The classification of main ncRNAs in plants. A, the classification of small RNAs, the display from left to right represents hierarchical relationships, every solid box presents one type of small RNA. B, the classification of medium-sized ncRNAs. C, the classification of lncRNAs, the green line and arrow represent lncRNAs transcription direction while the black line and arrow represent coding genes. D, the diagrammatic sketch of circular RNAs. hpRNA, hairpin RNA; siRNA, small interfering RNA; miRNA, microRNA; NAT-siRNA, natural antisense transcript siRNA; phasi RNAs, phased small interfering RNAs; U6 snRNA, U6 small nuclear RNA; snoRNA, small nucleolar RNA; TSS, transcription start site; TTS, transcription termination site.