Recently, Professor Cui Zining's team from the Group Microbial Research Center at South China Agricultural University, in collaboration with Professor Wu Zhibing from the Fine Chemical Research and Development Center at Guizhou University, made new progress in the research on Type III secretion inhibitors for citrus canker pathogen. The related findings were published online in the "Journal of Agricultural and Food Chemistry."
Citrus canker is an infectious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc), a pathogenic variant of Xanthomonas citri, and is one of the most serious bacterial diseases affecting citrus, classified as a globally quarantined disease. The pathogen affects the leaves, branches, and fruits of citrus plants, forming corky, slightly raised lesions on the affected organs. In severe cases, citrus canker can lead to decreased photosynthetic rates, branch dieback, defoliation, and premature fruit drop.
Currently, chemical control is the primary method for managing citrus canker, with fungicides and antibiotics commonly used due to their effective control of the pathogen. However, prolonged use of these pesticides can lead to the emergence and enrichment of resistant mutant strains. Therefore, developing novel pesticides that target the virulence factors of the pathogen without affecting its growth can significantly alleviate the development of pathogen resistance. The Type III secretion system is a common and conserved virulence factor in Gram-negative pathogenic bacteria, but it is not essential for the growth of the pathogen. Thus, the Type III secretion system serves as an ideal target for developing new pesticides to control citrus canker.
Supported by the National Key R&D Program, the National Natural Science Foundation, and the Guangdong Outstanding Youth Fund, the research team designed and synthesized a series of (30) amygdalic acid derivatives containing 2-mercapto-1,3,4-thiadiazole. Fluorescence reporter assays and growth curve results showed that seven compounds significantly inhibited the transcription of hpa1 without affecting Xcc growth. Subsequent in vivo inoculation experiments demonstrated that compound F9 significantly reduced Xcc hypersensitive response on tobacco and canker symptoms on citrus leaves. Additionally, compound F9 significantly inhibited the transcription of the CsLOB1 susceptibility gene in the host, further confirming its ability to suppress Xcc pathogenicity.
Reverse transcription real-time quantitative polymerase chain reaction (RT-qPCR) analysis revealed that compound F9 significantly inhibited the master regulatory genes hrpG and hrpX, the harpin protein-encoding gene hpa1, and the Type III secretion system apparatus-encoding genes hrpE, hrcC, and hrcT, indicating that the compound affected the transcription of genes related to the regulation pathway and structural components of the Type III secretion system. Furthermore, compound F9 was co-administered with the quenching bacterium Burkholderia anthina HN-8, which produces the diffusible signal factor (DSF). Experimental results showed that when compound F9 was used in combination with HN-8 at half the concentration, the control effect on citrus canker was comparable to using the compound alone, achieving a reduction in pesticide use while maintaining control efficacy.
This research provides a structural theoretical basis for the design and synthesis of Type III secretion inhibitors and offers new strategies for the prevention and control of citrus canker.
Related Paper Information: https://doi.org/10.1021/acs.jafc.3c07681
