Cross-Talk Between Calcium–Calmodulin and Brassinolide for Fungal Endophyte-Induced Volatile Oil Accumulation of Atractylodes lancea Plantlets

Using pharmacological and biochemical approaches, the signalling pathways between calcium (Ca²⁺)–calmodulin (CaM), brassinolide (BL), and nitric oxide (NO) for fungal endophyte-induced volatile oil accumulation were investigated in Atractylodes lancea plantlets. Gilmaniella sp. AL12 inoculation elevated the concentrations of BL, CaM, and Ca²⁺]_cyt, expression of the calmodulin 1 (CaM1) gene, and the levels of volatile oils. Treatment with AL12 or exogenous BL led to significant increases in the levels of cytosolic Ca²⁺ and CaM and CaM1 expression in plantlets. However, the upregulation of BL was almost completely blocked by pretreatments with CaM antagonists and Ca²⁺ channel blockers. Pretreatment with a BL inhibitor, brassinazole (BRz), did not influence the increase in levels of CaM induced by the endophyte. CaCl₂-induced increases in NO generation, CaM antagonists, and Ca²⁺ channel blockers were able to suppress NO production, and the NO-specific scavenger was not able to suppress the generation of Ca²⁺]_cyt in plantlets. Exogenous BL was not able to induce NO generation, and BRz had no effect on NO generation. Our results suggest that Ca²⁺–CaM induced by this endophyte mediates NO generation and BL concentration, and also functions downstream of BL signalling, resulting in the upregulation of volatile oil accumulation in A. lancea plantlets.