Blacksburg, Va. - 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 July 11, 2008;
<
http://www.plantcell.org/cgi/doi/10.1105/tpc.107.056093> . See also the
following commentary: Govers F, Bouwmeester K (2008) Effector
trafficking: RXLR-dEER as extra gear for delivery into plant cells. The
Plant Cell 10.1105/tpc.108.062075
** Jiang, RHY, Tripathy S, Govers F, Tyler BM (2008) RXLR effector
reservoir in two Phytophthora species is dominated by a single rapidly
evolving super-family with more than 700 members. Proceedings of the
National Academy of Sciences 105(12): 4874-4879.
Dou D, Kale SD, Wang X-L, Chen Y-B, Wang Q-Q, Wang X, Jiang RHY,
Arredondo FD, Anderson RG, Thakur PB, McDowell JM, Wang YC, Tyler BM
(2008) Conserved C-terminal motifs required for avirulence and
suppression of cell death by Phytophthora sojae effector Avr1b. Plant
Cell 20(4): 1118-1133.
