Cotton Genomics and Genetics 2025, Vol.16, No.5, 232-240 http://cropscipublisher.com/index.php/cgg 234 Figure 1 Biogenesis models of circular RNAs (circRNAs). Due to the emergence of differentially located breakpoints, primary RNA transcripts undergo “back-splicing” to produce the 5ʹ splicing donor site and 3ʹ splicing acceptor site. Subsequently, the 5ʹ splicing donor site is combined with the 3ʹ splicing acceptor site in reverse order to form a covalently closed loop without 5ʹ or 3ʹ polarities and poly (A) tails. (A) Exonic circRNAs (ecircRNAs) are composed exclusively of exons without flanking introns. The number of exons ranges from one to two or more depending on the breakpoints. Over 80% of circRNAs arise from ecircRNAs. (B) Exons (at least one) accompanied by flanking introns that have not been degraded during “back-splicing” compose exon-intron circRNAs (eiciRNAs). (C) Intron-derived circRNAs (ciRNAs) are produced through a lariat-derived mechanism depending on a consensus GU-rich domain near the 5ʹ splicing site and a C-rich domain near the breakpoint. The remaining noncircularized introns are sequestered. (D) tRNA intronic circRNAs (tricRNAs) originate from the exons and introns of pre-tRNAs cleaved by the tRNA splicing endonuclease complex. Abbreviations: ciRNA: circular intronic RNA; ecircRNA: exonic circRNA; eiciRNA: exon-intron circRNA; tricRNA: tRNA intronic circular RNA (Adopted from Chen et al., 2019) 3.2 CircRNA-miRNA-mRNA regulatory axis (ceRNA hypothesis) It's not only circRNA that can participate in this kind of "competition". According to the ceRNA (Competitive endogenous RNA) hypothesis, mRNA itself, lncRNA, etc. may also, like circRNA, compete to bind to miRNA. They do not operate independently of each other but form a complex regulatory network by competing for miRNA together. The role of circRNA in this process is like a "molecular bait"-it does not directly regulate any genes itself, but it can influence the destination of miRNA and indirectly interfere with the expression of other RNAs (especially mrnas). This "detour regulation" approach has been found in plant development, environmental stress response, and even certain pathological states (Han et al., 2020; Schwarzenbach, 2024). Ultimately, this is a "fine-tuning mechanism" characterized by crosstalk, rather than a simple switch-mode regulation. 3.3 Experimental approaches to validate sponge activity No matter how many theories there are, they all need experiments to support them. To prove that a certain circRNA is indeed "adsorbing" miRNA, researchers now commonly use several methods. The first is the
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