Plant Defense Responses to a Novel Plant Elicitor Candidate LY5-24-2

Plant elicitors enhance plant defense against pathogen attacks by inducing systemic acquired resistance (SAR) with no or low direct fungicidal activity. Here we report the synthesis of a novel plant elicitor candidate LY5-24-2 [3,4-dichloro-N-(3-chloro-5-(trifluoromethyl)pyridin-2-yl)isothiazole-5-carboxamide] and evaluation of its SAR inducing activity. Bioassays indicated that LY5-24-2 did not show significant anti-fungal activity but provided long-lasting resistance in Arabidopsis thaliana (A. thaliana) through promoting the accumulation of lignin, cellulose and pectin by 60.1%, 82.4% and 305.6%, respectively, at a concentration of 100 µM. LY5-24-2 also facilitated the closure of leaf stomata and increased the intracellular free Ca²⁺ by 47.8%, induced reactive oxygen species (ROS) accumulation, and inhibited the activity of ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6) by 38.9% and 34.0%, respectively, as compared with the control at a concentration of 100 µM. LY5-24-2 induced SAR in plants and was dependent on the NPR1-mediated SA pathway by up-regulating expression of 2273 genes in A. thaliana. Meanwhile, LY5-24-2 also improved cucumber (Cucumis sativus L.) defense against Pseudoperonospora cubensis (P. cubensis) through promoting ROS accumulation and inhibiting activity of APX and CAT by 30.7% and 23.1%, respectively. Its expression of SA signaling genes CsNPR1, CsPR4 and CsPR5 was enhanced by 10.8, 5.8 and 6.6 times, respectively. These results demonstrated that LY5-24-2 is a novel elicitor candidate for plant protection via inducing SAR.