The genome of the potato blight mould (Phytophthora infestans) has been successfully sequenced in an international collaboration between scientists. The work, published in Nature last week, also reveals how the potato pathogen has adapted to cause disease.
Phytophthora infestans is a water mould (or oomycete) and the destructive pathogen that causes potato blight (or late blight). Oomycetes are microscopic organisms that form long filaments that look like, but are evolutionarily distinct from, fungi. P. infestans is infamous in human history as the culprit behind the Irish potato famine between 1845 and 1852. 1 million people died in Ireland (reducing the population by 20 – 25%) and 1 million more emigrated as a consequence of the famine. P. infestans is still a significant worldwide agricultural problem; potatoes are the 4th largest agricultural crop and crop losses as a result of potato blight are estimated to cost $6.7 billion. This potato pathogen is incredibly difficult to manage as it can rapidly adapt to control strategies, such as genetically engineering blight resistant potatoes, and still cause disease that rots the leaves and tubers of the potato plant.
The P. infestans genome is the biggest genome, at 240 megabases (Mb), sequenced so far in the chromalveolates (a eukaryote super group that contains water moulds and algae). Interestingly, the genome is so big because it is full of repetitive DNA, which makes up 74% of the genome. This includes a large number of transposons (so called “jumping genes”, DNA sequences that can move around to different positions within the genome). The P. infestans genome was compared to two related genomes, P. sojae (the cause of soy bean root rot) and P. ramorum (the cause of sudden oak death). This revealed that Phytophthora genomes have an unusual organisation; blocks densely packed with conserved (i.e. identical) genes between the genomes, which are separated by regions with high numbers of repeated DNA with few genes present. Interestingly, rapidly evolving virulence genes (which are induced during infection and allow the organism to colonise the potato plant) are present in these gene-sparse areas in P. infestans.
Highly dynamic regions of the Phytophthora genome may be crucial for its rapid adaptability to host plants and how it has evolved as a plant pathogen. The full genome sequence of P. infestans allows scientists to identify essential genes that allow the pathogen to destroy potato crops. This could help researchers devise new ways to protect potato plants from P. infestans, such as breeding new types of resistant potato plants or developing new (hopefully environmentally-friendly) chemicals to kill the potato pathogen.