Ralstonia solanacearum pandemic lineage strain UW551 overcomes inhibitory xylem chemistry to break tomato bacterial wilt resistance

Transcriptome responses to Ralstonia solanacearum infection in tetraploid potato

Potato (Solanum tuberosum) is an important global food source, the growth of which can be severely impacted by Ralstonia solanacearum bacterial infection. Despite extensive research, the molecular mechanisms of potato resistance to this pathogen are imperfectly known. Huashu No. 12, a tetraploid potato genotype, is highly resistant to R. solanacearum. We inoculate Huashu No. 12 and Longshu No. 7 (highly susceptible to R. solanacearum) with R. solanacearum to compare disease resistance in these two potato varieties. Huashu No. 12 has significantly higher resistance to R. solanacearum infection than Longshu No. 7, with increased lignin content, and an abundance of callose and strong autofluorescence in the phloem sieve tube. Enzymes (e.g., superoxide dismutase, catalase, peroxidase, phenylalanine ammonia-lyase, and polyphenol oxidase) contribute to R. solanacearum resistance in Huashu No. 12. Transcriptome sequencing reveals 659 differentially expressed genes between the two varieties, with the ethylene responsive factor family containing the most differentially expressed genes. Gene ontology and KEGG analyses provided further insights into the genetic basis and molecular mechanisms underlying plant defense against R. solanacearum disease. By demonstrating the importance of enzymes and differential gene expression in Huashu No. 12 resistance to R. solanacearum infection, the breeding of disease-resistant potato becomes increasingly feasible.

Ubiquitination and degradation of plant helper NLR by the Ralstonia solanacearum effector RipV2 overcome tomato bacterial wilt resistance

The Ralstonia solanacearum species complex causes bacterial wilt in a variety of crops. Tomato cultivar Hawaii 7996 is a widely used resistance resource; however, the resistance is evaded by virulent strains, with the underlying mechanisms still unknown. Here, we report that the phylotype Ⅱ strain ES5-1 can overcome Hawaii 7996 resistance. RipV2, a type Ⅲ effector specific to phylotype Ⅱ strains, is vital in overcoming tomato resistance. RipV2, which encodes an E3 ubiquitin ligase, suppresses immune responses and Toll/interleukin-1 receptor/resistance nucleotide-binding/leucine-rich repeat (NLR) (TNL)-mediated cell death. Tomato helper NLR N requirement gene 1 (NRG1), enhanced disease susceptibility 1 (EDS1), and senescence-associated gene 101b (SAG101b) are identified as RipV2 target proteins. RipV2 is essential for ES5-1 virulence in Hawaii 7996 but not in SlNRG1-silenced tomato, demonstrating SlNRG1 to be an RipV2 virulence target. Our results dissect the mechanisms of RipV2 in disrupting immunity and highlight the importance of converged immune components in conferring bacterial wilt resistance.

Ralstonia solanacearum pandemic lineage strain UW551 overcomes inhibitory xylem chemistry to break tomato bacterial wilt resistance

Plant-pathogenic Ralstonia strains cause bacterial wilt disease by colonizing xylem vessels of many crops, including tomato. Host resistance is the best control for bacterial wilt, but resistance mechanisms of the widely used Hawaii 7996 tomato breeding line (H7996) are unknown. Using growth in ex vivo xylem sap as a proxy for host xylem, we found that Ralstonia strain GMI1000 grows in sap from both healthy plants and Ralstonia-infected susceptible plants. However, sap from Ralstonia-infected H7996 plants inhibited Ralstonia growth, suggesting that in response to Ralstonia infection, resistant plants increase inhibitors in their xylem sap. Consistent with this, reciprocal grafting and defence gene expression experiments indicated that H7996 wilt resistance acts in both above- and belowground plant parts. Concerningly, H7996 resistance is broken by Ralstonia strain UW551 of the pandemic lineage that threatens highland tropical agriculture. Unlike other Ralstonia, UW551 grew well in sap from Ralstonia-infected H7996 plants. Moreover, other Ralstonia strains could grow in sap from H7996 plants previously infected by UW551. Thus, UW551 overcomes H7996 resistance in part by detoxifying inhibitors in xylem sap. Testing a panel of xylem sap compounds identified by metabolomics revealed that no single chemical differentially inhibits Ralstonia strains that cannot infect H7996. However, sap from Ralstonia-infected H7996 contained more phenolic compounds, which are known to be involved in plant antimicrobial defence. Culturing UW551 in this sap reduced total phenolic levels, indicating that the resistance-breaking Ralstonia strain degrades these chemical defences. Together, these results suggest that H7996 tomato wilt resistance depends in part on inducible phenolic compounds in xylem sap.