Interactions between zinc and Phomopsis longicolla infection in roots of Glycine max

Phomopsis. longicolla is a hemibiotrophic fungus causing significant soybean yield loss worldwide. To reveal the role of zinc in plant-pathogen interactions, soybean seedlings were grown hydroponically with a range of Zn concentrations, 0.06 µM (deficient, Zn0), 0.4 µM (optimal growth), 1.5 µM, 4 µM, 12 µM, and toxic 38 μM, and were subsequently inoculated with P. longicolla via the roots. In vivo analysis of metal distribution in tissues by micro-X-ray fluorescence showed local Zn mobilization in the root maturation zone in all treatments. Decreased root and pod biomass, and photosynthetic performance in infected plants treated with 0.4 µM Zn were accompanied with accumulation of Zn, jasmonoyl-L-isoleucine (JA-Ile), jasmonic acid, and cell wall-bound syringic acid (cwSyA) in roots. Zn concentration in roots of infected plants treated with 1.5 µM Zn was seven-fold higher than in the 0.4 µM Zn treatment, which together with accumulation of JA-Ile, cwSyA, cell wall-bound vanilic acid and leaf jasmonates contributed to maintaining photosynthesis and pod biomass. Host-pathogen nutrient competition and phenolics accumulation limited the infection in Zn-deficient plants. The low infection rate in Zn 4 µM-treated roots correlated with salicylic and 4-hydroxybenzoic acid, and cell wall-bound p-coumaric acid accumulation. Zn toxicity promoted pathogen invasion and depleted cell wall-bound phenolics. The results show that manipulation of Zn availability improves soybean resistance to P. longicolla by stimulating phenolics biosynthesis and stress-inducible phytohormones.