Background Entomogenous fungi are important factors in biological control, but innate immunity of insects restricts the efficiency of fungus infection. 20‐hydroxyecdysone (20E) is involved in regulating the immune response of insects. Our previous studies have revealed that 20E enhances the expression of antibacterial peptides in the worldwide pest Locusta migratoria. However, the mechanism by which 20E controls innate immunity against entomogenous fungi is still unknown. Results In the present study, based on the transcriptome of L. migratoria fat bodies challenged by 20E, immulectin‐1 (LmIML‐1) was screened and identified to be involved in modulating antifungal immunity. Spatio‐temporal expression analysis showed LmIML‐1 was highly expressed in the fifth instar nymph stage, and mainly distributed in the fat bodies and hemolymph. Both exogenous and endogenous 20E could increase the transcription of LmIML‐1. In contrast, transcription of LmIML‐1 did not increase when the 20E signal was blocked by RNAi of LmEcR (ecdysone receptor). The expressed recombinant protein rLmIML‐1 possessed agglutination activity and promoted the encapsulation. RNA interference of LmIML‐1 reduced the encapsulation of hemocytes, decreased the antifungal activity of plasma against Metarhizium anisopliae and accelerated the death of nymphs under the stress of entomogenous fungus. Meanwhile, 20E did not increase the antifungal activity with silence of LmIML‐1 in L. migratoria. Conclusion 20E enhances antifungal immunity by activating immulectin‐1 in L. migratoria. Our findings indicate a potential mechanism of 20E systematically regulating innate immune response to resist pathogens and provide a well‐defined molecular target for improving biological control. © 2019 Society of Chemical Industry

BACKGROUNDPPO (prophenoloxidase) cascade plays an important role in resisting invasion of entomogenous fungus. The 20‐hydroxyecdysone (20E) exerts potent effect on the innate immunity in many insects. However, whether 20E controls the PPO cascade system against fungi and the regulatory mechanism in insects remains unclear.RESULTSIn this study, both the proteome and transcriptome of Locusta migratoria were determined followed by the induction of 20E. Pattern recognition receptor GNBP‐2 (Gram‐negative binding proteins) has been identified that responded to 20E at both messenger RNA (mRNA) and protein levels. The PPO gene expression in fat body and PO (phenoloxidase) activity in plasma was found significantly induced after 20E injection and during the high‐20E developmental stage. However, when 20E signal was blocked by RNA interference (RNAi) of ecdysone receptor, the expression level of PPO and PO activity failed to be increased by 20E. Thus, 20E could not significantly induce the expression of PPO gene and PO activity after RNAi of GNBP‐2. Furthermore, 20E treatment notably enhanced the resistance of L. migratoria against Metarhizium anisopliae. Followed by of GNBP‐2 silencing, the mortality of nymphs was significantly increased under the stress of Metarhizium anisopliae, and 20E injection could not increase the resistance.CONCLUSIONThe 20E regulates the PPO system to resist fungal invasion via regulating GNBP‐2 in worldwide pest L. migratoria. Our results provide insight into the mechanism of how 20E enhances the antimicrobial immunity, and will be beneficial for modification of entomogenous fungi targeting on hormones and the immune system. © 2020 Society of Chemical Industry

BACKGROUND Destruxin A (DA) is a mycotoxin secreted by entomogenous fungi, such as Metarhizium anisopliae, which has broad‐spectrum insecticidal activity. Insect innate immunity provides resistance against the invasion of entomopathogenic fungi. Previous studies have shown that DA could inhibit the immune response, however, the suppressive mechanism of DA on prophenoloxidase system is still unknown. RESULTS Based on the transcriptome of Aphis citricola, we screened the scavenger receptor class B(AcSR‐B)and identified that it significantly responds to DA. Spatio‐temporal expression analysis showed that AcSR‐B is highly expressed in adult stage and is mainly distributed in the abdominal region. We further revealed that both M. anisopliae and Escherichia coli could suppress the expression of AcSR‐B at 24 h, and that the expressed recombinant protein rAcSR‐B possessed agglutination activity to M. anisopliae and E. coli. DA could suppress the protein expression of AcSR‐B. In addition, RNA interference of AcSR‐B caused death of A. citricola in a dose‐dependent manner, and RNA interference of AcSR‐B increased mortality in A. citricola under the same lethal concentration of DA. The inhibiting effect of AcSR‐B silencing was similar with the DA treatment upon phenol oxidase (PO) activity of A. citricola hemolymph. DA could not decrease PO activity further after AcSR‐B silencing. CONCLUSION Destruxin A inhibits melanization by suppressing AcSR‐B in A. citricola. Our findings are helpful in understanding the underlying molecular mechanism of the DA suppressing immune system, and uncover a potential molecular target for double‐stranded RNA (dsRNA) insecticides.