RxLR Effectors: Master Modulators, Modifiers and Manipulators

Abstract

Cytoplasmic effectors with an Arg-any amino acid-Arg-Leu (RxLR) motif are encoded by hundreds of genes within the genomes of oomycete Phytophthora and downy mildew pathogens. There has been a dramatic increase in our understanding of the evolution, function and recognition of these effectors. Host proteins with a wide range of sub-cellular localisations and functions are targeted by RxLR effectors. Many processes are manipulated, including transcription, post-translational modifications, such as phosphorylation and ubiquitination, secretion and intracellular trafficking. This involves an array of RxLR effector modes-of-action, including stabilisation or destabilisation of protein targets; altering or disrupting protein complexes; inhibition or utility of target enzyme activities; and changing the location of protein targets. Interestingly, approximately 50% of identified host proteins targeted by RxLR effectors are negative regulators of immunity. Avirulence RxLR effectors may be directly or indirectly detected by nucleotide-binding leucine-rich repeat resistance (NLR) proteins. Direct recognition by a single NLR of RxLR effector orthologues conserved across multiple Phytophthora pathogens may provide wide protection of diverse crops. Failure of RxLR effectors to interact with or appropriately manipulate target proteins in nonhost plants has been shown to restrict host range. This knowledge can potentially be exploited to alter host targets to prevent effector-interaction, providing a barrier to host infection. Finally, recent evidence suggests that RxLR effectors, like cytoplasmic effectors from fungal pathogen Magnaporthe oryzae, may enter host cells via clathrin-mediated endocytosis.

Transcriptome analysis of Phytophthora cactorum infecting strawberry identified RXLR effectors that induce cell death when transiently expressed in Nicotiana benthamiana

“…Thirteen of the 20 cloned RXLR effectors did not induce cell death in N. benthamiana indicating that these RXLRs do not directly activate the plant's immune system. Some RXLR effectors that do not trigger cell death, instead can suppress PTI or ETI induced cell death thereby promoting pathogen virulence (Wang et al, 2023). The cell death suppression effect of RXLRs can be studied by co-infiltration with known effectors or elicitins that induce cell death in N. benthamiana (Zhan et al, 2022).…”

Chloroplast protein StFC-II was manipulated by a Phytophthora effector to enhance host susceptibility

“…Diverse host processes, including transcription, translation, post-translational modifications, and intracellular trafficking etc. were manipulated by RxLR effectors [ 13–15 , 55 ]. RxLR effectors regulate the activity of certain host enzymes.…”