Tea is a popular cash crop around the world that often endures low temperatures in the spring, thereby negatively affecting tea yield and quality. The objective of this study was to examine the effects of an arbuscular mycorrhizal fungus, Clariodeoglomus etunicatum, on plant growth, root morphology, leaf food quality, associated gene expression, and secondary metabolite profiles in tea (Camellia sinensis cv. Fuding Dabaicha) seedlings grown at low temperatures. A seven-day low temperature (4 degrees C) treatment significantly reduced root colonization rate by C. etunicatum by 21.08%. Uninoculated plants' growth was considerably impeded by low temperatures, whereas C. etunicatum inoculation significantly increased leaf number and leaf, stem and root biomass at low temperatures. Low temperatures had little effect on root morphology, whereas C. etunicatum inoculation significantly increased root surface area at low temperatures. In addition, low temperatures reduced leaf quality variables, whereas C. etunicatum significantly increased the levels of glucose, fructose, sucrose, tea polyphenols, catechin, free amino acids, and caffeine at low temperatures, along with up-regulated expression of CsGDH, CsGOGAT, and CsHMGR, as well as down-regulated expression of CsGs. Non-targeted metabolomics identified 767 differential metabolites, of which 65 differential metabolites were observed at low temperatures by C. etunicatum inoculation, including 42 (e.g., catechin, (-)-epicatechingallate, protoanemonin, epicatechin, and 4dodecylbenzenesulfonic acid) up-regulated and 23 (e.g., 4-hydroxybenzoic acid, raffinose, L-theanine, melezitose, and D-serine) down-regulated. Mycorrhiza-triggered differential metabolites enriched 268 metabolic pathways at low temperatures, including isoquinoline alkaloid biosynthesis, lysine biosynthesis, tyrosine metabolism, and others. These findings reveal that C. etunicatum can function as a fungal stimulant to improve the plant growth and leaf quality in tea while also changing the secondary metabolite profile at low temperatures.
