Defense responses and symbiotic functional initiation in trifoliate orange‒arbuscular mycorrhizal fungi interaction

Abstract

Arbuscular mycorrhizal fungi (AMF) trigger beneficial effects on their hosts, but it is unknown how plants modulate their defense responses during root colonization of AMF and the symbiotic benefits are initiated. The purpose of this study was to analyze the root mycorrhizal colonization process of trifoliate orange and the responsive patterns of plant growth, root peroxide hydrogen (H2O2), antioxidant enzymes and their encoding gene expression, and sugar, lipid and phosphate transporter protein gene expression at 7‒56 days of inoculation (doi) with Funneliformis mosseae (Fm). Fm developed appressoriums on the root surface at 7 doi, followed by abundant arbuscules in root cortical cells at 28 doi, intracellular vesicles at 42 doi, and root mycorrhizal colonization rate of 41.54% at 56 doi. Plant growth improvement by Fm started at 28 doi. The immune defense response of roots was initiated at 7 doi, as evidenced by the increase of H2O2 levels and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activity, as well as the up-regulation of PtMn-SOD, PtCu/Zn-SOD, PtPOD, and PtCAT expression, which lasted until 14 doi. Starting at 28 doi, a sugar transporter gene (PtSWEET), a lipid transporter gene (PtSTR), and a phosphate transporter gene (PtPT6) were initiated to be up-regulated, followed by the up-regulation of PtSTR2, PtPT3, and PtPT5 at 42 doi and PtFe-SOD at 56 doi. Arbuscule formation and plant growth improvement together at 28 doi suggested that arbuscules trigger improved growth responses of host plants. This study also reveals the initiation of host immune defense response and function in early root AMF colonization.Graphical