The innate immune system is essential for fish to survive in the aquatic environment with abundant microflora (Thirunavukkarasu et al., 2022). Indeed, due to the lack of a well-developed acquired immune system, innate immune system is the fundamental defense mechanism of fish (Magnadóttir, 2006). Particularly, the antimicrobial peptides (AMPs) exert important roles in the innate immune system in fishes (Shabir et al., 2018). Up-to-date, a large number of AMPs have been isolated and characterized in fish (Qiao et al., 2021), which could be classified into three groups according to their structures (Cuesta et al., 2008): α-helical linear AMPs (such as piscidin (Zhuang et al., 2017), pleurocidin (Cole et al., 1997), chrysophsin (Hsu et al., 2011), moronecidin (Salerno et al., 2007), and pardaxin (Adermann et al., 1998)); highly disulphide bonded β-sheets AMPs (such as defensin (Zhuang et al., 2021), hepcidin (Chaithanya et al., 2013b)); specific amino-acid-rich AMPs (such as parasin I (Park et al., 1998) and hipposin (Birkemo et al., 2003)).

Among these identified AMPs, hepcidin drawing attention of researchers due to its broad-spectrum antimicrobial activity and less possibility to cause bacterial resistance (Shirdel et al., 2019). Fish hepcidin was identified for the first time in hybrid striped bass early this century (Shike et al., 2002). Since then, the hepcidin cDNA has been successfully isolated and characterized in various fish species (Chaturvedi et al., 2014, Solstad et al., 2008). Besides, the tissue-specific expression patterns analysis further demonstrated that hepcidin are widely expressed in multiple tissues (liver, spleen, kidney and intestine) in fish with the highest expression normally being observed in the liver (Wang et al., 2009).

Furthermore, the homologous analysis showed the hepcidin genes are highly conserved among fishes, which encode the hepcidin precursor containing the signal peptide, the propeptide, and the cysteine-rich mature peptide (Kim et al., 2008). Functional studies revealed that the mature peptides of hepcidin often possesses desirable antimicrobial activity (Xu et al., 2018). For instance, the hepcidin peptides of various fish species (eg. Salmo caspius (Shirdel et al., 2019), Gobiocypris rarus (Ke et al., 2015), Puntius sarana (Das et al., 2015), Sparus aurata (Cuesta et al., 2008), Epinephelus coioides (Zhou et al., 2011), and Bostrychus sinensis (Shen et al., 2021)) demonstrated strong antibacterial and antifungal activities. Moreover, various studies have shown that hepcidin could play an important role in improving the immune system of fish and be used as an immunostimulant as well as a therapeutic agent to control fish diseases in the aquaculture industry (Lee et al., 2012, Shirdel et al., 2019).

Crescent sweetlips (Plectorhinchus cinctus) inhabit tropical and subtropical coastal reefs, near rocky and coral areas mainly distributed Indo-West Pacific (Liu et al., 2016). At present, it is an important marine fish cultured in southeast coast of China owe to its commercially valuable, rich nutrients and delicious taste (Qi et al., 2019). However, due to the increasing demands, the cultivation density of crescent sweetlips has been increasing annually. Indeed, this increase of aquaculture density often causes the excessive crowding, decreased water quality and inadequate nutrition, resulting the outbreak of fish disease (especially bacterial diseases), which poses a huge challenge to the aquaculture industry (Huang et al., 2019). Therefore, the identification of hepcidin peptides from crescent sweetlips and determination of its potential antibacterial activity may lay an important foundation to better understand the immune response in crescent sweetlips. Thus, the present study aimed to clone and characterize the hepcidin gene from Plectorhinchus cinctus. Furthermore, its potential antimicrobial activity will also be evaluated, with the key amino acid residues critical for antimicrobial activity being further analyzed. Meanwhile, the antimicrobial activity of hepcidin peptide from Plectorhinchus cinctus was compared to its counterparts in other fish species.