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Abstract Background Bacterial spot of pepper (BSP), caused by four different Xanthomonas species, primarily X. euvesicatoria (Xe), poses a significant challenge in pepper cultivation. Host resistance is considered the most important approach for BSP control, offering long-term protection and sustainability. While breeding for resistance to BSP for many years focused on dominant R genes, introgression of recessive resistance has been a more recent focus of breeding programs. The molecular interactions underlying recessive resistance remain poorly understood. Results In this study, transcriptomic analyses were performed to elucidate defense responses triggered by Xe race P6 infection by two distinct pepper lines: the Xe-resistant line ECW50R containing bs5, a recessive resistance gene that confers resistance to all pepper Xe races, and the Xe-susceptible line ECW. The results revealed a total of 3357 upregulated and 4091 downregulated genes at 0, 1, 2, and 4 days post-inoculation (dpi), with the highest number of differentially expressed genes (DEGs) observed at 2 dpi. Pathway analysis highlighted DEGs in key pathways such as plant-pathogen interaction, MAPK signaling pathway, plant hormone signal transduction, and photosynthesis – antenna proteins, along with cysteine and methionine metabolism. Notably, upregulation of genes associated with PAMP-Triggered Immunity (PTI) was observed, including components like FLS2, Ca-dependent pathways, Rboh, and reactive oxygen species (ROS) generation. In support of these results, infiltration of ECW50R leaves with bacterial suspension of Xe led to observable hydrogen peroxide accumulation without a rapid increase in electrolyte leakage, suggestive of the absence of Effector-Triggered Immunity (ETI). Furthermore, the study confirmed that bs5 does not disrupt the effector delivery system, as evidenced by incompatible interactions between avirulence genes and their corresponding dominant resistant genes in the bs5 background. Conclusion Overall, these findings provide insights into the molecular mechanisms underlying bs5-mediated resistance in pepper against Xe and suggest a robust defense mechanism in ECW50R, primarily mediated through PTI. Given that bs5 provides early strong response for resistance, combining this resistance with other dominant resistance genes will enhance the durability of resistance to BSP.

Background: Bacterial spot of pepper (BSP), caused by four different Xanthomonas species, primarily X. euvesicatoria (Xe), poses a significant challenge in pepper cultivation. Host resistance is considered the most important approach for BSP control, offering long-term protection and sustainability. While breeding for resistance to BSP for many years focused on dominant R genes, introgression of recessive resistance has been a more recent focus of breeding programs. The molecular interactions underlying recessive resistance remain poorly understood. Results: In this study, transcriptomic analyses were performed to elucidate defense responses triggered by Xe race P6 infection by two distinct pepper lines: the Xe-resistant line ECW50R containing bs5, a recessive resistance gene that confers resistance to all pepper Xe races, and the Xe-susceptible line ECW. The results revealed a total of 3357 upregulated and 4091 downregulated genes at 0, 1, 2, and 4 days post-inoculation (dpi), with the highest number of differentially expressed genes (DEGs) observed at 2 dpi. Pathway analysis highlighted DEGs in key pathways such as plant-pathogen interaction, MAPK signaling pathway, plant hormone signal transduction, and photosynthesis – antenna proteins, along with cysteine and methionine metabolism. Notably, upregulation of genes associated with PAMP-Triggered Immunity (PTI) was observed, including components like FLS2, Ca-dependent pathways, Rboh, and reactive oxygen species (ROS) generation. In support of these results, infiltration of ECW50R leaves with bacterial suspension of Xe led to observable hydrogen peroxide accumulation without a rapid increase in electrolyte leakage, suggestive of the absence of Effector-Triggered Immunity (ETI). Furthermore, the study confirmed that bs5 does not disrupt the effector delivery system, as evidenced by incompatible interactions between avirulence genes and their corresponding dominant resistant genes in the bs5 background. Conclusion: Overall, these findings provide insights into the molecular mechanisms underlying bs5-mediated resistance in pepper against Xe and suggest a robust defense mechanism in ECW50R, primarily mediated through PTI. Given that bs5 provides early strong response for resistance, combining this resistance with other dominant resistance genes will enhance the durability of resistance to BSP. [ABSTRACT FROM AUTHOR]

Pepper (Capsicum annuum L.) carrying the gds (corresponding to bs5) gene can prevent the development of bacterial leaf spot disease without HR. However, little is known regarding the development of the resistance mechanism encoded by gds, especially its influence on the bacterium. Here, the effect of gds was compared with pattern-triggered immunity (PTI), another form of asymptomatic resistance, to reveal the interactions and differences between these two defense mechanisms. The level of resistance was examined by its effect on the bacterial growth and in planta expression of the stress and pathogenicity genes of Xanthomonas euvesicatoria. PTI, which was activated with a Pseudomonas syringae hrcC mutant pretreatment, inhibited the growth of Xanthomonas euvesicatoria to a greater extent than gds, and the effect was additive when PTI was activated in gds plants. The stronger influence of PTI was further supported by the expression pattern of the dpsA bacterial stress gene, which reached its highest expression level in PTI-induced plants. PTI inhibited the hrp/hrc expression, but unexpectedly, in gds plant leaves, the hrp/hrc genes were generally expressed at a higher level than in the susceptible one. These results imply that different mechanisms underlie the gds and PTI to perform the symptomless defense reaction. [ABSTRACT FROM AUTHOR]

Pepper (Capsicum annuum L.) carrying the gds (corresponding to bs5) gene can prevent the development of bacterial leaf spot disease without HR. However, little is known regarding the development of the resistance mechanism encoded by gds, especially its influence on the bacterium. Here, the effect of gds was compared with pattern-triggered immunity (PTI), another form of asymptomatic resistance, to reveal the interactions and differences between these two defense mechanisms. The level of resistance was examined by its effect on the bacterial growth and in planta expression of the stress and pathogenicity genes of Xanthomonas euvesicatoria. PTI, which was activated with a Pseudomonas syringae hrcC mutant pretreatment, inhibited the growth of Xanthomonas euvesicatoria to a greater extent than gds, and the effect was additive when PTI was activated in gds plants. The stronger influence of PTI was further supported by the expression pattern of the dpsA bacterial stress gene, which reached its highest expression level in PTI-induced plants. PTI inhibited the hrp/hrc expression, but unexpectedly, in gds plant leaves, the hrp/hrc genes were generally expressed at a higher level than in the susceptible one. These results imply that different mechanisms underlie the gds and PTI to perform the symptomless defense reaction.

• Many phytopathogenic bacteria inject virulence effector proteins into plant cells. To identify novel virulence effectors of the bacterial plant pathogen Xanthomonas, a worldwide collection of pepper ( Capsicum annuum) pathogenic Xanthomonas strains was studied. • Xanthomonas gardneri strains produced in pepper enhanced watersoaking, a phenotype that is typical of a compatible interaction. Transfer of X. gardneri library clones into a Xanthomonas euvesicatoria recipient strain revealed that enhanced watersoaking was attributable to avrHah1 (avirulence ( avr) gene homologous to avrBs3 and hax2, No. 1), a novel avrBs3-like gene. • avrHah1 is a novel member of the avrBs3 family that encodes tandemly arranged repeat units of both 34 and 35 amino acid lengths. Although AvrHah1 is only distantly related to AvrBs3, it was shown to trigger a Bs3-dependent hypersensitive response (HR). When fused to a nuclear export signal, AvrHah1 is no longer capable of triggering a Bs3 HR, indicating that nuclear targeting of AvrHah1 is crucial to its recognition. • Phylogenetic analysis revealed that, although AvrBs3 and AvrHah1 are only distantly related, they share blocks of high homology within potentially solvent-exposed repeat units. Thus, these data suggest that the recognition specificity of AvrBs3-like proteins is predominantly determined by solvent-exposed residues, rather than by overall homology or repeat unit length. [ABSTRACT FROM AUTHOR]