Mycorrhizal symbiosis drives tolerance to potato cyst nematodes.

Host plant tolerance to pathogens is increasingly relevant as resistance sources and control options become scarce. Arbuscular mycorrhizal (AM) fungi are known to enhance plant stress tolerance, but it remains unclear whether they are essential for, or complement, innate tolerance. We observed that potato cultivars described as tolerant to G. pallida suffered yield loss under nematode pressure when grown in sterile soils, indicating a lack of tolerance. The introduction of Rhizophagus irregularis increased tuber biomass during nematode parasitism, with cultivars commercially labelled as tolerant exhibiting a stronger response to AM fungi. The data suggest cultivar differences in mycorrhizal responsiveness with the differential expression of a range of plant sugar transporter genes in "tolerant" cultivars inferring a role of sugar allocation in host tolerance. Overall, AM fungi are critical for conferring tolerance against G. pallida and revealing the underpinning genes may provide useful targets to explore in current commercially desirable yet intolerant cultivars.