Identification and expression analysis of cathepsin B genes in <i>Myzus persicae</i> (Hemiptera: Aphididae) and their response to environmental stresses

Cathepsin B (CTSB) is a cysteine protease that is widely found in eukaryotes and plays a role in insect growth, development, digestion, metamorphosis, and immunity. In the present study, we examined the role of CTSB in response to environmental stresses in <i>Myzus persicae</i> Sulzer (Hemiptera: Aphididae). Six <i>MpCTSB</i> genes, namely <i>MpCTSB-N, MpCTSB-16D1, MpCTSB-3098, MpCTSB-10270, MpCTSB-mp2</i>, and <i>MpCTSB-16</i>, were identified and cloned from <i>M. persicae</i>. The putative proteins encoded by these genes contained three conserved active site residues, i.e. Cys, His, and Asn. A phylogenetic tree analysis revealed that the six MpCTSB proteins of <i>M. persicae</i> were highly homologous to other Hemipteran insects. Real-time polymerase chain reaction revealed that the <i>MpCTSB</i> genes were expressed at different stages of <i>M. persicae</i> and highly expressed in winged adults or first-instar nymphs. The expression of nearly all <i>MpCTSB</i> genes was significantly upregulated under different environmental stresses (38°C, 4°C, and ultraviolet-B). This study shows that MpCTSB plays an important role in the growth and development of <i>M. persicae</i> and its resistance to environmental stress.