If you were to walk around asking people what they thought about mushrooms you would likely be met with varied opinions regarding their taste on top of pizza, whisked into soup, or grilled and sandwiched between two buns as a burger. If, however, you were to ask an agriculture worker or plant scientist what they thought about the diverse members of the fungal kingdom, they’d likely be pretty quick to shout the word “pathogen”! Fungi and their cousins, the oomycetes, are responsible for the devastation of a wide variety of agricultural crops each year, resulting in billions of dollars in losses and, in certain cases, famine and death. One of the most famous examples of a fungus-like phytopathogen is Phytophthera infestans, the causative agent responsible for the “Great Famine”. The “Great Famine”, as it has since been named, refers to a period between 1845 and 1852 in Ireland when potato blight wiped out potato crops throughout Europe. It has been estimated that over two-fifths of Ireland’s population was entirely dependent upon potato as a food source at the time and that the disease was directly responsible for the deaths and emigration of over two million Irish citizens.
While it is easy to be dismissive about the importance of potatoes as a food crop in Canada, the reliance on potato as a major source of calories by the world’s poor makes the search for a means of resisting potato blight of great importance. Advances in crop breeding for disease resistance (R) genes and fungicides effective against oomycetes have tempered the effects of potato blight; however, P. infestans continues to live up to its name – phthora, from the Greek “to ruin” and infestans, from the Latin “to attack” or “destroy”. Therefore, breeders have been desperate to discover a means of providing broad spectrum resistance for cultivated potato against P. infestans. It seems like Dr. Vivianne Vleeshouwers from Wageningen University in the Netherlands and her collaborators might have found just that.
In order to fend off pathogens, plants rely on two distinct types of immune response. In the first, extracellular receptors studded along the plant cell surface recognize conserved pathogen-associated molecular patterns (PAMPs), triggering a signalling cascade that results in transcriptional activation, callose deposition (to strengthen the plant cell wall), ion flux, and a variety of other defence responses. In the second, intracellular R proteins recognize pathogen avirulence proteins (or effectors), which again leads to a multifaceted, but unique, defence response. To date, defence against P. infestans has been limited to several species-specific R proteins. Unfortunately, the RxLR class of effectors recognized by these proteins is highly polymorphic and rapidly evolves to escape detection, resulting in disease.
Vleeshouwers and her group spearheaded by Juan Du at Huazhong Agricultural University in China have recently discovered that the elicitin response (ELR) protein from Solanum microdontum, a species of wild potato, is able to recognize a molecular pattern in elicitin, an extracellular protein conserved amongst Phytophthera species. They were able to demonstrate that upon treatment with P. infestans, ELR co-immunoprecipitated with immune co-receptors BAK1/SERK3, and that this association mediates broad-spectrum recognition and a defence response. They then transferred ELR into cultivated potato – a.k.a. the potato that is mass produced each year for human consumption – and were able to show enhanced resistance to two different P. infestans strains. While the level of resistance imparted to the cultivated potato species by ELR was markedly lower than that provided by previously described intracellular R proteins, the broader spectrum of recognition points towards ELR as an attractive candidate for resistance. Approaches that exploit the combination of ELR with the more easily evadable NB-LRR R proteins might be used to maximize resistance against oomycete phytopathogens, including P. infestans.
For many North Americans, the “Great Famine” represents the reason their great-great-grandparents migrated and set up shop on a new continent, a history far removed from the present. Unfortunately, history hasn’t just repeated itself but lives on, as P. infestans inflicts billions of dollars of damage each year to what remains one of the world’s most important food crops. While the research presented by Du et al. doesn’t yet solve the problem, it is an exciting starting point for providing the cultivated potato with broad-spectrum resistance to P. infestans and us with full and happy bellies.
1. Donnelly, J. S. (2008). The Great Irish Potato Famine. The History Press, Stroud, UK. p. 1-320.
2. Du, J. et al. (2015). Elicitin recognition confers enhanced resistance to Phytophthora infestans in potato. Nat Plants. 1:1-5.
3. Jones, J. D. & J. L. Dangl. (2006). The plant immune system. Nature. 444:323-329.