Adaptive Response of Flavonoids in Robinia Pseudoacacia L. Affected by the Contamination of Cadmium and Elevated CO2 to Arbuscular Mycorrhizal Symbiosis

As a key component in non-enzyme resistance system, flavonoids play a crucial role in the plant growth and defenses, which are significantly affected by biotic and abiotic factors such as fungi, bacteria, viruses, heavy metals, and atmospheric CO2. Arbuscular mycorrhizal fungi (AMF) play an important role in enhancing plant tolerance to adverse environments, which can significantly affect the synthesis of flavonoids by forming mycorrhizal symbionts with plant roots. However, few studies explored the combined effects of AMF, elevated CO2, and heavy metals on flavonoids in plants. Here, we investigated the adaptive response of flavonoids accumulation in Robinia pseudoacacia L. seedlings affected by the contamination of cadmium (Cd) and elevated CO2 to arbuscular mycorrhizal symbiosis. The results showed that G. mosseae decreased (p < 0.05) Cd content in leaves by 62.2% under elevated CO2. Moreover, G. mosseae colonization led to significant decreases in robinin, quercetin, kaempferol and acacetin by 17.4%, 11.1%, 15.5% and 23.1% under elevated CO2 + Cd, respectively. Additionally, G. mosseae down-regulated (p < 0.05) expression levels of phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) genes under elevated CO2 + Cd, and CHS and uridine diphosphate flavonoid glucosyltransferase (UFGT) activities decreased (p < 0.05). Quercetin, kaempferol and acacetin showed positive (p < 0.05) correlation with PAL and CHS genes expression and PAL, CHS, and UFGT activities. Cadmium, C/N ratio, carotenoids, leaf biomass, total chlorophyll, P, and starch in leaves and G. mosseae colonization rate in roots influenced (p < 0.05) flavonoids content. Overall, G. mosseae reduced flavonoids synthesis by down-regulating gene expression levels and activities of key enzymes under elevated CO2 + Cd. The results improved our understanding of the regulation of AMF on non-enzymatic resistance of plants grown in heavy metal-contaminated soils under increasing atmospheric CO2 scenarios.