Fig. 6: Conservation of reproductive 21 nt phasiRNA in seed plants.

a Phylogeny showing the representative species of seed plants according to estimated divergence times based on the time tree of life (Kumar et al.⁵¹). The miR2118/482 superfamily consists of miR2118, miR482, miR8558, and miR472. MYA means million years ago. √ indicates the presence and X indicates the absence; na indicates “not analyzed”. b Model of generation of 21 nt reproductive phasiRNAs in seed and flowering plants. miRNA triggers initiate the cleavage of Pol II transcripts with the help of AGO proteins resulting 3′-end cleaved fragments, which are processed by RDR6 to generate double-stranded RNA molecules. DCL4 chops these double-stranded molecules into 21 nt duplexes, initiated from the cleavage site (denoted by dotted lines). In gymnosperms, monocots, and some eudicots such as flax, miR2118 is the only trigger for the production of 21 nt reproductive phasiRNAs, whereas in columbine, rose, and wild strawberry, there is a lineage-specific miRNA trigger for this role. Based on the roles in other species, family members of AGO1 and AGO5 might load the trigger miRNA and 21 nt phasiRNAs, respectively. The resulting phasiRNAs from these mechanisms were predicted to act in both cis and trans to modulate the gene expression based on the studies in monocots. Panel b was created with BioRender.com under an academic license.