Mycorrhiza-induced plant defence responses in trifoliate orange infected by Phytophthora parasitica

Arbuscular mycorrhizal fungi (AMF) reduce disease incidence of host plants through the competition of carbon sources and direct inhibition of pathogens, as well as through induction of biochemical and molecular responses. However, it is not known whether AMF enhance the resistance to Phytophthora parasitica-induced root rot in citrus and what the underlying mechanisms are. This study was carried out to analyze roles of Funneliformis mosseae (a mycorrhizal fungus) in plant defence responses of Poncirus trifoliata infected by P. parasitica. A week after the pathogen infection, mycorrhizal seedlings possessed higher expression of root mitogen-activated protein kinase 3 (PtMAPK3) regardless of P. parasitica infection. F. mosseae induced higher root salicylic acid (SA) concentrations, accompanied with up-regulation of SA synthesis genes (PtPAL1 and PtEPS1), regardless of being infected with P. parasitica or not. Jasmonic acid (JA) synthesis genes were down-regulated by mycorrhization in the absence of P. parasitica and up-regulated (except for PtAOC) by mycorrhization under P. parasitica infection. Moreover, F. mosseae stimulated higher expression of pathogenesis-related protein gene 1 (PtPR1), PtPR4, and PtPR5, especially under P. parasitica infection. F. mosseae inoculation increased levels of root lignin, calmodulin, and total soluble phenol and activities of root chitinase, phenylalanine ammonialyase, and β-1,3-glucanase, and decreased concentrations of root nitric oxide with or without P. parasitica infection. These results implied that F. mosseae elicited MAPKs cascades as well as SA- and calmodulin-mediated signal pathways to activate disease-defence genes, proteins, and compounds to early-warn P. parasitica infection for enhancing tolerance of root rot in trifoliate orange.