Researchers from the Virginia Bioinformatics Institute (VBI) at Virginia
Tech have identified the region of a large family of virulence proteins in
oomycete plant pathogens that enables the proteins to enter the cells of
their hosts. The protein region contains the amino acid sequence motifs RXLR
and dEER and has the ability to carry the virulence proteins across the
membrane surrounding plant cells without any additional machinery from the
pathogen. Once inside the plant cell, the proteins suppress the immune
system of the plant allowing the infection to progress. The work, which
focused on the virulence protein Avr1b from the soybean plant pathogen
Phytophthora sojae, is published in the advance online edition of The Plant
Cell.
Oomycetes are fungal-like organisms related to marine algae that cause tens
of billions of dollars of losses to agriculture, forestry and natural
ecosystems every year. The oomycete Phytophthora infestans caused the Irish
potato famine in the nineteenth century. Another Phytophthora species, P.
ramorum, is causing Sudden Oak Death disease in California's coastal
forests. P. sojae results in $200-300 million in annual losses for
commercial soybean farmers in the United States and estimated annual soybean
losses of $1-2 billion worldwide. All of these oomycete species contain
hundreds of genes that encode for virulence proteins that have the RXLR-dEER
region.**

The virulence proteins, including Avr1b, enter the soybean host where they
are capable of suppressing an important process in plant immunity called
programmed cell death.*** Programmed cell death is an in-built suicide
mechanism that kills infected plant tissue, filling it with toxins so the
pathogen can no longer feed on it. By preventing this protective mechanism
in the host, the virulence proteins ensure that the pathogen can establish
an unassailable foothold in the plant tissue from which the pathogen can
pursue its destructive path.

Postdoctoral fellow Dr. Daolong Dou, the lead author of the article,
commented: "We have suspected for a long time that these virulence proteins
had some way of slipping inside plant cells to suppress immunity. Our
findings finally nail down that mechanism and enable us to focus on how to
block the entry mechanism."

The researchers also demonstrated that the RXLR and dEER motifs could be
replaced by similar targeting sequences found in effector proteins produced
by the malarial parasite Plasmodium. This hints that the targets of the
effectors in the soybean and human hosts may be very ancient.

VBI Professor Brett Tyler remarked: "The finding that virulence proteins
from oomycetes and the malaria parasite Plasmodium use the same entry
mechanism means that we may be able to use the same or similar drugs to
block infection by both groups of pathogens. This type of approach may also
be relevant to other groups of pathogens, such as fungi, which we also
suspect of slipping virulence proteins into host cells."

The breakthrough was enabled by an ingenious device for introducing DNA into
living tissues invented by a Virginia Tech undergraduate, Shiv Kale. Kale,
who has subsequently joined Dr. Tyler's research team as a graduate student,
remarked: "The double-barreled Gene Gun enabled us to make much more
accurate measurements of the Avr1b protein than were previously possible,
which made it practicable to measure the action of the RXLR and dEER
motifs." Kale was co-lead author of the article.

The research was supported by funding from the National Research Initiative
of the United States Department of Agriculture's Cooperative State Research,
Education and Extension Service, the National Science Foundation, the
Netherlands Genomics Initiative, and the Virginia Bioinformatics Institute.

* Daolong D, Kale SD, Wang X, Jiang RHY, Bruce NA, Arredondo FD, Zhang X,
Tyler BM (2008) RXLR-mediated entry of Phytophthora sojae effector Avr1b
into soybean cells does not require pathogen-encoded machinery. The Plant
Cell Published on: www.plantcell.org/cgi/doi/10.1105/tpc.107.056093.