Microbial Pathogen Effectors in Plant Disease

Abstract

Microbial plant pathogens use secreted effector proteins for successful infection of their host. This evolved state is rather exceptional as most microbes do not cause disease in the vast majority of plant species. An important primary activity of effectors is to interfere with a range of plant immune processes to evade and suppress pathogen detection, or to block immune signalling and downstream responses. Furthermore, effectors can enhance disease susceptibility by altering cellular processes and modulating host transcription. For most of these activities, effectors specifically target plant proteins that are central in these processes. An advanced virulence strategy is the post‐translational modification by effectors of plant targets to change their activity or stability. The knowledge gathered on the molecular mechanisms underlying effector‐triggered susceptibility of plants provides great potential for novel approaches of resistance breeding. Key Concepts Pathogens secrete and/or translocate effector proteins to promote plant disease. Besides their primary role in promoting disease, effectors or effector‐modified plant proteins can be recognised by resistance proteins to activate an effector‐triggered immune response. Many effectors block pathogen‐associated molecular pattern (PAMP)‐triggered immunity and/or effector‐triggered immunity. Other effectors rewire signalling pathways and reprogramme the plant cell to promote pathogen growth. Certain effectors can affect the activity or function of host proteins by post‐translational modifications e.g. (de)phosphorylation or targeting for proteosomal degradation.

Direct and Indirect Visualization of Bacterial Effector Delivery into Diverse Plant Cell Types during Infection

“…Pathogenic microbes manipulate their plant host by interfering with a plethora of cellular processes (Asai & Shirasu, 2015;Pelgrom & Van den Ackerveken, 2016). To hijack the cellular machinery of the plant, pathogens deliver effector proteins inside host cells by mechanisms that are often not fully understood.…”

Bacterial nucleomodulins: A coevolutionary adaptation to the eukaryotic command center

“…Since the discovery of the first nucleomodulins in Agrobacterium , other phytopathogen species were observed to control host genes through the use of transcription activator-like effector nucleases (TALENs) [ 28 , 29 , 35 ]. Xanthomonas and Ralstonia phytopathogens inject TALENs into host plant cells via a type III secretion system (T3SS).…”

“…Xanthomonas and Ralstonia phytopathogens inject TALENs into host plant cells via a type III secretion system (T3SS). Within the cell cytosol, the TALENs translocate to the nucleus where they bind TAL-specific DNA sequences in the host genome and induce specific host gene expression by mimicking host transcription activators [ 35 , 36 ]. While the contribution of TALENs to Xanthomonas pathogenesis has been largely characterized, their functional role during Ralstonia pathogenesis has yet to be explored [ 37 ].…”

“…The first nucleomodulin identified with specificity for binding directly to a eukaryotic promotor element, and the founder of the TALEN family, is AvrBs3 of Xanthomonas [ 35 , 37 ]. AvrBs3 was first discovered during the study of Xanthomonas campestris infection in pepper plant cells [ 37 ].…”

Hunting modulators of plant defence: the grapevine trunk disease fungus Eutypa lata secretes an amplifier for plant basal immunity

“…These elicitor-triggered defence responses can be manipulated by effectors secreted by the pathogen. Pathogen effectors can support infection success by indirectly modulating targets outside of innate immunity ( Pelgrom and Van den Ackerveken, 2001 ).…”