Design and Synthesis of Mandelic Acid Derivatives for Suppression of Virulence via T3SS against Citrus Canker.

Citrus canker, a highly contagious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc), poses a substantial threat to citrus crops, leading to serious reductions in fruit yield and economic losses. Most commonly used bactericides against Xcc lead to the rapid development of resistant subpopulations. Therefore, it is imperative to create novel drugs, such as type III secretion system (T3SS) inhibitors, that specifically target bacterial virulence factors rather than bacterial viability. In our study, we designed and synthesized a series of mandelic acid derivatives including 2-mercapto-1,3,4-thiazole. Seven substances were found to reduce the level of transcription of hpa1 without affecting bacterial viability. In vivo bioassays indicated that compound F9 significantly inhibited hypersensitive response and pathogenicity. RT-qPCR assays showed that compound F9 visibly suppressed the expression of Xcc T3SS-related genes as well as citrus canker susceptibility gene CsLOB1. Furthermore, the combination with compound F9 and quorum-quenching bacteria HN-8 can also obviously alleviate canker symptoms.

Control of Phytophthora melonis damping-off treated with 24-epibrassinolide and a histological study of cucumber hypocotyl.

The oomycete Phytophthora melonis causes a severe disease in cucumber plants in Asia. In this study, the diameter of cucumber hypocotyl in the resistant variety 'Shantou qing gua' was significantly larger than that of the susceptible variety 'Zhongnong No. 20'. The significantly lower incidence of disease and less invasive hyphae on the epidermis and transverse section of hypocotyls in P plants of the resistant variety than those in susceptible cultivars were also observed. Brassinosteroids are a class of phytohormones that affect plant growth and development and are involved in regulating plant resistance to a variety of biotic and abiotic stresses. 24-Epibrassinolide root drenching significantly enhanced the thickening of cucumber hypocotyl. Thick hypocotyls showed strong resistance to P. melonis, indicating that it significantly reduced the incidence of disease and retarded the hyphae extension for both resistant and susceptible cucumbers. 24-Epibrassinolide pretreatment had no significant effect on the elongation of cucumber hypocotyl. Further histological observation showed that under the condition of infection with P. melonis, exogenous 24-epibrassinolide could induce lignin deposition in external phloem and xylem vessel cell wall of the cucumber hypocotyl vascular bundle. There is also an accumulation of callose in the external phloem sieve plate, which activates the resistance responses in cell walls. It is worth mentioning that in both inoculated and uninoculated conditions, exogenous 24-epibrassinolide enhanced lignin formation in external phloem and xylem vessel cell wall of the vascular bundle. This increased the content of lignin in hypocotyl as well as the number of vascular bundles in the hypocotyl base. The above results show that 24-epibrassinolide constitutively regulates the thickening of cucumber hypocotyl and the development of vascular bundle, hence preventing phytophthora infection and inducing plant resistance to disease.