Isolation of anti-Saprolegnia lignans from Magnolia officinalis and SAR evaluation of honokiol/magnolol analogs

Abstract To control the fish fungal pathogen Saprolegnia, the effects of the petroleum ether extracts of Magnolia officinalis were evaluated by a rapeseed ( Brassicanapus ) microplate method in vitro . By loading on an open silica gel column and eluting with petroleum ether-ethyl acetate-methanol, honokiol (C 18 H 18 O 2 ) and magnolol (C 18 H 18 O 2 ) were isolated from Magnolia officinalis. Saprolegnia parasitica growth was inhibited significantly when honokiol concentration was >8.0 mg/L, and magnolol concentration was >9.0 mg/L, with EC 50 values of 4.38 and 4.92 mg/L, respectively. Six honokiol and magnolol derivatives were designed, synthesized and evaluated for their anti -Saprolegnia activity . According to the results, double bond and hydroxyl played an important role in inhibiting Saprolegnia . Mechanistically, through the scanning electron microscope observation, honokiol and magnolol could cause the Saprolegnia parasitica mycelium tegumental damage including intensive wrinkles and nodular structures. Moreover, compared to traditional drugs kresoxim-methyl (LC 50  = 0.66 mg/L) and azoxystrobin (LC 50  = 2.71 mg/L), honokiol and magnolol showed a lower detrimental effect on zebrafish, with the LC 50 values of 6.00 and 7.28 mg/L at 48 h, respectively. Overall, honokiol and magnolol were promising lead compounds for the development of commercial drugs anti- Saprolegnia.