Molecular Dialog of Ralstonia solanacearum and Plant Hosts with Highlights on Type III Effectors

Abstract

Ralstonia solanacearum is a highly destructive soil-borne bacterium that causes bacterial wilt disease in more than 310 plant species worldwide. The pathogenicity of the bacteria is closely associated with type III effectors (T3Es), a class of virulence factors that are delivered to host plant cells by the type III secretion system. In spite of the complex evolutionary history and genetic diversity of the R. solanacearum species complex (RSSC), more than 100 different T3Es have been identified from the genomes of various strains. Based on the available functional studies, certain T3Es interact with host plant proteins and suppress host cell immunity, whereas other T3Es are recognized by the host plant to trigger specific resistance mechanisms. This review summarizes the mechanisms by which T3Es interfere with plant immune responses and the activation of the plant defense system upon T3E recognition. This in-depth review of the molecular interactions between R. solanacearum and its host plants offers insights into the complexity of plant-pathogen interactions and provides a scientific rationale and theoretical support for the future breeding of resistant crops.

Keywords: Ralstonia solanacearum; pathogenesis; plant immunity; type III effectors.

Figure 1

Mechanisms of R. solanacearum T3E-mediated suppression of plant immunity. For detailed information, please refer to the main text. ⨂ represents that the corresponding pathways is inhibited, and ? indicates that the underlying mechanism is unknown. EPS, exopolysaccharide; CWDEs, cell wall-degrading enzymes; GSH, glutathione; FBN1, Fibrillin 1; PUB4, plant U-box protein 4; ATPA, ATP synthase F1 subunit α; RBOHs, respiratory burst oxidase homologs; CAT, catalase; TOR, the target of rapamycin; GABA, γ-aminobutyric acid; GADs, glutamate decarboxylases.

Figure 2

Diverse host range and specific recognition of R. solanacearum T3Es by different Solanaceae and Nicotiana species. These T3Es act as avirulence effectors and may be utilized to attenuate R. solanacearum virulence and enhance the host resistance. Rip, Ralstonia injected protein; Avr, avirulence protein.