Bacterial wilt, a typical vascular disease caused by Ralstonia solanacearum, is one of the most devastating diseases and dramatically reduces crop yield and quality. The most effective strategy for controlling wilt disease is breeding disease-resistant varieties, which requires understanding the molecular mechanisms underlying plant immune responses against R. solanacearum. However, more and more evidence suggests that vascular immune responses are cell type specific. After sensing of R. solanacearum, the cell wall lignification of vascular tissues plays a vital role in restricting the spread of R. solanacearum. Lignin biosynthesis pathway genes are strictly controlled at the transcriptional, translational, and spatial-temporal specific expression aspects. Here, we summarized the current understanding of the recognition and signal transductionupon R. solanacearum infection and the research progress of pathogen-induced vascular lignification on regulating resistance to R. solanacearum, including the expression of lignin biosynthesis genes, the transport and polymerization of monolignols, and the generation of different types of lignin. We hope that this review will provide a theoretical basis for breeding bacterial wilt disease-resistant cultivars by modifying vascular lignification.

Ralstonia solanacearum causes bacterial wilt disease in multiple crops, which severely threatens the global crop safety production. This pathogen exhibits high genetic diversity and evolves rapidly, and there is a lack of effective disease-resistant varieties in production, which brings great challenges for effective disease control. Identifying receptor proteins in plants that recognize associated molecular patterns or effectors of R. solanacearum and elucidating their molecular recognition mechanisms can provide clues to understand the mechanisms of plant-pathogen interaction, and lay a basis for the development of broad-spectrum disease resistance in plants. This paper reviewed the recent progress on the molecular basis of plant-R. solanacearum recognition. We mainly focused on the identification and functional analysis of membrane and intracellular receptors that recognize R. solanacearum in plants, as well as the mechanism behind receptor recognition of the associated molecular patterns or effectors from R. solanacearum. Besides, we provide research prospects for the exploration and utilization of disease-resistant resources against R. solanacearum in the future.

Ralstonia solanacearum is a very harmful plant pathogenic bacterium, and the plant bacterial wilt caused by it seriously affects the healthy production of tomato and potato crops. It has broad host varieties and can acquire new virulence through horizontal gene transfer and gene recombination to extend the host range. The pathogenic mechanism of R. solanacearum is complex, type Ⅲ secretion system (T3SS) is the key pathogenic factor, and the type Ⅲ effectors (T3Es) secreted by it play important roles in the pathogenic processand inhibit innate immune response of hosts at different levels. Moreover, plant hosts can recognize R. solanacearum effectors and activate effector-triggered immunity (ETI) to achieve disease resistance. In this review, the virulence and avirulence mechanisms of R. solanacearum T3Es were discussed and summarized, providing insights for further understanding the pathogenesis of R. solanacearum and the mechanisms of plant resistance to bacterial wilt.

In order to clarify the pathogenic function of uridine diphosphate (UDP)-glucose 4-epimerase encoding gene galE on Ralstonia pseudosolanacearum, the association between galE and the pathogenic-related phenotypes, and the gene deletion effect on physiological and biochemical metabolism of R. pseudosolanacearum were explored by constructing galE gene knock-out strain ΔgalE and its complementary strain CΔgalE. The results showed that the deletion of galE gene significantly decreased the pathogenicity of R. pseudosolanacearum on sweet potatoes and affected pathogenic-related phenotypes. The colony fluidity, swimming mobility, exopolysaccharide content and biofilm formation of ΔgalE reduced compared to those of wild-type SPRS911 and CΔgalE. In the galactose metabolism pathway, after deletion of galE gene, the expression levels of galU, pgm, and glk genes involved in the metabolism between uridine diphosphate glucose (UDPG) and glucose decreased, and D-glucose 6-phosphate accumulated. The expression levels of UDP-galactose (UDP-Gal) metabolism related genes dgoK, dgoAa, dgoAb, malZ and galM increased. The deletion of galE gene also enhanced the assimilation of malic acid in R. pseudosolanacearum. These results indicate that the galE gene has significant effects on the pathogenicity and the carbon metabolism of R. pseudosolanacearum.

Chinese wheat mosaic virus (CWMV) is one of the most important pathogens causing mosaic disease in wheat and has threatened the yield and quality of wheat for a long time. Cysteine-rich protein (CRP) of CWMV plays important and complex roles in viral infection. To further study CRP functions and CWMV infection mechanisms, the CRP coding region was amplified by reverse transcription-polymerase chain reaction (RT-PCR) from leaves of CWMV-infected wheat and cloned into the prokaryotic expression vector pET-32a. The recombinant plasmid pET-CRP was transformed into Escherichia coli BL21 (DE3) for inducible expression. The recombinant CRP was purified by nickel-column affinity chromatography and used as an antigen to immunize New Zealand white rabbits for polyclonal antibody preparation. A series of immunological assays, including Western blot, indirect enzyme-linked immunosorbent assay (ELISA) and dot ELISA, showed that the purified CRP antibody had high specificity, and its titer was as high as 1∶4 096 000, which was four times higher than that of the unpurified antibody. The antibody could recognize 0.5 ng antigen, showing its high sensitivity. In addition, the purified CRP antibody could specifically and sensitively recognize native CRP even at a 1∶120 000 dilution. In conclusion, the polyclonal antibody can be not only used for precise diagnosis of the CWMV-infected plant samples from fields, but also applied to detect CRP expressed transiently in plants, which lays a foundation for subsequent detection, quantification and subcellular localization of CRP.

Watermelon diseased leaves exhibiting leaf curl and yellowing symptoms were collected from Shanghai. To clarify the pathogenic type, small RNA high-throughput sequencing of the diseased leaves was performed, and the diseased samples were confirmed to be infected by tomato leaf curl New Delhi virus (ToLCNDV) through sequence splicing and dot-enzyme linked immunosorbent assay (Dot-ELISA). The DNA was further enriched by rolling circle replication (RCR) and was amplified by polymerase chain reaction (PCR) using back-to-back primers to obtain the full-length DNA-A and DNA-B sequences of two ToLCNDV watermelon isolates, which were tentatively named as ToLCNDV SH-WM1 and ToLCNDV SH-WM2. Phylogenetic analysis using MEGA 11.0 and SDT v1.2 softwares showed that ToLCNDV SH-WM1 was closely related to ToLCNDV SH-WM2 and other reported ToLCNDV isolates from China, with a total nucleotide sequence similarity of 98.99%-99.70%. Whole genome polymorphism analysis and population variation analysis showed that the single nucleotide variation rate of ToLCNDV isolates from China was less than 3%, with a total of 46 synonymous mutation sites and 29 non-synonymous mutation sites. There were no insertion or deletion mutations, and the amino acid sequence of AC4 protein was the most conservative among all encoded proteins. This study reveals that the ToLCNDV isolates from China have low intraspecific genetic variation.

Cucurbit aphid-borne yellows virus (CABYV) is one of the most important viruses infecting cucurbits, such as melon (Cucumis melo), and severely affects the yield and quality of crops. Identification of CABYV-responsive genes can provide target genes for breeding of melon resistant to the viral disease. In this study, we used the CABYV infectious cloning vector to inoculate the melon XZM, and the disease identification and transcriptome analysis in the melon XZM after CABYV inoculation were performed at 0 dpi (days post inoculation), 5 dpi, 10 dpi, 15 dpi and 20 dpi. The results indicated that melon leaves showed typical disease symptoms of leaf chlorosis, yellowing and leaf thickening at 20 dpi. A total of 1 654 differentially expressed genes (DEGs) in response to CABYV infection were identified by transcriptome sequencing analysis, including 677 up-regulated genes and 977 down-regulated genes. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed that the responsive genes were mainly enriched in plant-pathogen interaction, photosynthesis, starch and sucrose metabolism, glyoxylate and dicarboxylate metabolism, etc., pathways and processes. Co-expression and interaction analysis of DEGs revealed that RIN4, a key gene in pathogen defense responses, may negatively regulate responses to CABYV infection in melon. This study demonstrates the possible molecular mechanism of responses to CABYV infection and provides a basis for breading melon against CABYV.