Wild potato genes: the future of late blight control
Researchers are studying the genes of wild potato species in the hopes of finding a novel approach to mitigating crop losses caused by the persistent late blight disease.
The destructive potential of the late blight crop disease pathogen Phytophthora infestans impacted and shaped global society throughout the 19th century. It caused unprecedented potato crop failures in Ireland which led to the Great Famine. Despite it happening over 175 years ago, the disease has still not been eradicated, demonstrating the aggressiveness, adaptability and fitness of this significant crop pathogen.
In Ireland, the potato industry remains a crucial part of the arable sector, with an estimated annual contribution to the national economy exceeding €111 million at farm gate level. Yet, each season, potato farmers are wholly dependent on the use of crop protectants to mitigate loss of yield and quality because of the late blight. This typically equates to upwards of 12 sprays each season, primarily due to the absence of robust genetic resistance against the disease in commonly grown potato varieties.
To combat this, researchers from Teagasc and the University of Dundee are employing novel enrichment sequencing technologies to get a unique insight into P. infestans and identify durable host resistance genes in wild potato species. This research is being completed through the ESoLaB project.
Fungicide resistance and the emergence of new strains P. infestans was introduced to Ireland through infected potato shipments from South America in the early 1840s. Its genotype (genetic material) dominated the global P. infestans population, but was displaced by another genotype in the
mid-20th century. Later on, so too was this genotype replaced by more aggressive P. infestans lineages, showing a continuous adaptation and evolution of the pathogen.
In the 19th century, a strategy to combat the disease was to use a copper-based fungicide. This was followed by the introduction of highly efficient fungicides such as metalaxyl in the 20th century. Unfortunately, the rapidly evolving P. infestans developed resistance to the fungicide, with the first confirmed case of metalaxyl resistance reported in 1981.
This vicious cycle has recently been repeated, with the emergence of a P. infestans strain that has the capacity to resist the inhibitory effects of fluazinam, an important active ingredient heavily used in modern-day fungicides. Consequently, the Irish potato sector faces a challenging future, with the efficacy of the chemistry toolbox decreasing and an absence of varieties with durable genetic resistance.
Novel strategies to target resistance
A natural wealth of P. infestans resistance (R) genes are available in wild potato species, which can be exploited for the deployment of R genes in commonly used potato cultivars through breeding approaches. To date, around 56 commercial potato cultivars are available in Ireland, out of which 34 are commercially grown.
Studies conducted as part of the ESoLaB project have revealed a noticeable gap between the number of functional R genes cloned and those that are utilised in the agronomically most important cultivars. The data shows that potato breeding has largely focused on the continuous use of four R genes (albeit with some geographical differences), whereas at least 17 distinct R genes against late blight have been cloned and described in scientific literature. This indicates that a number of novel resistances are available for consideration as part of an integrated crop protection strategy.
During P. infestans infection, plants undergo an immune response with various changes at both cellular and molecular level. This includes the activation of a plant’s first line of defence, after its cells recognise P. infestans cell surface receptors. To overcome this immune response, P. infestans releases specific proteins (termed ‘effectors’) to suppress the immune system of the plant. In turn, and as a counter defence mechanism, plants have developed a second line of defence that includes the recognition of these effectors by specific potato R genes.
Effectors recognised by R genes are in effect dysfunctional. As such, they are not able to further support the infection and are labelled as Avirulence (Avr) factors. In fact, the virulence potential or ‘profile’ of
P. infestans is determined by the variation in its effectors or Avr genes, driven by widely utilised R genes. As a result, the effector profile of P. infestans is an important predictive tool to assist breeders in deciding what R genes to incorporate into the breeding of novel potato varieties.
In spite of this, effector diversity within P. infestans has largely been ignored in the deployment strategies of new resistances against the disease. To address this, ESoLaB has utilised recently developed and validated genome technologies to study R gene deployment in top grown potato cultivars and to quantify the effector diversity within the Irish P. infestans population. This information will be utilised as a means to inform and guide the future breeding and commercialisation of novel potato varieties with durable resistance to late blight disease.
Phytophthora is Latin for plant (phyto) destroyer (phthora).
ESoLaB has received funding from the Research Leaders 2025 programme (cofunded by Teagasc and the European Union’s Horizon 2020 research and innovation programme) under the Marie Skłodowska-Curie grant agreement number 754380.
The project team would like to thank Steven Kildea (Teagasc) for his contribution to this research.
This article was first published in TResearch Winter 2022