# Innovation and Application of the Type III Secretion System Inhibitors in Plant Pathogenic Bacteria
## 4 Conclusions and Perspectives  
Bacterial plant diseases, such as bacterial wilt, fire blight, soft rot, citrus greening, and bacterial leaf blight of rice, cause significant economic losses ($100M) annually on a global scale. Current control options are limited and involve applying chemicals, copper, and antibiotics, and biological control agents like bacteriophage [113; 153]. In the field of fire blight management, for instance, three antibiotics have been used, including streptomycin, oxytetracycline, and a newly registered antibiotic kasugamycin. They are proven to be the most effective method in controlling fire blight in apple and pear orchards in the United States [154; 155]. However, the widespread of resistance to antibiotics has placed major constraints on antibiotic usage. An increasing number of studies has implied the preexistence of antibiotic-resistance genes in environmental microbiome, such as soil, ground water, phyllosphere, and animal gut, before introducing antibiotics [114]. In addition, the extensive application of antibiotics in agriculture could greatly endanger human health as antibiotic-resistance genes are able to transfer between bacterial species via horizontal gene transfer [156]. Thus, the discovery and development of alternative control methods for controlling bacterial plant diseases are urgently needed. T3 SS is an essential virulence factor in many gram-negative bacterial pathogens [1; 2]. Plant pathogenic bacteria use T3 SSs to translocate various effector proteins into host cells to manipulate plant signaling behaviors and repress host immune responses [4]. The innovation of novel molecules that specifically target and inhibit primary virulence factors, such as T3 SS, without lethal selective pressure serves as a compelling control option compared with conventional antibiotics. However, unlike antibiotics, often with known modes of action, the mechanisms of newly discovered virulence inhibitors are not well established in bacteria. This could be due to the lack of fundamental knowledge for the regulation of virulence factors and will be facilitated by analyzing the landscape of bacterial responses to virulence inhibitors at the whole-genome transcription level via next-generation sequencing techniques [136]. In the model microorganism _D. dadantii_, several regulatory components, including transcriptional and post-transcriptional regulators, sRNAs, and bacterial second messengers, have been reported to regulate the T3 SS (Figure 1). The majority of these regulators, like c-di-GMP, are genetically and functionally conserved between different bacterial species, implicating that small molecules proven to be effective in one bacterium can also be applied in other bacteria. Several exemplary studies have already proved this concept via studying the impact of plant phenolic compounds and salicidene acylhydrazides on T3 SSs in various animal and plant pathogens [118; 121; 122]. Both small peptides and molecules (natural or synthetic) have been extensively studied in human pathogens _P. aeruginosa_ and _V. cholerae_ due to their impact on the c-di-GMP signaling. These molecules exert their function via binding to c-di-GMP directly, affecting the enzymatic activity of DGCs that synthesize c-di-GMP, or mimicking c-di-GMP as a competitor in bacteria [157; 158; 159; 160]. However, similar studies have not been reported in plant pathogenic bacteria, and the idea of using small molecules that target c-di-GMP for the management of plant diseases needs to be further evaluated. On the other hand, controlling bacterial diseases using T3 SS inhibitors may raise a concern whether they will interfere with the T3 SS in some non-pathogenic or host beneficial bacteria that is also important for host interactions [161]. Ecologically, T3 SS has been shown to contribute to the interaction between bacteria and fungi in soil and related habitats [162].