Proposed model for an alternate tissue-specific miRNA biogenesis pathway.
In the SAM region, the established DCL1/DRB1 functional partnership is responsible for the biogenesis of miRNAs with unchanged accumulation in drb235 plants (upper middle dark grey panel). For the drb235 enhanced miRNA accumulation class, DRB2 is antagonistic to DRB1. We propose that these pri-miRNA transcripts express a DRB2-associating feature (dashed bumpy blue line) that prevents folding of the pre-miRNA stem-loop region and subsequent recognition and entry into the DCL1/DRB1 biogenesis pathway (upper left light grey panel). DRB2 is also synergistic to DRB1 for the biogenesis of miRNAs with reduced accumulation in drb235 plants. These pri-miRNA transcripts are proposed to express features that direct their association with either DRB1 (solid bumpy blue line) or DRB2 (dashed bumpy blue line), allowing for their entry and processing by either the canonical DCL1/DRB1 partnership or the alternate tissue-specific DCL1/DRB2 partnership (upper right light grey panel).