Exonic Circular RNAs Are Involved in Arabidopsis Immune Response Against Bacterial and Fungal Pathogens and Function Synergistically with Corresponding Linear RNAs.

Circular RNAs (circRNAs) are a group of covalently closed RNAs, and their biological function is largely unknown. In this study, we focused on circRNAs that are generated from exon back-splicing (exonic circRNAs). The linear RNA counterparts encode functional proteins so that we can compare and investigate the relationship between circular and linear RNAs. We compared circRNA expression profiles between untreated and Pseudomonas syringae-infected Arabidopsis and identified and experimentally validated differentially expressed exonic circRNAs by multiple approaches. We found that exonic circRNAs are preferentially enriched in biological processes that associate with biotic and abiotic stress responses. We discovered that circR194 and circR4022 are involved in plant response against P. syringae infection, whereas circR11208 is involved in response against Botrytis cinerea infection. Intriguingly, our results indicate that these exonic circRNAs function synergistically with their corresponding linear RNAs. Furthermore, circR4022 and circR11208 also play substantial roles in Arabidopsis tolerance to salt stress. This study extends our understanding of the molecular functions of plant circRNAs.

Osa-miR164a targets OsNAC60 and negatively regulates rice immunity against the blast fungus Magnaporthe oryzae.

Exploring the regulatory mechanism played by endogenous rice miRNAs in defense responses against the blast disease is of great significance in both resistant variety breeding and disease control management. We identified rice defense-related miRNAs by comparing rice miRNA expression patterns before and after Magnaporthe oryzae strain Guy11 infection. We discovered that osa-miR164a expression reduced upon Guy11 infection at both early and late stages, which was perfectly associated with the induced expression of its target gene, OsNAC60. OsNAC60 encodes a transcription factor, over-expression of which enhanced defense responses, such as increased programmed cell death, greater ion leakage, more reactive oxygen species accumulation and callose deposition, and upregulation of defense-related genes. By using transgenic rice over-expressing osa-miR164a, and a transposon insertion mutant of OsNAC60, we showed that when the miR164a/OsNAC60 regulatory module was dysfunctional, rice developed significant susceptibility to Guy11 infection. The co-expression of OsNAC60 and osa-miR164a abolished the OsNAC60 activity, but not its synonymous mutant. We further validated that this regulatory module is conserved in plant resistance to multiple plant diseases, such as the rice sheath blight, tomato late blight, and soybean root and stem rot diseases. Our results demonstrate that the miR164a/OsNAC60 regulatory module manipulates rice defense responses to M. oryzae infection. This discovery is of great potential for resistant variety breeding and disease control to a broad spectrum of pathogens in the future.

CropCircDB: a comprehensive circular RNA resource for crops in response to abiotic stress.

Circular RNA (circRNAs) may mediate mRNA expression as miRNA sponge. Since the community has paid more attention on circRNAs, a lot of circRNA databases have been developed for plant. However, a comprehensive collection of circRNAs in crop response to abiotic stress is still lacking. In this work, we applied a big-data approach to take full advantage of large-scale sequencing data, and developed a rich circRNA resource: CropCircDB for maize and rice, later extending to incorporate more crop species. We also designed a metric: stress detections score, which is specifically for detecting circRNAs under stress condition. In summary, we systematically investigated 244 and 288 RNA-Seq samples for maize and rice, respectively, and found 38 785 circRNAs in maize, and 63 048 circRNAs in rice. This resource not only supports user-friendly JBrowser to visualize genome easily, but also provides elegant view of circRNA structures and dynamic profiles of circRNA expression in all samples. Together, this database will host all predicted and validated crop circRNAs response to abiotic stress.