Roadmap for future research on plant pathogen effectors

Abstract

Bacterial and eukaryotic plant pathogens deliver effector proteins into plant cells to promote pathogenesis. Bacterial pathogens containing type III protein secretion systems are known to inject many of these effectors into plant cells. More recently, oomycete pathogens have been shown to possess a large family of effectors containing the RXLR motif, and many effectors are also being discovered in fungal pathogens. Although effector activities are largely unknown, at least a subset suppress plant immunity. A plethora of new plant pathogen genomes that will soon be available thanks to next-generation sequencing technologies will allow the identification of many more effectors. This article summarizes the key approaches used to identify plant pathogen effectors, many of which will continue to be useful for future effector discovery. Thus, it can be viewed as a 'roadmap' for effector and effector target identification. Because effectors can be used as tools to elucidate components of innate immunity, advances in our understanding of effectors and their targets should lead to improvements in agriculture.

Figure 1

‘Roadmap’ for discovery of plant pathogen effectors and their targets. The flowchart shows the approaches used for effector discovery (green boxes), target discovery (yellow boxes) and target function discovery (light blue boxes). Starting, intermediate and stopping points in the flowchart are depicted as dark blue boxes. Pink boxes depict comparisons that can be carried out after effector repertoires and targets have been isolated that may give insights to virulence and nonhost resistance. The lines connecting each box indicate that it may be used in combination with other approaches in the network, not necessarily just the adjacent boxes. Useful tools for studying eukaryotic effectors are the ‘effector detector’ screens that fuse candidate effectors to type III secretion signals, such that they are injected into plant cells by a bacterial strain containing a type III protein secretion system (Rentel et al., 2008; Sohn et al., 2007). Effectors can be expressed transiently or transgenically in planta to determine whether they produce phenotypes, such as enhanced susceptibility to pathogens, or whether they suppress plant innate immunity outputs. T‐DNA knock‐out and RNA interference knock‐down plants or plants over‐expressing effector targets can be used to determine target function and to confirm that they are effector targets. See text for additional details.