Common and contrasting themes in host cell-targeted effectors from bacterial, fungal, oomycete and nematode plant symbionts described using the Gene Ontology

Abstract

A wide diversity of plant-associated symbionts, including microbes, produce proteins that can enter host cells, or are injected into host cells in order to modify the physiology of the host to promote colonization. These molecules, termed effectors, commonly target the host defense signaling pathways in order to suppress the defense response. Others target the gene expression machinery or trigger specific modifications to host morphology or physiology that promote the nutrition and proliferation of the symbiont. When recognized by the host's surveillance machinery, which includes cognate resistance (R) gene products, defense responses are engaged to restrict pathogen proliferation. Effectors from diverse symbionts may be delivered into plant cells via varied mechanisms, including whole organism cellular entry (viruses, some bacteria and fungi), type III and IV secretion (in bacteria), physical injection (nematodes and insects) and protein translocation signal sequences (oomycetes and fungi). This mini-review will summarize both similarities and differences in effectors and effector delivery systems found in diverse plant-associated symbionts as well as how these are described with Plant-Associated Microbe Gene Ontology (PAMGO) terms.

Figure 1

Effector delivery structures of Gram-negative bacterium, oomycete, fungus, and nematode in plant cell. (A) Type III secretion system in Gram-negative bacterium injects effectors into the host cell. (B) The haustorium in biotrophic and hemibiotrophic filamentous pathogens is believed to be the site of effector release into the host cell. (C) Gland secretions, which include effectors, are injected into the plant cell via the stylet of the nematode. Effectors (E) thus delivered, can either suppress host defenses and/or trigger host cell defenses, which include programmed cell death (PCD) upon recognition by resistance (R) proteins. Recognition of effectors by R proteins may occur directly (observed with some fungal effectors) or indirectly as a result of interaction of the effectors with other host protein(s) (observed with a number of bacterial effectors). Potential subcellular locations of effectors such as the nucleus and chloroplasts are also shown.