A translocation signal for delivery of oomycete effector proteins into host plant cells.
Bacterial, oomycete and fungal plant pathogens establish disease by translocation of effector proteins into host cells, where they may directly manipulate host innate immunity. In bacteria, translocation is through the type III secretion system, but analogous processes for effector delivery are uncharacterized in fungi and oomycetes. Here we report functional analyses of two motifs, RXLR and EER, present in translocated oomycete effectors. We use the Phytophthora infestans RXLR-EER-containing protein Avr3a as a reporter for translocation because it triggers RXLR-EER-independent hypersensitive cell death following recognition within plant cells that contain the R3a resistance protein. We show that Avr3a, with or without RXLR-EER motifs, is secreted from P. infestans biotrophic structures called haustoria, demonstrating that these motifs are not required for targeting to haustoria or for secretion. However, following replacement of Avr3a RXLR-EER motifs with alanine residues, singly or in combination, or with residues KMIK-DDK--representing a change that conserves physicochemical properties of the protein--P. infestans fails to deliver Avr3a or an Avr3a-GUS fusion protein into plant cells, demonstrating that these motifs are required for translocation. We show that RXLR-EER-encoding genes are transcriptionally upregulated during infection. Bioinformatic analysis identifies 425 potential genes encoding secreted RXLR-EER class proteins in the P. infestans genome. Identification of this class of proteins provides unparalleled opportunities to determine how oomycetes manipulate hosts to establish infection.
Whisson
Stephen C
SC
Plant Pathology Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK. Steve.Whisson@scri.ac.uk
Boevink
Petra C
PC
Moleleki
Lucy
L
Avrova
Anna O
AO
Morales
Juan G
JG
Gilroy
Eleanor M
EM
Armstrong
Miles R
MR
Grouffaud
Severine
S
van West
Pieter
P
Chapman
Sean
S
Hein
Ingo
I
Toth
Ian K
IK
Pritchard
Leighton
L
Birch
Paul R J
PR
D000409:Alanine / Q000378:metabolism; D020418:Algal Proteins / Q000737:chemistry* / Q000378:metabolism*; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D019295:Computational Biology; D044043:Pectobacterium / Q000235:genetics; D010838:Phytophthora / Q000737:chemistry / Q000378:metabolism*; D021382:Protein Sorting Signals; D021381:Protein Transport; D044224:Pseudomonas syringae / Q000235:genetics; D011198:Solanum tuberosum / Q000378:metabolism* / Q000382:microbiology; D014026:Nicotiana / Q000378:metabolism* / Q000382:microbiology
D020418:Algal Proteins; D021382:Protein Sorting Signals; D000409:Alanine