Soft rot Pectobacteriaceae (SRP), typical of Pectobacterium and Dickeya, are a class of Gram-negative bacterial pathogens that cause devastating diseases on a wide range of crops and ornamental plants worldwide. Quorum sensing (QS) is a cell-cell communication mechanism regulating the expression of specific genes by releasing QS signal molecules associated with cell density, in most cases, involving in the vital process of virulence and infection. In recent years, several types of QS systems have been uncovered in Dickeya pathogens to control diverse biological behaviors, especially bacterial pathogenicity and transkingdom interactions. This review depicts an integral QS regulation network of Dickeya, elaborates in detail the regulation of specific QS system on different biological functions of the pathogens and hosts, aiming at providing a systematic overview of Dickeya pathogenicity and interactions with hosts, and, finally, expects the future prospective of effectively controlling the bacterial soft rot disease caused by Dickeya by quenching the key QS signal.
Rice bacterial blight is a devastating bacterial disease threatening rice yield all over the world and Xanthomonas oryzae pv. oryzae (Xoo) is traditionally believed as the pathogen. In recent years, we have received diseased rice samples with symptoms of blight leaves from Sichuan and Guangdong Provinces, China. Pathogen isolation and classification identified two different enterobacteria as the causal agents, namely as Enterobacter asburiae and Pantoea ananatis. Among them, E. asburiae was isolated from samples of both Provinces, and P. ananatis was only isolated from the Sichuan samples. Different from rice foot rot pathogen Dickeya zeae EC1 and rice bacterial blight pathogen Xoo PXO99A, strains SC1, RG1 and SC7 produced rare cell wall degrading enzymes (CWDEs) but more extrapolysaccharides (EPS). E. asburiae strains SC1 and RG1 produced bacteriostatic substances while P. ananatis strain SC7 produced none. Pathogenicity tests indicated that all of them infected monocotyledonous rice and banana seedlings, but not dicotyledonous potato, radish or cabbage. Moreover, strain RG1 was most virulent, while strains SC1 and SC7 were similar virulent on rice leaves, even though strain SC1 propagated significantly faster in rice leaf tissues than strain SC7. This study firstly discovered E. asburiae as a new pathogen of rice bacterial blight, and in some cases, P. ananatis could be a companion pathogen. Analysis on production of virulence factors suggested that both pathogens probably employ a different mechanism to infect hosts other than using cell wall degrading enzymes to break through host cell walls.
Stenotrophomonas maltophilia is ubiquitous in diverse environmental habitats. It alerts significant concern due to its increasing incidence of nosocomial and community-acquired infection in immunocompromised patients and multiple drug resistance. It is rarely reported as a phytopathogen except causing white stripe disease of rice in India and postharvest fruit rot of Lanzhou Lily. Recently, Dickeya zeae and S. maltophilia strains were simultaneously isolated from soft rot leaves of Clivia miniata in Guangzhou, China, and were both demonstrated pathogenic to the host. Compared with the D. zeae strains, S. maltophilia strains propagated faster for greater growth in LB medium and produced no cellulases or polygalacturonases, more proteases and fewer extracellular polysaccharides. Furthermore, S. maltophilia strains swam and swarmed dramatically less on semi-solid media, but formed extraordinarily more biofilms. Both D. zeae and S. maltophilia strains isolated from clivia caused rot symptoms on other monocot hosts, but not on dicots. Similar to previously reported S. maltophilia strains isolated from other sources, strain JZL8 survived under many antibiotic stresses. Complete genome sequence of S. maltophilia strain JZL8 consists of a chromosome of 4,635,432 bp without plasmid. Pan-genome analysis of JZL8 and 180 other S. maltophilia strains identified 50 JZL8-unique genes, seven of which implicates potential contribution of JZL8 pathogenicity on plants. JZL8 also contains 3 copies of T1SS, likely responsible for its greater production of proteases. Findings from this study extend our knowledge on the host range of S. maltophilia and provide insight into phenotypic and genetic features underlying the plant pathogenicity of JZL8.
Bacterial soft rot is one of the most destructive diseases of taro (Colocasia esculenta) worldwide. In recent years, frequent outbreaks of soft rot disease have seriously affected taro production and became a major constraint to the development of taro planting in China. However, little is known about the causal agents of this disease, and the only reported pathogens are two Dickeya species and P. carotovorum. In this study, we report taro soft rot caused by two novel Pectobacterium strains, LJ1 and LJ2, isolated from taro corms in Ruyuan County, Shaoguan City, Guangdong Province, China. We showed that LJ1 and LJ2 fulfill Koch’s postulates for taro soft rot. The two pathogens can infect taro both individually and simultaneously, and neither synergistic nor antagonistic interaction was observed between the two pathogens. Genome sequencing of the two strains indicated that LJ1 represents a novel species of the genus Pectobacterium, for which the name “Pectobacterium colocasium sp. nov.” is proposed, while LJ2 belongs to Pectobacterium aroidearum. Pan-genome analysis revealed multiple pathogenicity-related differences between LJ1, LJ2, and other Pectobacterium species, including unique virulence factors, variation in the copy number and organization of Type III, IV, and VI secretion systems, and differential production of plant cell wall degrading enzymes. This study identifies two new soft rot Pectobacteriaceae (SRP) pathogens causing taro soft rot in China, reports a new case of co-infection of plant pathogens, and provides valuable resources for further investigation of the pathogenic mechanisms of SRP.
Ipomoea aquatica is a leafy vegetable widely cultivated in tropical Asia, Africa, and Oceania. Bacterial leaf canker disease has been attacking the planting fields and seriously affecting the quality of I. aquatica in epidemic areas in China. This study examined the microbial composition of I. aquatica leaves with classical symptoms of spot disease. The results showed that Xanthomonas was overwhelmingly dominant in all four diseased leaf samples but rarely present in rhizospheric soil or irrigation water samples. In addition, Pantoea was also detected in two of the diseased leaf samples. Pathogen isolation, identification, and inoculation revealed that both Xanthomonas sp. TC2-1 and P. ananatis were pathogenic to the leaves of I. aquatic, causing crater-shaped ulcerative spots and yellowing with big brown rot lesions on leaves, respectively. We further sequenced the whole genome of strain TC2-1 and showed that it is a member of X. perforans. Overall, this study identified X. perforans as the causal pathogen of I. aquatica bacterial leaf canker, and P. ananatis as a companion pathogen causing yellowing and brown rot on leaves. The correct identification of the pathogens will provide important basis for future efforts to formulate targeted application strategy for bacterial disease control.
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