Growth and secondary metabolite modulation by soil beneficial microorganisms in Artemisia annua plants

The family Asteraceae includes many genera among which Artemisia is one of the most widespread in the world. Artemisia annua, a herbaceous annual plant commonly named “annual absinthe”, produces a high number of secondary metabolites among which VOCs (Volatile Organic Compounds) and artemisinin, a sesquiterpene lactone with antimalarial action. Since the inoculation with soil beneficial microorganisms, such as arbuscular mycorrhizal fungi (AMF) and plant growth promoting bacteria (PGPB), is one of the strategies used to improve plant growth and metabolism, in this work different AMF and PGPB inocula were tested, alone or in combination, on two clones of A. annua (CL7 and CL26) grown in controlled conditions. Plants were irrigated three times a week with a Long Ashton nutrient solution at 32µM of P and harvested after two months of culture. Then, the mycorrhizal colonization (M%) in the plant root system, the shoot and root fresh and dry weight, the leaf chlorophyll and carotenoid concentrations and the quantitative (HPLC) analysis of artemisinin were evaluated. In addition, the composition of the leaf volatile profile of CL26 plants was analyzed by GC-MS. Data were statistically analyzed by two-way ANOVA using “fungus” (F) and “bacterium” (B) as factors. A one-way ANOVA followed by Fisher test with significance cut-off at p<0.05 was used to assess differences among the treatments. Results showed that the various bacterial and/or fungal inocula had different impact on growth and metabolism of the two different clones. Some bacterial strains were found to increase the ability of AMF to colonize the plant roots, even if the M% remained rather low. In general, no significant differences for the artemisinin content were recorded between the different treatments of each clone, however the same bacterial strain (Pf7) led to a decrease content of artemisinin in CL7 clone, while it increased its production in CL26. Regarding the volatile profile, 120 molecules (some of which detected for the first time in this plant species) were identified. The volatilome composition varied according to the different inocula since some molecules were exclusive of a certain treatment and others showed variations in their proportions. Therefore, plants of each treatment had their own specific particular odor due to the interaction between plant and soil beneficial microorganisms.