Discovery and Mechanism of Novel 7-Aliphatic Amine Tryptanthrin Derivatives against Phytopathogenic Bacteria

Abstract

Rice bacterial leaf blight is a destructive bacterial disease caused by Xanthomonas oryzae pv. oryzae (Xoo) that seriously threatens crop yields and their associated economic benefits. In this study, a series of improved dissolubility 7-aliphatic amine tryptanthrin derivatives was designed and synthesized, and their potency in antibacterial applications was investigated. Notably, compound 6e exhibited excellent activity against Xoo, with an EC₅₀ value of 2.55 μg/mL, compared with the positive control bismerthiazol (EC₅₀ = 35.0 μg/mL) and thiodiazole copper (EC₅₀ = 79.4 μg/mL). In vivo assays demonstrated that 6e exhibited a significant protective effect on rice leaves. After exposure, the morphology of the bacteria was partially atrophied by SEM. Furthermore, 6e increased the accumulation of intracellular reactive oxygen species, causing cell apoptosis and the formation of bacterial biofilms. All the results indicated that 6e could be a potential agrochemical bactericide for controlling phytopathogenic bacteria.

Keywords: 7-aliphatic amine tryptanthrin derivatives; SEM; Xanthomonas oryzae pv. oryzae; apoptosis.

Figure 1

(A) Natural source of typtanthrin derivatives; (B) The design of target compounds.

Scheme 1

Synthetic route for the titular compounds 6a–6z.

Figure 2

Protection and curative activities of compound 6e and BT against rice bacterial leaf blight under controllable greenhouse conditions at 200 μg/mL. CK (control check) was the control group; BT (bismerthiazol) was the positive control.

Figure 3

SEM images for Xoo after incubated in 10 × EC₅₀ and 20 × EC₅₀ concentrations of compound 6e: (A) control (0 μg/mL); (B) 10 × EC₅₀; (C) 20 × EC₅₀. Scale bars are 1 μm. Red arrows point to areas where there are significant structural changes.

Figure 4

Compound 6e induced ROS production. (A) Intracellular ROS generation in Xoo cells was measured via flow cytometry. (B) Statistical results of ROS. Three asterisks (***) indicate p < 0.001 compared with the control.

Figure 5

Flow cytometric analysis of apoptotic cells after treatment of Xoo cells with different concentrations of compound 6e. The control group consisted of untreated cells. The histogram with different colors represents the apoptotic ratio of compound 6e at various concentrations. Three asterisks (***) indicate p < 0.001 compared with the control.

Figure 6

Inhibition of biofilm formation incubated with compound 6e was analyzed using crystal violet assay. The symbols ** and *** stand for p < 0.01 and p < 0.001, respectively, compared with the controls.