Developing an efficient in vitro elicitation system using UV-B radiation for elevated biomass and azadirachtin production in callus culture of Melia azedarach L. – an important multipurpose industrial plant

Melia azedarach L. is an important multipurpose plant (ornamental, landscape, shade-tree, timber industry) with biopesticide and medicinal potential due to natural compounds, mainly limonoids that have insecticide and antimicrobial effect. Propagation of M. azedarach through conventional methods is difficult and azadirachtin production low, therefore in vitro tissue culture can constitute an effective alternative for stable, continuous and high-yield secondary metabolites production. For this purpose, in the present study, the effects of explant type (leaves, immature flowers), plant growth regulators [2,4-D or TDZ (0, 1, 2 mg/L), 2,4-D + TDZ (1 + 1, 1 + 2, 2 + 1, 2 + 2 mg/L)], UV-B radiation exposure time (0, 1, 2, 3, 4 h/day), and incubation period (2, 4 weeks) on producing azadirachtin and growth parameters (fresh weight, dry weight, growth index %) in vitro callus culture of M. azedarach were assessed. Results showed that leaf explants gave superior percentage for callus induction (100%) (4 weeks) and fresh weight (54.77 mg) (8 weeks) compared with immature flower explants (96.67%, 51.20 mg) under 2 mg/L 2,4-D + 2 mg/L TDZ. Leaf-derived calli exhibited significantly higher growth parameters and azadirachtin content than immature flower-derived calli under the same UV-B exposure time and incubation period in MS medium under 2 mg/L 2,4-D + 2 mg/L TDZ. The maximum increase in azadirachtin and growth parameters was achieved in leaf-derived callus by the highest UV-B exposure time of 4 h/day and the longest incubation period of 4 weeks (fresh weight: 1139.95 mg, dry weight: 115.35 mg, growth index: 279.98%, azadirachtin: 14.93 mg/g dry weight). The process of callus culture in association with UV-B irradiation as an elicitor can be a viable option for the production of azadirachtin in a large-scale bioreactor fulfilling the ever escalating industrial demand for plant-derived extracts. These results can further be manipulated as a sustainable method for the production of a natural and environmentally friendly pesticide (e.g. azadirachtin).