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.