The adaptive mechanisms that organisms have evolved to rapidly respond to complex and variable external environmental conditions are critical to their reproduction, health and survival (Price et al., 2003; Yang and Joho, 2019). In insects, phenotypic plasticity as an important adaptive mechanism has been widely and intensively studied in orders such as Lepidoptera, Coleoptera, Hymenoptera, Diptera, Orthoptera, and Hemiptera (Simpson et al., 2011). Wing dimorphism in insects is considered to be a form of phenotypic plasticity and there are many factors that induce the formation of wing differentiation (Zhang et al., 2019). Aphids are capable of using many different cues, including density, host plant conditions, temperature, interspecific interactions, and photoperiod, to exert influence on the switch between wingless and winged morphs (Braendle et al., 2005). Among them, the strongest clue to wing morphology induction is tactile stimulation, which resulted in more wing morphology as aphid density increased. The factors affecting the dimorphism of crickets' wings are different among species, and the main environmental factors are density, stress and photoperiod (Olvido et al., 2003; Zeng et al., 2010; Zeng et al., 2014). In the brown planthopper (Nilaparvata lugens, BPH), the most destructive insect pest of rice (Oryza sativa), wing dimorphism can shift in response to biotic or abiotic stresses (An et al., 2014, Backus et al., 2005, Lin et al., 2018, Xu et al., 2015). Long-winged adults have the ability to migrate long distances and invade rice-growing areas, while short-winged adults are less capable of flight but more reproductive, capable of rapid population growth in a short period of time and cause serious damage to regional rice (Denno and Roderick, 1990, Zera and Denno, 1997, Harrison, 1980). This kind of wing differentiation in BPH is probably regulated by multiple genes (Mochida, 1975, Roff, 1986). Therefore, it is necessary to screen for genes associated with wing differentiation.

The main signaling pathways associated with the regulation of wing differentiation in insects are the insulin/insulin-like signaling pathway (IIS), juvenile hormone (JH) and ecdysone signaling pathway (Zhang et al., 2019). The insulin signaling pathway and the juvenile hormone signaling pathway have been well studied and confirmed to be related to wing dimorphism in BPH (Xu et al., 2015, Chen et al., 2023, Zhao et al., 2017). Functional analysis of genes based on RNA interference (RNAi) revealed that IIS pathway acts as a key regulator of wing morphological switching in BPH (Xu et al., 2015, Zhang et al., 2019). Two insulin receptors (NlInR1 and NlInR2) have opposing roles in regulating wing differentiation, and activation of NlInR1 activity in BPH stimulates the downstream NlPI(3)K-NlAkt signaling cascade pathway, contributing to the formation of long-winged BPH. However, NlInR2 negatively regulates the activity of NlInR1-NlPI (3)K-NlAkt signaling by modulating the activity of NlInR1, which leads to the formation of the short-winged BPH (Xu et al., 2015). Later studies have shown that the proportion of long-winged females undergoes significant changes by simulating the glucose concentration during the maturation of rice plants (Lin et al., 2018). The application of JH and its agonists also induced short-winged morphs at wing differentiation sensitive stages (Iwanaga and Tojo, 1986, Ayoade et al., 1999), while treatment with JH antagonists (precocene II) had the opposite effect (Ayoade et al., 1996, Bertuso et al., 2002). Previous studies have shown that knockdown of the key gene of the juvenile hormone (JH) catabolic pathway, juvenile hormone epoxide hydrolase (Nljheh), significantly increased the proportion of short-winged morphs in female BPH (Zhao et al., 2017).

Previously, we have found that NlODC may respond to abiotic stress (low temperature and short photoperiod conditions) for wing dimorphism after differential expression gene and WGCNA analysis of transcriptome data (Chen et al., 2023). ODCs are widely existent in many insects and the function of ODCs varies from insect to insect. The ODC transcript was temporally regulated throughout embryogenesis in Musca domestica (Toutges and Santoso, 2011). ODC activity was significantly elevated during the larval-pupal development of Manduca sexta (Birnbaum et al., 1988). DFMO inhibited ODC activity in Aedes aegypti, resulting in suppression of oocyte development (Kogan & Hagedorn, 2000). Some studies have found that ODC activity may be regulated by phosphorylation/activation of Akt (Zhang et al., 2014), which is involved in BPH wing differentiation through the insulin signaling pathway. Interestingly, ODCs are hormonally controlled in insects (Wyatt et al.,1973) and wing differentiation in BPH is associated with hormones (Ayoade et al., 1996, Bertuso et al., 2002, Zhao et al., 2017). Hence, the potential molecular mechanism of how NlODC regulates wing differentiation in BPH remains unknown.

Here, we present evidence that NlODC and its regulated polyamines are involved in wing differentiation of BPH as confirmed by silencing NlODC and polyamine rescue experiments. In the regulation of wing differentiation in BPH, NlODC mutually antagonistic to NlAkt may act through other signaling pathways such as the juvenile hormone (JH) pathway, rather than the classical insulin signaling pathway. These findings not only assigned a functional role to NlODC, but also will help researchers to study the mechanisms regulating the wing differentiation of BPH and to exploit this characteristic for pest control.