Despite the best efforts of plants to defend themselves against pathogens, these destructive organisms have evolved an array of methods to overcome plant defense systems.
There is a constant co-evolution between plants and their pathogens in a finely tuned biochemical warfare that leads to the evolution of genetic changes in the hosts and pathogens. One of the most dramatic episodes of this warfare was the Irish potato famine of the 19th century.
Potatoes grew in abundance in Ireland in the mid-1800s and were a life-saving crop. An insidious disease first discovered in the United States in 1843, and then utterly destroyed the potato crops in Ireland from 1845 to 1849, causing the severe Irish famine of the time.
The identification of this new organism named Phytophtora infestans, which nineteenth century scientists originally thought was a fungus, ushered in the science of plant pathology. While it shares many characteristics with fungi, this Oomycete is more closely related to brown algae. Taxonomists have placed it in a kingdom separate from the fungi.
Potato Blight Still a Problem Throughout the 21st Century
Over the intervening century and a half, plant breeders have been hard at work creating new varieties of potatoes resistant to this blight, only to see them eventually succumb to the disease. One of the more devastating outbreaks in recent time took place in Great Britain in 2006 when a new strain of potato blight caused the worst outbreak of the disease in 50 years.
Plant pathologists have been avidly studying the biology and more recently the genetics of this organism to try and determine how it is so effective at overcoming the resistance mechanisms of plants.
Improvements in the technology of sequencing DNA have enabled researchers to examine all of an organism’s DNA, a technique known as genome sequencing. This technique has proven powerful in helping researchers to determine which genes are present and compare the genetic structure between different strains of the same organism.
The first published large-scale sequence of the DNA of a strain of P. infestans appeared in 2009 in Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans, and provided clues as to how it rapidly evolved to cause disease on new types of potatoes.
In 2013, Dr. Michael D. Martin and his colleagues published the first sequence of this organism from herbarium specimens of the time of the famine in their study, Reconstructing genome evolution in historic samples of the Irish potato famine pathogen. This enabled them to perform molecular detective work on the strains of the 1800s to compare them to those of today and provided clues into the evolution of virulence in this pathogen.
The Basics of Potato Blight Molecular Warfare
One way in which biochemical warfare takes place is within the cells of the plant itself. Many plant pathogens secrete a suite of molecules known as effectors. These molecules are typically proteins that manipulate the chemistry of the infected plant cells.
Plants frequently evolve the ability to recognize specific effectors and respond with resistance to the organism. Such R genes (for resistance) are then bred into new varieties of plants to make them resistant. Generally, it is just a matter of time before pathogens evolve the ability to cause disease on resistance plants. P. infestans is particularly effective at evolving to overcome resistance.
Scientists believe that their production of a variety of effectors results in their ability to rapidly evolve to cause disease on resistant plants. The potato blight organism produces a suite of effectors that vary in their effectiveness to help cause disease, depending on the genetics of the particular cultivar of potato they are attacking. A number of these effectors have a signature amino acid motif of RXLR.
The Rapid Evolution of the Potato Blight Organism
The function of many genes involved in pathogenicity have yet to be revealed. Dr. Martin and his team found that the European strain from 1899 lacked 26 genes that have a known function. Of these missing genes, 88% are known to have a role in causing disease on plants. Clearly, the genetic makeup of this pathogen has changed dramatically over time.
Dr. Brian J. Haas’ publication of the first genome wide sequence of this organism identified regions of the DNA that were rapidly changing, indicating that the genes in those regions were rapidly evolving to form new types. The effector genes were localized to these regions, suggesting a mechanism for the rapid changes in these genes that affect the pathogenicity of the organism.
While the strains from the 19th century were advanced for their time and able to decimate the potato varieties of that era, Dr. Martin’s genome analysis shows that these historical strains lack many of the effector genes present in today’s varieties.
In particular, more RXLR effectors were absent from the historical strains than from those of today. Dr. Martin identified five new genes of this type among modern isolates from the US that were lacking in the 19th century strains. Dr. Martin told Decoded Plants that “I think the absence of many of the currently important effector genes in the historical isolates suggests that the strains of today are even more virulent than those found in 19th century Europe.”
Rapid Changes in the Population of Effectors
The potato blight organism is highly effective at evolving to the ability to cause disease on resistant varieties of potatoes. Scientists believe that the production of a large suite of effector molecules that perturb the hosts’ defense mechanisms results in the ability of the strains to adapt to new types of resistance.
A comparison of the genomic sequences of historical isolates that caused the Irish famine of the mid 1880s to those of today has revealed significant changes in the genetic makeup of the different isolates. The newer ones differ greatly from the older isolates in their genetic coding for effector molecules. The rapid changes in the genetic composition of these molecules appears to explain the ability of the potato blight organism to rapidly evolve to overcome man’s efforts to control this disease.