Arbuscular mycorrhizal fungi (AMF) increase growth and secondary metabolism in cucumber subjected to low temperature stress

• AM fungi alleviated chilling stress-induced reduction of plant growth.
• AMF induced higher secondary metabolites content and enzymatic activities.
• Inoculation with AMF caused a reduction in H2O2 content under low temperature.
• Secondary metabolites participate in AMF-mediated alleviation of chilling stress.

The widely present symbiotic association between arbuscular mycorrhizal fungi (AMF) and plant roots contributes considerably to improve plant growth, nutrient uptake and stress responses. The present study addressed the potential of AM fungus Funneliformis mosseae in the alleviation of chilling stress for cucumber seedlings. The results showed that the AMF-inoculated cucumber seedlings had significant higher fresh weight and dry weight than non-AMF inoculated control plants under both normal (25/15 °C) and low temperature (15/10 °C) treatment. Under chilling stress, AMF inoculation significantly improved the content of related secondary metabolites including phenols, flavonoids, lignin, DPPH activity and phenolic compounds compared with the non-AMF control. Furthermore, large increments were observed in a number of enzymatic activities related to secondary metabolism and antioxidant system in AMF-inoculated seedlings under low temperature, such as glucose-6-phosphate dehydrogenase (G6PDH), shikimate dehydrogenase (SKDH), phenylalanine ammonia-lyase (PAL), cinnamyl alcohol dehydrogenase (CAD), polyphenol oxidase (PPO), guaiacol peroxidase (G-POD), caffeic acid peroxidase (CA-POD) and chlorogenic acid peroxidase (CGA-POD). As well, the expression of stress-related marker genes was enhanced in AMF-inoculated seedlings in comparison with the non-AMF control. Furthermore, AMF symbiosis decreased hydrogen peroxide (H2O2) content under low temperature. Taken together, these results indicated that the enhanced secondary metabolism and integrated transcriptional regulation might play a crucial role in AMF-mediated alleviation of chilling stress in cucumber.