Implication of the Type III Effector RipS1 in the Cool-Virulence of Ralstonia solanacearum Strain UW551.

Jeffrey K Schachterle,Qi Huang

doi:10.3389/fpls.2021.705717

Abstract

Members of the Ralstonia solanacearum species complex cause a variety of wilting diseases across a wide range of hosts by colonizing and blocking xylem vessels. Of great concern are race 3 biovar 2 strains of R. solanacearum capable of causing brown rot of potato at cool temperatures, which are select agents in the United States. To gain a better understanding of cool-virulence mechanisms, we generated libraries of transposon mutants in the cool-virulent R. solanacearum strain UW551 and screened 10,000 mutants using our seedling assay for significantly reduced virulence at 20°C. We found several mutants that exhibited reduced virulence at 28 and 20°C and also mutants that were only affected at the cooler temperature. One mutant of the latter chosen for further study had the transposon inserted in an intergenic region between a type III secretion system effector gene ripS1 and a major facilitator superfamily (MFS) protein gene. Gene expression analysis showed that expression of ripS1 was altered by the transposon insertion, but not the MFS protein gene. An independent mutant with this insertion upstream of ripS1 was generated and used to confirm virulence and gene expression phenotypes. The effector, RipS1, has unknown function and is part of a family of effectors belonging to the largest known type III effectors. The functional connection between RipS1 and cool-virulence of R. solanacearum UW551 suggests that RipS1 (and/or its upstream promoter element) may serve as a potential target for development of cool-virulence-specific diagnostic tools to differentiate the highly regulated cool-virulent strains from non-cool-virulent strains of R. solanacearum. Our results provide important information for continued work toward a better understanding of cool-virulence of R. solanacearum and development of proper control strategies to combat this important plant pathogen.

Highlights

The Ralstonia solanacearum species complex (Rssc) – a group of related Gramnegative beta-proteobacteria – is pathogenic to many plant species, including several crops important economically and to food security across the globe (Hayward, 1991)
Using a newly developed high-throughput virulence screening method (Schachterle and Huang, 2021) to screen 10,000 mutants of R. solanacearum strain UW551 in N. glutinosa grown in water agar wells of 96-well plates led to the discovery of 26 mutants with reduced virulence, suggesting the effectiveness of the screening method
UW551-TnR1, was chosen for further study and displayed significantly reduced virulence under cool temperatures but maintained full virulence under warm temperature conditions in the screen assay, and such virulence phenotype was confirmed by subsequent tomato seedling assays

Summary

Introduction

The Ralstonia solanacearum species complex (Rssc) – a group of related Gramnegative beta-proteobacteria – is pathogenic to many plant species, including several crops important economically and to food security across the globe (Hayward, 1991). Rssc strains that exhibit the cool-virulence phenotype with potato as the host were initially designated as race 3 biovar 2 strains on the basis of host range and biochemical properties and subsequently designated as phylotype IIB sequevars 1 and 2 based on molecular characters from multiplex PCR (Fegan and Prior, 2005). We identified a DNA region associated with the cool-virulence of R. solanacearum strain UW551 (Stulberg et al, 2018), and we developed the first PCR assay targeting the cool-virulence associated region for specific detection and identification of race 3 biovar 2 strains of R. solanacearum (Stulberg et al, 2018)

Methods

Results

Conclusion