Three amino acid residues are required for the recognition of Ralstonia solanacearum RipTPS in Nicotiana tabacum

Molecular Dialog of Ralstonia solanacearum and Plant Hosts with Highlights on Type III Effectors

Ralstonia solanacearum is a highly destructive soil-borne bacterium that causes bacterial wilt disease in more than 310 plant species worldwide. The pathogenicity of the bacteria is closely associated with type III effectors (T3Es), a class of virulence factors that are delivered to host plant cells by the type III secretion system. In spite of the complex evolutionary history and genetic diversity of the R. solanacearum species complex (RSSC), more than 100 different T3Es have been identified from the genomes of various strains. Based on the available functional studies, certain T3Es interact with host plant proteins and suppress host cell immunity, whereas other T3Es are recognized by the host plant to trigger specific resistance mechanisms. This review summarizes the mechanisms by which T3Es interfere with plant immune responses and the activation of the plant defense system upon T3E recognition. This in-depth review of the molecular interactions between R. solanacearum and its host plants offers insights into the complexity of plant-pathogen interactions and provides a scientific rationale and theoretical support for the future breeding of resistant crops.

A robust high-throughput functional screening assay for plant pathogen effectors using the TMV-GFP vector

Uncovering the function of phytopathogen effectors is crucial for understanding mechanisms of pathogen pathogenicity and for improving our ability to protect plants from diseases. An increasing number of effectors have been predicted in various plant pathogens. Functional characterization of these effectors has become a major focus in the study of plant-pathogen interactions. In this study, we designed a novel screening system that combines the TMV (tobacco mosaic virus)-GFP vector and Agrobacterium-mediated transient expression in the model plant Nicotiana benthamiana. This system enables the rapid identification of effectors that interfere with plant immunity. The biological function of these effectors can be easily evaluated by observing the GFP fluorescence signal using a UV lamp within just a few days. To evaluate the TMV-GFP system, we initially tested it with well-described virulence and avirulence type III effectors from the bacterial pathogen Ralstonia solanacearum. After proving the accuracy and efficiency of the TMV-GFP system, we successfully screened a novel virulence effector, RipS1, using this approach. Furthermore, using the TMV-GFP system, we reproduced consistent results with previously known cytoplasmic effectors from a diverse array of pathogens. Additionally, we demonstrated the effectiveness of the TMV-GFP system in identifying apoplastic effectors. The easy operation, time-saving nature, broad effectiveness, and low technical requirements of the TMV-GFP system make it a promising approach for high-throughput screening of effectors with immune interference activity from various pathogens.

Comparative genomics and host range analysis of four Ralstonia pseudosolanacearum strains isolated from sunflower reveals genomic and phenotypic differences

BACKGROUND

Bacterial wilt caused by Ralstonia solanacearum species complex (RSSC) is one of the devastating diseases in crop production, seriously reducing the yield of crops. R. pseudosolanacearum, is known for its broad infrasubspecific diversity and comprises 36 sequevars that are currently known. Previous studies found that R. pseudosolanacearum contained four sequevars (13, 14, 17 and 54) isolated from sunflowers sown in the same field.

RESULTS

Here, we provided the complete genomes and the results of genome comparison of the four sequevars strains (RS639, RS642, RS647, and RS650). Four strains showed different pathogenicities to the same cultivars and different host ranges. Their genome sizes were about 5.84 ~ 5.94 Mb, encoding 5002 ~ 5079 genes and the average G + C content of 66.85% ~ 67%. Among the coding genes, 146 ~ 159 specific gene families (contained 150 ~ 160 genes) were found in the chromosomes and 34 ~ 77 specific gene families (contained 34 ~ 78 genes) in the megaplasmids from four strains. The average nucleotide identify (ANI) values between any two strains ranged from 99.05% ~ 99.71%, and the proportion of the total base length of collinear blocks accounts for the total gene length of corresponding genome was all more than 93.82%. Then, we performed a search for genomic islands, prophage sequences, the gene clusters macromolecular secretion systems, type III secreted effectors and other virulence factors in these strains, which provided detailed comparison results of their presence and distinctive features compared to the reference strain GMI1000. Among them, the number and types of T2SS gene clusters were different in the four strains, among which RS650 included all five types. T4SS gene cluster of RS639 and RS647 were missed. In the T6SS gene cluster, several genes were inserted in the RS639, RS647, and RS650, and gene deletion was also detected in the RS642. A total of 78 kinds of type III secreted effectors were found, which included 52 core and 9 specific effectors in four strains.

CONCLUSION

This study not only provided the complete genomes of multiple R. pseudosolanacearum strains isolated from a new host, but also revealed the differences in their genomic levels through comparative genomics. Furthermore, these findings expand human knowledge about the range of hosts that Ralstonia can infect, and potentially contribute to exploring rules and factors of the genetic evolution and analyzing its pathogenic mechanism.

Ubiquitin E3 ligase activity of Ralstonia solanacearum effector RipAW is not essential for induction of plant defense in Nicotiana benthamiana

As one of the most destructive bacterial phytopathogens, Ralstonia solanacearum causes substantial annual yield losses of many important crops. Deciphering the functional mechanisms of type III effectors, the crucial factors mediating R. solanacearum-plant interactions, will provide a valuable basis for protecting crop plants from R. solanacearum. Recently, the NEL (novel E3 ligase) effector RipAW was found to induce cell death on Nicotiana benthamiana in a E3 ligase activity-dependent manner. Here, we further deciphered the role of the E3 ligase activity in RipAW-triggered plant immunity. We found that RipAW^C177A, the E3 ligase mutant of RipAW, could not induce cell death but retained the ability of triggering plant immunity in N. benthamiana, indicating that the E3 ligase activity is not essential for RipAW-triggered immunity. By generating truncated mutants of RipAW, we further showed that the N-terminus, NEL domain and C-terminus are all required but not sufficient for RipAW-induced cell death. Furthermore, all truncated mutants of RipAW triggered ETI immune responses in N. benthamiana, confirming that the E3 ligase activity is not essential for RipAW-triggered plant immunity. Finally, we demonstrated that RipAW- and RipAW^C177A-triggered immunity in N. benthamiana requires SGT1 (suppressor of G2 allele of skp1), but not EDS1 (enhanced disease susceptibility), NRG1 (N requirement gene 1), NRC (NLR required for cell death) proteins or SA (salicylic acid) pathway. Our findings provide a typical case in which the effector-induced cell death can be uncoupled with immune responses, shedding new light on effector-triggered plant immunity. Our data also provide clues for further in-depth study of mechanism underlying RipAW-induced plant immunity.