Plant Cell Advance Online Publication
Published on July 11, 2008; 10.1105/tpc.107.056093
Received October 11, 2007
Returned for revision June 6, 2008
Accepted June 27, 2008
RXLR-Mediated Entry of Phytophthora sojae Effector Avr1b into Soybean Cells Does Not Require Pathogen-Encoded Machinery
Daolong Dou 1, Shiv D. Kale 1, Xia Wang 1, Rays H.Y. Jiang 1, Nathan A. Bruce 1, Felipe D. Arredondo 1, Xuemin Zhang 1, and Brett M. Tyler 1*
1 Virginia Bioinformatics Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
* To whom correspondence should be addressed. E-mail: bmtyler{at}vt.edu.
Effector proteins secreted by oomycete and fungal pathogens have been inferred to enter host cells, where they interact with host resistance gene products. Using the effector protein Avr1b of Phytophthora sojae, an oomycete pathogen of soybean (Glycine max), we show that a pair of sequence motifs, RXLR and dEER, plus surrounding sequences, are both necessary and sufficient to deliver the protein into plant cells. Particle bombardment experiments demonstrate that these motifs function in the absence of the pathogen, indicating that no additional pathogen-encoded machinery is required for effector protein entry into host cells. Furthermore, fusion of the Avr1b RXLR-dEER domain to green fluorescent protein (GFP) allows GFP to enter soybean root cells autonomously. The conclusion that RXLR and dEER serve to transduce oomycete effectors into host cells indicates that the >370 RXLR-dEER–containing proteins encoded in the genome sequence of P. sojae are candidate effectors. We further show that the RXLR and dEER motifs can be replaced by the closely related erythrocyte targeting signals found in effector proteins of Plasmodium, the protozoan that causes malaria in humans. Mutational analysis of the RXLR motif shows that the required residues are very similar in the motifs of Plasmodium and Phytophthora. Thus, the machinery of the hosts (soybean and human) targeted by the effectors may be very ancient.
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