Overview of Plant Small RNAs
Discovery History of Small RNAs
About 30 years ago, the laboratories of Dr. Gary Ruvkun and Dr. Victor Ambros simultaneously discovered that a small RNA (named lin-4) targets and regulates another RNA (named lin-14). The interaction between lin-4 and lin-14 regulates the developmental timing of C. elegans. Drs. Ruvkun and Ambros, together with plant small RNA biologist Dr. David Baulcombe, were awarded the 2008 Albert Lasker Award. Their scientific discovery is considered the beginning of the small RNA field. Around the same time, Drs. Andrew Fire and Craig Mello discovered that RNA interference (RNAi) is mediated by double-stranded RNA, for which they received the 2006 Nobel Prize. RNAi is intrinsically related to small RNAs, and since then the sRNA field has developed rapidly. In 2024, Nobel Prize in Physiology or Medicine was awarded to Drs. Ruvkun and Ambros who discovered microRNAs.
Overview of Plant Small RNAs
Plant small RNAs mainly include two types: microRNAs (miRNAs) and small interfering RNAs (siRNAs). miRNA precursors can form stem-loop structures and are then cleaved by Dicer (DCL) proteins to produce mature miRNAs. The 3' end of mature miRNAs undergoes 2'-O-methylation and is then transported to the cytoplasm. miRNAs are loaded onto Argonaute (AGO) proteins and exert their functions by cleaving target RNAs or inhibiting translation. The majority of endogenous plant siRNAs are 24-nt long siRNAs generated by DCL3, originating from heterochromatin, transposons, and repetitive sequence regions of the genome. These 24-nt siRNAs are key molecules in the RNA-directed DNA methylation (RdDM) pathway, which mediates de novo methylation of transposon DNA. Another important class of plant siRNAs is known as tasiRNAs or phasiRNAs. TAS transcripts are first cleaved by trigger miRNAs, then amplified into double-stranded RNAs by RNA-dependent RNA polymerase (RdRP), and subsequently cleaved by DCL4 to produce 21-nt siRNAs, namely tasiRNAs. Such siRNAs are widespread in plants, and many of them are called phasiRNAs because their trigger miRNAs remain unknown.
