Exploring mycorrhizal fungi in walnut with a focus on physiological roles

Walnuts are an economically important forest tree used for timber and nut production, and the nut of fruits is rich in various nutrients, becoming one of the four important nuts in the world. Walnuts have deep roots, which can be colonized by either ectomycorrhizal fungi or arbuscular mycorrhizal fungi in the soil. These mycorrhizal fungi form beneficial symbioses in roots of walnut. A large number of ectomycorrhizal fungi have been identified, whilst Boletus edulis, Calvatia uiacina, and Cantharelles cibariusisolated from walnut orchards stimulated plant growth and gave the capacity of stress tolerance in walnut. Moreover, Carya illinoensis is a very good host plant for commercial production of truffles, especially Tuber indicum. In addition, ectomycorrhizal fungi accelerate plant growth and enhance potential stress tolerance of walnuts. Inoculation with arbuscular mycorrhizal fungi also showed the improvement of plant growth and nutrient acquisition of walnut, the enhancement of drought tolerance in walnut, nutrient redistribution under walnut interplanting patterns, and the delivery of juglone by mycorrhizal hyphae. A culturable in vitro arbuscular mycorrhizal like fungus Piriformospora indica also enhanced salt tolerance of walnut plants. In this mini-review, the physiological roles of mycorrhizal fungi, including arbuscular mycorrhizal fungi, ectomycorrhizal fungi and arbuscular mycorrhizal like fungus (P. indica) on walnut plants are summarized, and future outlooks in the field are proposed.