Common mycorrhizal networks activate salicylic acid defense responses of trifoliate orange (Poncirus trifoliata)

Abstract

Citrus canker, caused by Xanthomonas axonopodis pv. citri (‘Xac’), is an important quarantine disease in citrus crops. Arbuscular mycorrhizal fungi (AMF) form symbiotic interactions with host plants and further affect their disease resistance, possibly by modulating the activity of salicylic acid (SA), a key phytohormone in disease resistance. Common mycorrhizal networks (CMNs) can interconnect plants, but it is not yet clear whether CMNs promote resistance to citrus canker and, if so, whether SA signaling is involved in this process. To test this possibility, we used a two‐chambered rootbox to establish CMNs between trifoliate orange (Poncirus trifoliata) seedlings in chambers inoculated (treated) or not (neighboring) with the AMF, Paraglomus occultum. A subset of the AMF‐inoculated seedlings were also inoculated with Xac (+AMF+Xac). At 2 d post‐inoculation (dpi), compared with the +AMF−Xac treatment, neighboring seedlings in +AMF+Xac treatment had lower expression levels of the SA biosynthetic genes, PtPAL, PtEPS1, and PtPBS3, but higher SA levels, which attributed to the upregulation of PtPAL and PtPBS3 in treated seedlings and the transfer of SA, via CMNs, to the neighboring seedlings. At 4 dpi, the pathogenesis‐related (PR) protein genes, PtPR1, PtPR4, and PtPR5, and the transcriptional regulatory factor gene, PtNPR1, were activated in neighboring seedlings of +AMF+Xac treatment. At 9 dpi, root phenylalanine ammonia‐lyase activity and total soluble phenol and lignin concentrations increased in neighboring seedlings of +AMF+Xac treatment, likely due to the linkage and signal transfer, via CMNs. These findings support the hypothesis that CMNs transfer the SA signal from infected to neighboring healthy seedlings, to activate defense responses and affording protection to neighboring plants against citrus canker infection.

Mycorrhiza Regulates Signal Substance Levels and Pathogen Defense Gene Expression to Resist Citrus Canker

“…In our previous studies, JA and SA concentrations were significantly increased when suberification was formed in the Xac-infected site (Zhang et al, 2017b(Zhang et al, , 2019. However, in this work, only the JA content was increased when water stain appeared in Xac-treated plants.…”

Inoculation with Clariodeoglomus etunicatum improves leaf food quality of tea exposed to P stress

“…Among them, AMF in soil can form symbiotic arbuscular mycorrhizas (AMs) with most terrestrial plants. In AM symbiotic system, AM fungi can not only directly improve the absorption of soil nutrients through external hyphae (Mei et al, 2019;He et al, 2020a;Wu et al, 2020), enhance the ability of plants to resist abiotic stresses (Wu et al, 2019a;He et al, 2020b;Zhang et al, 2020;Zou et al, 2020Zou et al, , 2021, but also improve the disease resistance of plants (Zhang et al, 2019b), thus improving the yield and quality of plants (Wu et al, 2020). In sustainable agricultural development, the role of AMF in improving plant growth and quality has been widely concerned (Wu et al, 2020).…”

A friendly-environmental strategy: application of arbuscular mycorrhizal fungi to ornamental plants for plant growth and garden landscape

“…In general, soil nutrients are always low under excessive heavy metals condition, while mycorrhizal ornamental plants usually maintain higher nutrient levels. For example, mycorrhizal Medicago sativa had higher biomass and N, P, K and Ca contents than non-mycorrhizal plants under Cd stress (Zhang et al, 2019b). AMF also secrete a special glycoprotein, glomalin, to contribute soil nutrient pools and form protein-metal compounds for reducing the level of heavy metal in soil (Chern et al, 2007;Gonzalez-Chavez et al, 2009;He et al, 2020;Meng et al, 2020).…”