This 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.
After entering the plant cell, the proteins suppress the immune system of the plant allowing the infection to progress.
The study, which focused on the virulence protein Avr1b from the soybean plant pathogen Phytophthora sojae, was made possible by an ingenious device invented by a Virginia Tech undergraduate, Shiv Kale. This device is used for introducing DNA into living tissues.
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. All the 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 suppress an important process in plant immunity called programmed cell death, which 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.
"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," said postdoctoral fellow Dr. Daolong Dou, the lead author of the article.
It was also shown that the RXLR and dEER motifs could be replaced by similar targeting sequences found in effector proteins produced by the malarial parasite Plasmodium. This indicates that the targets of the effectors in the soybean and human hosts may be very ancient.
"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," said Kale.
The study was published in the advance online edition of The Plant Cell.