MicroRNAs for insect resistance
Insect pests impart a great economic loss to rice productivity. More than 200 insect pests attack rice at different stage of the plant’s life cycle. Current advances in genomics have greatly expanded the understanding of the role of miRNAs in the regulation of insect development, moulting, fecundity, and cellular homeostasis (Sattar and Thompson, 2016). Silencing of target genes by ingesting miRNA or siRNA causes perturbation in these traits. Transgenic plants expressing double stranded RNA (dsRNA) or miRNA have been developed that target different genes such as cytochrome P450, vacuolar ATPase subunit A and ecdysone receptor and tested as proofs of concept for insect pest management (Maoet al. 2007; Yu et al. 2014). Chitin synthase (CHS) is one of the principal enzymes for chitin biosynthesis in insects. Transgenic plants expressing artificial miRNA (miR-24 ) has been reported in cotton against Helicoverpa armigera. This was shown to target chitinase genes, which led to the suppression of molting (Agrawal, Rajamani, Reddy, Mukherjee & Bhatnagar 2015). The larvae of cotton bollworm feeding on bacteria expressing an artificial miRNA showed significantly greater mortality and developmental defects (Yogindran & Rajam 2016). The Nlu -miR-2703 targetingNilaparvata lugen chitin synthase A gene plays a role in the regulation of chitin synthesis in brown planthopper (BPH) (Li, Chen, Fan, Chen & Zhang 2017a). The transcription of Nlu -miR-173 is known to regulate proper molting function in BPH by targetingN. lugens Ftz-F1 (NlFtz-F1 ) gene involved in the 20-hydroxyecdysone signaling pathway (Chen et al. 2018). Heet al. (2019) identified an insect-specific miR-14 that targets two genes, i.e., the spook (Spo ) and ecdysone receptor (EcR ) in the ecdysone signalling network. Significantly higher mortality and developmental defects have been observed in rice stem borer after feeding on transgenic plants expressing the miR-14 . Feeding on transgenic rice expressing miR-15 has been shown to delay pupation and inhibit growth of striped stem borer (Jiang et al. 2017).
Differentially expressed miRNAs were identified by deep sequencing of an introgression line with the Bph15 gene in comparison to susceptible genotype during brown planthopper (BPH) infestation. Several known miRNAs along with 183 other novel BPH-responsive miRNAs have been identified to be involved in the regulation of the basal defenses and specific resistance to BPH (Wu et al. 2017b). An integrated miRNA and mRNA expression profiling analysis identified numerous differentially expressed BPH-responsive miRNAs between transgenic line carrying Bph6 and wild type Nipponbare (Tan et al. 2020).Osa-miR156 was investigated as a negative regulator of the BPH resistance in rice. Sequestration of the Osa-miR156 using target mimic mutant of Osa-miR156 displayed enhanced resistance to BPH by promoting the expression of OsMPK3 and OsMPK6 genes. The transcript level of the OsWRKY70 transcription factor was significantly reduced in Osa-miR156 -sequestered target mimic mutant that represses the jasmonic acid (JA) biosynthesis and signaling (Ge et al. 2018). Another miRNA, Osa-miR396 also acts as a negative regulator of the BPH resistance. Downregulation ofOsa-miR396 enhances the expression of growth regulating factor 8 (OsGRF8 ) gene which positively regulate the expressionflavanone 3-hydroxylase (OsF3H ) gene involved in the flavonoid biosynthetic pathway. The OsF3H positively regulates the BPH resistance by overexpressing the salicylic acid (SA) pathway and repressing the jasmonic acid (JA) pathway.