Pyramiding resistances to potato cyst nematodes to produce potato cultivars with durable and broad-spectrum resistance
Resistance to cyst nematodes in potatoes is based on complex interactions between molecules produced by the pest and the plant’s immune system. The outcome is dependent on the species and subpopulations of PCN involved. Several distinct subpopulations (pathotypes) have been reported for G. rostochiensis and G. pallida that are differentiated according to their ability to infect different potato species (e.g., wild relatives of cultivated potatoes) containing different resistances. In some cases the genetic basis of the resistance is understood, for example resistance to G. rostochiensis (Ro1 and Ro4) is conferred by a single copy dominant gene (H1) as occurs in Maris Piper. The vast majority of the G. rostochiensis present in GB is of the Ro1 pathotype and resistant varieties have provided good control of this species - but favoured the multiplication of G. pallida.
Resistance to G. pallida has been more difficult to find and there is no single dominant gene conferring resistance to the common pathotypes Pa2/3. Sources that confer partial resistance have been identified (H3, Gpa5) and used in breeding programmes resulting in varieties such as Vales Everest and Innovator. Another resistance source (H2) that was first identified in ~1960’s provides resistance against pathotype Pa1 whilst also conferring a lower level of control of pathotypes Pa2/3. The aim of this study was to obtain a better understanding of the genetic basis of the H2resistance, identifying and mapping the genomic location of the resistance. This is the first step to more easily incorporate the resistance into new varieties. The ultimate aim (but outside the scope of the studentship) would be to combine H2-mediated resistance with resistance from other sources as this is expected to generate broad spectrum, strong and durable resistance to G. pallida. The student also investigated the genetic basis for the recognition of the nematode by a plant’s immune system. This involved studies of potential effector genes which encode proteins (effectors) that circumvent a plant’s first layer of defence but may also trigger a resistance response.
H2 is a single dominant gene – The progeny of the resistant : susceptible cross segregated with a close 1:1 ratio which is what is expected for a single dominant gene in cultivated potato. A single gene is ideal for breeding purposes.
H2 maps to potato chromosome 5 – Identifying genetic differences between resistant and susceptible samples allowed the location of the functional H2 resistance gene to be mapped to the top end of potato chromosome 5
Identified a marker which segregated 100% with the resistant allele – Markers designed based on the genetic differences between resistant and susceptible samples allowed an area which contains two resistance genes to be identified, one of the markers was based on a genetic difference within the gene, so this gene became the candidate H2 gene
Breeding material is very resistant to PCN – in parallel with the mapping of the H2resistance gene, preliminary breeding was undertaken using resistant P55/7 with a breeding line (10.Z.3.8.a) which contains resistance to G. rostochiensis (H1) and G. pallida pathotype Pa2/3 (H3). Initial testing of a sub-set of the progeny revealed that they were highly resistant to both G. rostochiensis and G. pallida.
Identify candidate effectors which have the potential to induce H2-mediated resistance- the research focussed on an effector localised to cells below the nematode stylet region. Further work is required to clarify the effector's role in activation of H2-mediated resistance.