Acidovorax citrulli is a seed-borne pathogen causing bacterial fruit blotch of cucurbits including melon and watermelon. We investigated the roles of quorum sensing in the wild-type group II strain Aac-5 of A. citrulli by generating aacR and aacI knockout mutants and their complementation strains. We found that twitching motility and virulence were reduced, but biofilm formation and seed attachment were increased significantly in the two mutants as compared to the wild type strain. Deletion of aacR and aacI, however, had no effect on swimming motility and polar flagella formation of Aac-5. Furthermore, deletion of aacR resulted in reduced gene expression of hrpE, hrcN and pilT, while deletion of aacI affected only the expression of hrpE and pilT, not hrcN.
Cabbage Fusarium Wilt (CFW) is a serious disease caused by Fusarium oxysporum f. sp. conglutinans in many parts of the world. The use of chemical fungicides has placed a heavy burden on the environment and is prone to drug resistance in plant pathogens. As a method with great potential, biological control has attracted the attention of many academics both at home and abroad. In this study, we have found that strains B5 and B6 had a strong inhibitory effect on various pathogens and significantly inhibited mycelium growth. They were both identified as Bacillus velezensis by morphological features, biochemical determinations, 16S rRNA gene and gyrA gene sequence analysis. When different concentrations of bacterial suspension were applied to cabbage seeds, hypocotyl and taproot length increased to varying degrees. The in vivo results showed that B5 and B6 decreased the incidence of cabbage seedling wilt disease, with B6 performing significantly better. Furthermore, B. velezensis B6 had the ability to colonize cabbage plants and rhizosphere soil. Thus, strain B6 has great potential for biocontrol development and this research could lead to the development of a promising biological agent for CFW.
Colletotrichum capsici is one of the most important pathogens on chili peppers. Unreasonable application of chemical fungicides will lead to threats to human and animal health, environmental damage, and increased fungicide resistance to pathogens. As an alternative strategy, biological control has been paid more and more attention by academics. In this study, the Yb-1 strain was isolated from healthy cucumber leaves, which could significantly restrict the mycelium growth of C. capsici and Botrytis cinerea, with inhibition rates of 93.49% and 74.03%, respectively. Strain Yb-1 was identified as Bacillus velezensis by morphological features and 16S rRNA gene, gyrA gene, and gyrB gene sequence analysis. When pepper seeds were treated with different concentrations of bacterial suspension, we found that a medium-concentration treatment (104 CFU/mL and 105 CFU/mL) could promote seed germination and growth, while high-concentration treatments (108 CFU/mL) limited seed germination and growth. In addition, the root-irrigation method, acupuncture-leaf method, and leaf-cutting method were used to evaluate the safety of strain Yb-1 on peppers. The results indicated that Yb-1 did not affect the normal growth of peppers. The results of further field trials showed that the control effect of strain Yb-1 on pepper anthracnose was 59.45%. Thus, the strain B. velezensis Yb-1 has great biocontrol potential for pepper anthracnose and promotes plant growth.
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