Silicon Controls Bacterial Wilt Disease in Tomato Plants and Inhibits the Virulence-Related Gene Expression of Ralstonia solanacearum

Silicon nanoparticles (SiNPs) stimulated secondary metabolism and mitigated toxicity of salinity stress in basil (Ocimum basilicum) by modulating gene expression: a sustainable approach for crop protection

The underlying mechanisms through which silicon oxide nanoparticles (SiNPs) can confer salinity resistance to plants are poorly understood. This study explored the efficacy of supplementing nutrient solution with SiNPs (20-30 nm; 10 mg kg-1 soil) to stimulate metabolism and alleviate the risks associated with salinity (0.73 g kg-1 soil) in basil seedlings. For this purpose, variations in photosynthetic indices, proline osmoprotectant, antioxidant markers, phenylpropanoid metabolism, and transcriptional behaviors of genes were investigated. SiNPs increased shoot fresh weight (38%) and mitigated the risk associated with the salinity stress by 14%. SiNPs alleviated the inhibitory effects of salinity on the total chlorophyll concentration by 15%. The highest increase (twofold) in proline content was recorded in the SiNP-treated seedlings grown under salinity. The nano-supplement enhanced the activity of enzymatic antioxidants, including peroxidase (2.5-fold) and catalase (4.7-fold). SiNPs induced the expression of gamma-cadinene synthase (CDS) and caffeic acid O-methyltransferase (COMT) genes by 6.5- and 18.3-fold, respectively. SiNPs upregulated the eugenol synthase (EGS1) and fenchol synthase (FES) genes by six- and nine-fold, respectively. Salinity transcriptionally downregulated the geraniol synthase (GES) gene, while this gene displayed an upward trend in response to SiNPs by eight-fold. The nano-supplement transcriptionally stimulated the R-linalool synthase (LIS) gene by 3.3-fold. The terpinolene synthase (TES) gene displayed a similar trend to that of the GES gene. The highest expression (25-fold) of the phenylalanine ammonia-lyase (PAL) gene was recorded in seedlings supplemented with SiNPs. The physiological and molecular assessments demonstrated that employing SiNPs is a sustainable strategy for improving plant primary/secondary metabolism and crop protection.

Cloning, Expression, and Functional Analysis of the MYB Transcription Factor SlMYB86-like in Tomato

MYB transcription factors (TFs) have been shown to play a key role in plant growth and development and are in response to various types of biotic and abiotic stress. Here, we clarified the structure, expression patterns, and function of a MYB TF, SlMYB86-like (Solyc06g071690) in tomato using an inbred tomato line exhibiting high resistance to bacterial wilt (Hm 2-2 (R)) and one susceptible line (BY 1-2 (S)). The full-length cDNA sequence of this gene was 1226 bp, and the open reading frame was 966 bp, which encoded 321 amino acids; its relative molecular weight was 37.05055 kDa; its theoretical isoelectric point was 7.22; it was a hydrophilic nonsecreted protein; and it had no transmembrane structures. The protein also contains a highly conserved MYB DNA-binding domain and was predicted to be localized to the nucleus. Phylogenetic analysis revealed that SlMYB86-like is closely related to SpMYB86-like in Solanum pennellii and clustered with other members of the family Solanaceae. Quantitative real-time PCR (qRT-PCR) analysis revealed that the expression of the SlMYB86-like gene was tissue specific and could be induced by Ralstonia solanacearum, salicylic acid, and jasmonic acid. The results of virus-induced gene silencing (VIGS) revealed that SlMYB86-like silencing decreased the resistance of tomato plants to bacterial wilt, suggesting that it positively regulates the resistance of tomatoes to bacterial wilt. Overall, these findings indicate that SlMYB86-like plays a key role in regulating the resistance of tomatoes to bacterial wilt.

A Mutation of a Putative NDP-Sugar Epimerase Gene in Ralstonia pseudosolanacearum Attenuates Exopolysaccharide Production and Bacterial Virulence in Tomato Plant

Ralstonia solanacearum species complex (RSSC) is a soil borne plant pathogen causing bacterial wilt on various important crops, including Solanaceae plants. The bacterial pathogens within the RSSC produce exopolysaccharide (EPS), a highly complicated nitrogen-containing heteropolymeric polysaccharide, as a major virulence factor. However, the biosynthetic pathway of the EPS in the RSSC has not been fully characterized. To identify genes in EPS production beyond the EPS biosynthetic gene operon, we selected the EPS-defective mutants of R. pseudosolanacearum strain SL341 from Tn5-inserted mutant pool. Among several EPS-defective mutants, we identified a mutant, SL341P4, with a Tn5-insertion in a gene encoding a putative NDP-sugar epimerase, a putative membrane protein with sugar-modifying moiety, in a reverse orientation to EPS biosynthesis gene cluster. This protein showed similar to other NDP-sugar epimerases involved in EPS biosynthesis in many phytopathogens. Mutation of the NDP-sugar epimerase gene reduced EPS production and biofilm formation in R. pseudosolanacearum. Additionally, the SL341P4 mutant exhibited reduced disease severity and incidence of bacterial wilt in tomato plants compared to the wild-type SL341 without alteration of bacterial multiplication. These results indicate that the NDP-sugar epimerase gene is required for EPS production and bacterial virulence in R. pseudosolanacearum.

A screening identifies harmine as a novel antibacterial compound against Ralstonia solanacearum

Ralstonia solanacearum, the causal agent of bacterial wilt, is a devastating plant pathogenic bacterium that infects more than 450 plant species. Until now, there has been no efficient control strategy against bacterial wilt. In this study, we screened a library of 100 plant-derived compounds for their antibacterial activity against R. solanacearum. Twelve compounds, including harmine, harmine hydrochloride, citral, vanillin, and vincamine, suppressed bacterial growth of R. solanacearum in liquid medium with an inhibition rate higher than 50%. Further focus on harmine revealed that the minimum inhibitory concentration of this compound is 120 mg/L. Treatment with 120 mg/L of harmine for 1 and 2 h killed more than 90% of bacteria. Harmine treatment suppressed the expression of the virulence-associated gene xpsR. Harmine also significantly inhibited biofilm formation by R. solanacearum at concentrations ranging from 20 mg/L to 60 mg/L. Furthermore, application of harmine effectively reduced bacterial wilt disease development in both tobacco and tomato plants. Collectively, our results demonstrate the great potential of plant-derived compounds as antibacterial agents against R. solanacearum, providing alternative ways for the efficient control of bacterial wilt.

Plant and soil-associated microbiome dynamics determine the fate of bacterial wilt pathogen Ralstonia solanacearum

MAIN CONCLUSION

Plant and the soil-associated microbiome is important for imparting bacterial wilt disease tolerance in plants. Plants are versatile organisms that are endowed with the capacity to withstand various biotic and abiotic stresses despite having no locomotory abilities. Being the agent for bacterial wilt (BW) disease, Ralstonia solanacearum (RS) colonizes the xylem vessels and limits the water supply to various plant parts, thereby causing wilting. The havoc caused by RS leads to heavy losses in crop productivity around the world, for which a sustainable mitigation strategy is urgently needed. As several factors can influence plant-microbe interactions, comprehensive understanding of plant and soil-associated microbiome under the influence of RS and various environmental/edaphic conditions is important to control this pathogen. This review mainly focuses on microbiome dynamics associated with BW disease and also provide update on microbial/non-microbial approaches employed to control BW disease in crop plants.

Silicon Application for the Modulation of Rhizosphere Soil Bacterial Community Structures and Metabolite Profiles in Peanut under Ralstonia solanacearum Inoculation

Silicon (Si) has been shown to promote peanut growth and yield, but whether Si can enhance the resistance against peanut bacterial wilt (PBW) caused by Ralstonia solanacearum, identified as a soil-borne pathogen, is still unclear. A question regarding whether Si enhances the resistance of PBW is still unclear. Here, an in vitro R. solanacearum inoculation experiment was conducted to study the effects of Si application on the disease severity and phenotype of peanuts, as well as the microbial ecology of the rhizosphere. Results revealed that Si treatment significantly reduced the disease rate, with a decrement PBW severity of 37.50% as compared to non-Si treatment. The soil available Si (ASi) significantly increased by 13.62-44.87%, and catalase activity improved by 3.01-3.10%, which displayed obvious discrimination between non-Si and Si treatments. Furthermore, the rhizosphere soil bacterial community structures and metabolite profiles dramatically changed under Si treatment. Three significantly changed bacterial taxa were observed, which showed significant abundance under Si treatment, whereas the genus Ralstonia genus was significantly suppressed by Si. Similarly, nine differential metabolites were identified to involve into unsaturated fatty acids via a biosynthesis pathway. Significant correlations were also displayed between soil physiochemical properties and enzymes, the bacterial community, and the differential metabolites by pairwise comparisons. Overall, this study reports that Si application mediated the evolution of soil physicochemical properties, the bacterial community, and metabolite profiles in the soil rhizosphere, which significantly affects the colonization of the Ralstonia genus and provides a new theoretical basis for Si application in PBW prevention.

First Report of Enterobacter cloacae Causing Stem, Leaf and Fruit Rot on Tomato in China

Tomato (Solanum lycopersicum L.), as one of the most economically important and highly nutritious vegetable crops across the world, is widely cultivated in China, one of the largest tomato-concuming countries in the world (Ye et al., 2020; Wang and Liu, 2021). At present, major bacterial diseases in tomato include bacterial speck disease, tomato bacterial wilt and bacterial canker, all of which affect the tomato production around the world (Rosli et al., 2021; Peritore-Galve et al., 2021; Wang et al., 2022). In April 2022, a new bacterial disease was discovered on leaves, stems and fruits of tomato in a farmer's greenhouse located in Longfeng District in DaQing (125°07`-125°15`E, 46°28`-46°32`N), Heilongjiang Province, China. This field had tomato disease incidences approximately 50%. Apparent brown discolorations were found on fruits, leaves and stems in tomato plants. Symptoms were similar to fungal brown spots caused by Phytophthora infestans of tomato (Zhi et al.,2021; Liu et al.,2021) (Supplementary Figure S1). To isolate and identify the pathogen, the tissues of infected fruits, leaves and stems with typical symptoms were excised from diseased plants separately, and were disinfected with 75% ethanol for 10 s followed by 2% NaClO for 3 min and then washed five to eight times with sterile water (Wang et al., 2017). Afterwards, the samples were plated on nutrient agar (NA) solid medium and incubated. After incubation at 30°C for 2-3 days, bacterial colonies were isolated, then purified on nutrient agar (NA) solid medium at least twice by a streak plate method (Dou et al., 2019; Li et al, 2021; Zhao et al., 2022). White colonies grew on the NA medium after incubating for 2 days, showing round, opaque and smooth, which was similar to characteristics described as Enterobacter cloacae (García-González et al., 2018; Li et al, 2021). To further confirm the speculation on the identity of the isolated bacterium, the fragments of 16S rRNA were amplified and sequenced. The sequence of 16S rRNA was uploaded into GeneBank with accession numbers (OP077195.1). BLAST analysis of the sequence showed 97.68% identity with one corresponding sequence of E. cloacae in GeneBank (namely MK937637.1). Furthermore, a phylogenetic tree based on the sequence of 16S rRNA gene revealed that the isolate was grouped in the same clade as E. cloacae (Supplementary Figure S2). Based on Koch postulates to test pathogenicity of isolated bacteria, bacteria were inoculated on 30 day-old healthy tomato plants with three leaves stages, and the re-isolation of bacteria were carried out after 2 days of inoculation. To confirm pathogenicity, the isolates were cultured on LB medium at 30℃ for 2 days to prepare suspensions and adjusted to an optical density (OD) of 0.2 at A600, with a final concentration of 1ⅹ108 CFU/ml. Eight potted tomato plants were sprayed with bacteria suspensions, and eight control potted plants were sprayed with sterile distilled water. These seedlings were incubated in a chamber at 30°C with a 12 h light/dark photoperiod, with 85% relative humidity. After 2 days, inoculated tomato seedlings showed irregular small spots in leaves and brown necrosis at blade tips, and 8 to 10 days later, the leaves of tomato plants browned and died. The symptoms were the same with those of the initial diseased leaves of tomato plants (Supplementary Figure S1). No symptoms were observed on the control leaves (Supplementary Figure S3). Pathogenicity tests were repeated three biological times with same results. Meanwhile, the bacteria strains were re-isolated from symptomatic inoculated seedlings and confirmed as E. cloacae by culture and sequence methods as above. In China, there are no detailed records about the causal agent of this disease on tomato in a published paper in Chinese and English. To our knowledge, this is the first report of Enterobacter leaf brown necrosis caused by E. cloacae on tomato in China. Those results are of great significance for the production and management of tomato in greenhouse and control of the disease.