Identification of a Putative DNA-Binding Protein in Arabidopsis That Acts as a Susceptibility Hub and Interacts With Multiple Pseudomonas syringae Effectors

Solanum pimpinellifolium exhibits complex genetic resistance to Pseudomonas syringae pv. tomato

Pseudomonas syringae pv. tomato (Pst) is the causal agent of bacterial speck disease in tomatoes. The Pto/Prf gene cluster from Solanum pimpinellifolium was introgressed into several modern tomato cultivars and provided protection against Pst race 0 strains for many decades. However, virulent Pst race 1 strains that evade Pto-mediated immunity now predominate in tomato-growing regions worldwide. Here we report the identification of resistance to a Pst race 1 strain (Pst19) in the wild tomato accession S. pimpinellifolium LA1589 (hereafter LA1589), using our rapid high-throughput seedling screen. LA1589 supports less bacterial growth than cultivars, and does not exhibit a hypersensitive response to Pst19. We tested an existing set of 87 Inbred Backcross Lines (IBLs) derived from a cross between susceptible Solanum lycopersicum E-6203 and Solanum pimpinellifolium LA1589 for resistance to Pst19. Using single-marker analysis, we identified three genomic regions associated with resistance. Bacterial growth assays on IBLs confirmed that these regions contribute to resistance in planta. We also mapped candidate genes associated with resistance in a cross between the Solanum lycopersicum var. lycopersicum cultivar Heinz BG-1706 and S. pimpinellifolium LA1589. By comparing candidates from the two mapping approaches, we were able to identify 3 QTL and 5 candidate genes in LA1589 for a role in resistance to Pst19. This work will assist in molecular marker-assisted breeding to protect tomato from bacterial speck disease.

Diverse interactions of five core type III effectors from Ralstonia solanacearum with plants

Ralstonia solanacearum is a widespread plant bacterial pathogen that can launch a range of type III effectors (T3Es) to cause disease. In this study, we isolate a pathogenic R. solanacearum strain named P380 from tomato rhizosphere. Five out of 12 core T3Es of strain P380 are introduced into Pseudomonas syringae DC3000D36E separately to determine their functions in interacting with plants. DC3000D36E that harbors each effector suppresses FliC-triggered Pti5 and ACRE31 expression, ROS burst, and callose deposition. RipAE, RipU, and RipW elicit cell death as well as upregulate the MAPK cascades in Nicotiana benthamiana. The derivatives RipC1ΔDXDX(T/V) and RipWΔDKXXQ but not RipAEK310R fail to suppress ROS burst. Moreover, RipAEK310R and RipWΔDKXXQ retain the cell death elicitation ability. RipAE and RipW are associated with salicylic acid and jasmonic acid pathways, respectively. RipAE and RipAQ significantly promote the propagation of DC3000D36E in plants. The five core T3Es localize in diverse subcellular organelles of nucleus, plasma membrane, endoplasmic reticulum, and Golgi network. The suppressor of G2 allele of Skp1 is required for RipAE but not RipU-triggered cell death in N. benthamiana. These results indicate that the core T3Es in R. solanacearum play diverse roles in plant-pathogen interactions.

Diversity, Evolution, and Function of Pseudomonas syringae Effectoromes

Pseudomonas syringae is an evolutionarily diverse bacterial species complex and a preeminent model for the study of plant-pathogen interactions due in part to its remarkably broad host range. A critical feature of P. syringae virulence is the employment of suites of type III secreted effector (T3SE) proteins, which vary widely in composition and function. These effectors act on a variety of plant intracellular targets to promote pathogenesis but can also be avirulence factors when detected by host immune complexes. In this review, we survey the phylogenetic diversity (PD) of the P. syringae effectorome, comprising 70 distinct T3SE families identified to date, and highlight how avoidance of host immune detection has shaped effectorome diversity through functional redundancy, diversification, and horizontal transfer. We present emerging avenues for research and novel insights that can be gained via future investigations of plant-pathogen interactions through the fusion of large-scale interaction screens and phylogenomic approaches.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.