Abstract
Many plant-associated organisms, including microbes, nematodes, and insects, deliver effector proteins into the apoplast, vascular tissue, or cell cytoplasm of their prospective hosts. These effectors function to promote colonization, typically by altering host physiology or by modulating host immune responses. The same effectors however, can also trigger host immunity in the presence of cognate host immune receptor proteins, and thus prevent colonization. To circumvent effector-triggered immunity, or to further enhance host colonization, plant-associated organisms often rely on adaptive effector evolution. In recent years, it has become increasingly apparent that several effectors of plant-associated organisms are repeat-containing proteins (RCPs) that carry tandem or non-tandem arrays of an amino acid sequence or structural motif. In this review, we highlight the diverse roles that these repeat domains play in RCP effector function. We also draw attention to the potential role of these repeat domains in adaptive evolution with regards to RCP effector function and the evasion of effector-triggered immunity. The aim of this review is to increase the profile of RCP effectors from plant-associated organisms.
Highlights
Many plant-associated organisms, including microbes, nematodes, and insects, deliver effector proteins into the apoplast, vascular tissue, or cell cytoplasm of their prospective hosts
Analyses of protein sequence and tertiary structure have revealed that several effectors of plant-associated organisms are repeat-containing proteins (RCPs)
Repeat domains may contribute to the rapid adaptive evolution of RCP effectors, providing a source of functional diversity, flexibility, and/or a means of evading host recognition
Summary
Many plant-associated organisms, including microbes, nematodes, and insects, deliver effector proteins into the apoplast, vascular tissue, or cell cytoplasm of their prospective hosts. Diverse plant-associated organisms, including bacteria, fungi, oomycetes, nematodes, and insects, secrete or inject a suite of proteins, termed effectors, into the tissues of their prospective hosts (Bozkurt et al, 2012; Deslandes and Rivas, 2012; Mitchum et al, 2013; Jaouannet et al, 2014; Lo Presti et al, 2015) These effectors, which localize to the host apoplast, or are targeted to various plant cell compartments, function to promote colonization, typically by altering host physiology or by modulating host immune responses (Hogenhout et al, 2009; Win et al, 2012a). This in turn provides the effector with a larger surface area-to-volume ratio than that of a typical globular protein of equivalent amino acid length, a feature that is well-suited to certain functional roles. This is elegantly illustrated by the transcription activator-like (TAL) effectors of the bacterial plant pathogens, Xanthomonas spp., which interact with host DNA in the plant cell nucleus to hijack host genes (by transcriptional activation) whose expression
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