The novel plant elicitors, 3-benzyl-5-[1-(2-oxo-4-phenyl-1-oxaspiro[4.5]dec-3-en-3-yl)ethylidene]-2-aminoimidazolin-4-one
derivatives, were designed based on the diversity-oriented synthesis
strategy and synthesized in four steps via the Knoevenagel condensation
reaction as the key step. They were characterized by 1H
NMR, 13C NMR, HR-ESI-MS, and X-ray diffraction. The position
of the CN bond of Z- and E-configuration compounds was determined by X-ray diffraction. The
in vivo fungicidal activity evaluation revealed that most of these
compounds exhibited remarkable activities (100%) against Pseudoperonospora cubensis at 400 μg/mL, among
which compound 8e still exhibited excellent protective
activity with a 50% inhibition rate at 0.1 μg/mL. Because the
in vitro effect on tested phytopathogens was poor, the mechanism to
induce the immune responses and reinforce the resistance of cucumber
against Botrytis cinerea was studied.
The results indicated that the compound 8e-mediated defense
response against B. cinerea was based
on the accumulation of pathogenesis-related proteins and cell wall
reinforcement by callose deposition. Quantitative analysis of salicylic
acid (SA) and jasmonic acid (JA) and the increased expression of induced
resistance-related genes and the defense-associated phenylalanine
ammonia lyase revealed that the immune response triggered by compound 8e was highly associated with the SA signaling pathway. Significant
upregulation of JA-related genes Cs-AOS2 indicated
that the JA signaling pathway was also influenced. It was also shown
that the plants treated with compound 8e promoted primary
root elongation, which resulted in enhanced plant growth. Most importantly,
these compounds have completely new structures compared with the traditional
plant elicitors. Further research of 8e-mediated plant
disease resistance might have a great influence on the development
of plant elicitors.