Small RNA ArcZ Regulates Oxidative Stress Response Genes and Regulons in Erwinia amylovora

Evaluation of Plant Defense Inducers and Plant Growth Regulators for Fire Blight Management Using Transcriptome Studies and Field Assessments

Fire blight, caused by Erwinia amylovora, is a destructive disease of pome fruit trees. In the United States, apple and pear growers rely on applications of copper and antibiotics during bloom to control fire blight, but such methods have already led to regional instances of resistance. In this study, we used transcriptome analyses and field trials to evaluate the effectiveness of three commercially available plant defense elicitors and one plant growth regulator for fire blight management. Our data indicated that foliar applications of acibenzolar-S-methyl (ASM; Actigard 50WG) triggered a strong defense-related response in apple leaves, whereas applications of Bacillus mycoides isolate J (LifeGard WG) or Reynoutria sachalinensis extract (Regalia) did not. Genes upregulated by ASM were enriched in the biological processes associated with plant immunity, such as defense response and protein phosphorylation. The expression of several pathogenesis-related (PR) genes was induced by ASM as well. Surprisingly, many differentially expressed genes in ASM-treated apple leaves overlapped with those induced by treatment with prohexadione-calcium (ProCa; Apogee), a plant growth regulator that suppresses shoot elongation. Further analysis suggested that ProCa likely acts similarly to ASM to stimulate plant immunity because genes involved in plant defense were shared and significantly upregulated (more than twofold) by both treatments. Our field trials agreed with the transcriptome study, demonstrating that ASM and ProCa exhibit the best control performance relative to the other biopesticides. Taken together, these data are pivotal for the understanding of plant response and shed light on future improvements of strategies for fire blight management.

Genetic Dissection of the Erwinia amylovora Disease Cycle

Fire blight, caused by the bacterial phytopathogen Erwinia amylovora, is an economically important and mechanistically complex disease that affects apple and pear production in most geographic production hubs worldwide. We compile, assess, and present a genetic outlook on the progression of an E. amylovora infection in the host. We discuss the key aspects of type III secretion-mediated infection and systemic movement, biofilm formation in xylem, and pathogen dispersal via ooze droplets, a concentrated suspension of bacteria and exopolysaccharide components. We present an overall outlook on the genetic elements contributing to E. amylovora pathogenesis, including an exploration of the impact of floral microbiomes on E. amylovora colonization, and summarize the current knowledge of host responses to an incursion and how this response stimulates further infection and systemic spread. We hope to facilitate the identification of new, unexplored areas of research in this pathosystem that can help identify evolutionarily susceptible genetic targets to ultimately aid in the design of sustainable strategies for fire blight disease mitigation.

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

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.