Scientists have unravelled the genome of Phytophthora infestans, the pathogen that poses the most serious threat to the world’s potato crop. Nearly one hundred researchers from 34 institutes in six countries, including scientists at Wageningen University, took part in the project to determine the DNA sequence and to map the genes. They discovered that the genome of the causal agent of potato late blight is twice to four times the size of comparable organisms. Moreover, the divergent structure of the genome offers an insight into the pathogen’s success in besieging potato plants. Nature will publish the results on 9 September.
Potato late blight caused by P. infestans is a dreaded disease for potato growers: the plants die within five to seven days after infection. Crop failures result in an annual loss of 9.4 million euros in the Netherlands alone. Growers spend more than 115 million euros a year on almost weekly treatments with fungicides in endeavouring to limit their losses. The Dutch potato crop is treated with a larger amount of fungicide than any other crop.
The international research team, managed by the Broad Institute, the USA, counted 240 million base pairs in the Phytophthora infestans genome. This genome is particularly large in comparison with two related species, P. sojae, which infects soybean (95 million) and P. ramorum, which thrives on oak and other tree species (65 million). The majority of the almost 18,000 P. infestans genes are clustered in gene dense regions: they are mostly housekeeping genes that have remained unchanged for million years and are involved, for example, in the organism’s metabolism. They play virtually no role in the colonisation of the host plant. However, the majority of the genome (74 %) is comprised of long regions consisting of copies of the same DNA elements, without many genes. The rare genes that are located in these repeat-rich regions play a crucial role in the infection of the plant. The gene desert is particularly dynamic: genes appear, mutate or die out at a rapid pace. The changeability of the genes in these genome regions ensures that some strains among the many different P, infestans strains present in nature will have the weapons needed for a successful attack on the plants. These strains then reproduce very rapidly.
Attack
When Phytophthora comes into contact with a plant it launches its attack by secreting proteins that establish a bridgehead in the plant cell to prepare for the pathogen’s further penetration of the tissue. The research team identified more than 500 RXLR genes and 200 CRN genes in the P. infestans genome, which are referred to as ‘effector genes’ that play a role in the pathogen’s attack on the potato plant. A better insight into the mechanisms by which the pathogen infects the plant will enable researchers to improve the plant’s defense machinery and, ultimately, this may result in a substantial reduction in the potato growers’ use of chemical crop protection agents.
Illustration:
» Download: jpg of a larger image
» Download: pdf of a larger image
Family tree of the CRN proteins which Phytophthora deploys to colonise a potato plant. The variously-coloured branches emanating from the centre show the mutual relationships between P. infestans (blue), P. sojae (yellow) and P. ramorum (red). The multicoloured rim shows the attacking proteins (effectors), comprised of a number of domains. Some of these have remained unchanged during evolution (blue triangles and green rhombi), whilst others are highly variable (other forms and colours). The diversity in effector proteins and the dynamics of the Phytophthora genome contribute to P. infestans’ ability to circumvent late blight resistance genes in potato. (Source: Haas et al. Nature, 9 September 2009).
Note
More information is available from Professor Dr Francine Govers, Laboratory of Phytopathology, Wageningen University, tel. +31 (0)317 483138, e-mail francine.govers@wur.nl, or from Jac Niessen, Science Information Officer, Wageningen University, tel. +31 (0)317 485003, e-mail jac.niessen@wur.nl. (the DOI of the publication will be 10.1038/nature08358).