Identification of cucumber circular RNAs responsive to salt stress

Background: Circular RNAs (circRNAs) are 3-5 head-to-tail covalently closed non-coding RNA that have been proved to play essential roles in many cellular and developmental processes. However, no information relate to cucumber circRNAs is available currently, especially under salt stress condition. Results: In this study, we sequenced circRNAs in cucumber and a total of 2787 were identified, with 1934 in root and 44 in leaf being differentially regulated under salt stress. Characteristics analysis of these circRNAs revealed following features: most of them are exon circRNAs (79.51%) and they prefer to arise from middle exon(s) of parent genes (2035/2516); moreover, most of circularization events (88.3%) use non-canonical-GT/AG splicing signals; last but not least, pairing-driven circularization is not the major way to generate cucumber circRNAs since very few circRNAs (18) contain sufficient flanking complementary sequences. Annotation and enrichment analysis of both parental genes and target mRNAs were launched to uncover the functions of differentially expressed circRNAs induced by salt stress. The results showed that circRNAs may be paly roles in salt stress response by mediating transcription, signal transcription, cell cycle, metabolism adaptation, and ion homeostasis related pathways. Moreover, circRNAs may function to regulate proline metabolisms through regulating associated biosynthesis and degradation genes. Conclusions: The present study identified large number of cucumber circRNAs and function annotation revealed their possible biological roles in response to salt stress. Our findings will lay a solid foundation for further structure and function studies of cucumber circRNAs.