The authors believe that the use of melatonin pretreatment may be an effective strategy for limiting fungal Marssonin infections in No ills. Theoretically, how melatonin guarantees the acquisition of resistance is not yet clear. This is the first report in which melatonin is useful in counteracting biotic stress in plant species.
A different approach has been taken by Lee and colleagues  to examine the effectiveness of melatonin in the fight against biotic stress. Rather than providing melatonin supplementation, they inactivated the serotonin N-acetyltransferase gene in two Arabidopsis mutant cell lines with a T-DNA insertion that led to a substantial reduction in endogenous melatonin levels in these plants. As a result, the plants showed a high susceptibility of the avirulent pathogen, Pseudomonas syringae. Loss of resistance to this pathogen has also been associated with reduced induction of PR1, ICS1 and PDF1 genes. 2, which normally provide a defense against the bacterial pathogen. Since melatonin acts upstream of salicylic acid (SA) synthesis, the low levels of melatonin in Arabidopsis knockout may have been responsible for the reduced levels of SA which, in turn, make these plants more sensitive to the pathogen. The Figure above illustrates the multiple means by which melatonin reduces abiotic stresses in plants as illustrated in the previous paragraphs.
Abstract of the thesis "Melatonin in the plant world (Phytomelatonin): therapeutic properties and future prospects" by Dr. Giorgio Guerrini
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