miRNAs target conserved motifs of diverse genes encoding NB-LRRs to trigger trans-acting siRNAs in legumes.

Trans-acting siRNAs (tasiRNAs) negatively regulate target transcripts and are characterized by siRNAs spaced in 21-nucleotide 'phased' intervals. TasiRNAs have not been extensively described in many plant species. We identified dozens of new miRNAs in Medicago and soybean and confirmed 119 Medicago targets. A search for phased tasiRNA-like small RNAs ('phasiRNAs') found at least 114 Medicago loci, the majority of which were NB-LRR encoding genes. Notably, conserved domains in NB-LRR-encoding RNAs are targeted by several 22-nt miRNA families to trigger phasiRNA production. DCL2 and SGS3 transcripts were also cleaved by these 22-nt miRNAs, generating phased small RNAs, suggesting synchronization between silencing and pathogen defense pathways. A second example of 'two-hit' phasiRNA processing was identified, utilizing miR156-miR172 sites. Our data illustrate a complex tasiRNA-mediated regulatory circuit that potentially modulates plant-microbe interactions. A few miRNA triggers regulate an extremely large gene family by targeting highly conserved protein motif-encoding sequences, representing a new paradigm for miRNA function. Overall design: Examination of different tissue types in legumes (Medicago, soybean, common bean, peanut) by high throughput sequencing for small RNA profiling

16

Zhai J, Jeong DH, De Paoli E, Park S, Rosen BD, Li Y, González AJ, Yan Z, Kitto SL, Grusak MA, Jackson SA, Stacey G, Cook DR, Green PJ, Sherrier DJ, Meyers BC