Characterization of Serpin Family Genes in Three Rice Planthopper Species and Their Expression Profiles in Response to <i>Metarhizium</i> Infection

Rice planthoppers, including <i>Nilaparvata lugens</i>, <i>Sogatella furcifera</i>, and <i>Laodelphax striatellus</i>, are major agricultural pests. Serpins, which function as serine protease inhibitors, play a pivotal role in the immune systems of these insects, especially within the Toll signaling pathway and the prophenoloxidase (PPO) cascade. This study presents a comparative analysis of <i>serpin</i> genes among these species, highlighting their roles in immunity and development. Utilizing genomic and bioinformatics approaches, we identified 11, 11, and 14 <i>serpin</i> genes in <i>N. lugens</i>, <i>S. furcifera</i>, and <i>L. striatellus</i>, respectively. Phylogenetic analysis revealed a close evolutionary relationship between these <i>serpin</i> genes and <i>Bombyx mori Bm</i>Serpins, emphasizing the functional diversity of the serpin family. Structural analysis confirmed the presence of the reactive center loop (RCL) in all serpin proteins, with the Serpin7 subfamily showing a unique dual RCL configuration. Expression profiling showed species-specific <i>serpin</i> expression patterns across different life stages and adult tissues. Moreover, transcriptional analysis of <i>serpin</i> genes in the three planthoppers following <i>Metarhizium</i> infection uncovered distinct immune regulatory patterns two days post-infection. Notably, the expression of <i>NlSerpin2-2</i>/<i>6</i>, <i>SfSerpin4</i>/<i>6/7-1, and LsSerpin4/5-2/6</i> was upregulated post-infection, potentially enhancing antifungal capabilities. In contrast, the expressions of <i>NlSerpin1/7-1/9</i> and <i>LsSerpin1/2/3/8/13</i> were downregulated, possibly suppressing immune responses. Moreover, <i>Serpin6s</i>, which share a conserved phylogenetic lineage, exhibited enhanced immune activity in response to fungal invasion. These insights into serpin-mediated immune regulation could contribute to the development of novel pest-control strategies.