Strain-Specific Hypersensitive and Extreme Resistance Phenotypes Elicited by Potato virus Y Among 39 Potato Cultivars Released in Three World Regions Over a 117-Year Period

Abstract

Strain-specific hypersensitive (HR) and extreme resistance (ER) phenotypes elicited in potato plants by three Potato virus Y (PVY) isolates in strain groups PVYO (BL and DEL3) and PVYD (KIP1) were studied. PVYO and PVYD isolates elicit HR genes Ny or putative Nd, respectively, and all three isolates elicit ER gene Ry. They were inoculated to 39 Australasian, European, or North American potato cultivars released over a 117-year period and harvested tubers were replanted. Both primary and secondary symptoms were recorded. Two European cultivars always developed ER following sap and graft inoculation and, thus, carried comprehensive PVY resistance gene Ry. One Australasian and two European cultivars always developed susceptible phenotypes and, thus, lacked genes Ry, Ny, and putative Nd. Sap inoculation with isolate KIP1 elicited localized HR (LHR) in 31 cultivars and both LHR and systemic HR (SHR) in three others; thus, all carried putative Nd. Isolates BL and DEL3 both elicited susceptible phenotypes in 11 of these 34 cultivars but LHR alone, SHR alone, or both LHR and SHR in the other 23 which, therefore, all carry Ny. With these two isolates, SHR expression ranged from very severe to very weak, with the greatest numbers of isolate–cultivar combinations occurring in the severe category with BL (n = 11) and moderate category (n = 12) with DEL3. Within the same isolate–cultivar combination, overall, SHR symptom expression was weaker with secondary than primary infection. With both primary and secondary infection, SHR expression was most severe with KIP1 and weakest with DEL3. Genes Ny and putative Nd were present in cultivars released between 1939 and 2010 or 1893 and 2010, respectively, occurring in cultivars from all three world regions. These findings have important implications concerning breeding new PVY-resistant potato cultivars, especially for countries lacking healthy seed potato stocks, or where subsistence farmers cannot afford them. An alternative to including gene Ry is incorporating as many strain-specific PVY resistance genes as possible.

Viral Diseases in Potato

“…Strain groups PVY C , PVY O and PVY Z elicit HR phenotypes with hypersensitivity genes Nc, Ny, or Nz, respectively. Strain groups PVY N and PVY E overcome all three hypersensitivity genes, but differ in the phenotypes they induce in tobacco, only PVY N eliciting veinal necrosis (Chikh-Ali et al 2014;Karasev and Gray 2013;Rowley et al 2015;Kehoe and Jones 2016;Jones and Vincent 2018). Such biological strains do not necessarily coincide with the phylogenetic lineages named after them.…”

“…For example, previously potato biological strain groups PVY C and PVY O were thought to coincide with major lineages PVY C and PVY O , respectively. However, this proved incorrect as isolates within biological strain group PVY D fitted within phylogroup PVY C and those in PVY Z within phylogroup PVY O (Kehoe and Jones 2016;Jones and Vincent 2018). As the number of complete PVY genome sequences from different world regions grows, phylogenetic nomenclature based on biological, geographical, and sequence names is becoming increasingly unsustainable so substituting Latinised numerals for current PVY subgroup names was suggested (Jones 2014;Kehoe and Jones 2016;Jones and Kehoe 2016).…”

“…There is still a need to determine how common resistance genes Ry, Nc, Nz, Ny, and putative Nd are among Andean potato cultivars and the degree of protection they provide against PVY in Andean potato crops. This applies not only to commercial plantings grown with or without access to healthy seed programs but also to Andean native potato landraces belonging to the four potato species that grow in Andean subsistence plantings (Jones 1981;Jones and Vincent 2018).…”

Potato virus Y; the Andean connection

“…PVY N and PVY E overcome Nc, Ny , and Nz (yet to be tested with putative Nd ), but are separated by the symptoms they cause in infected tobacco plants, only PVY N inducing veinal necrosis in this species (Jones 1990; Singh et al. 2008; Karasev and Gray 2013; Gray and Power 2018; Jones and Vincent 2018).…”

Epidemiology and control of potato virus Y in Kenya

“…Nytbr and Nctbr genes from S. tuberosum limits virus movement within the plant by inducing systemic necrosis on virus infected tissues (Nie et al 2012: Jones andStuart 2018). This is a form of resistance which results in little or no virus translocation to the daughter tubers from the mother plant after initial infection (Bertschinger et al 2017).…”