The Arabidopsis thaliana SICKLE (SIC) protein is proline-rich but largely uncharacterized. sic-1 mutants have a miRNA deficiency, alterations in transcript splicing, and increased intron accumulation. sic-3 mutants have global changes in transcript splicing and circadian clock defects likely caused by altered splicing of circadian clock transcripts. Here we show that 13 of 20 putative SIC-interacting proteins are splicing-associated, and that Arabidopsis SIC and DBR1 interact directly. DBR1 is the only lariat debranching enzyme in Arabidopsis and knockout mutants are embryo lethal. The weak dbr1-2 mutant accumulates intron lariats, which interfere with miRNA processing, and has morphological phenotypes similar to sic-1. Human cells deficient in DBR1 also have lariat accumulation and this causes exon skipping. Here we show that sic-1, sic-3, and a new dbr1 mutant allele, dbr1-3, accumulate lariats like the dbr1-2 mutant, and that sic-3 and dbr1-2 have similar changes in splicing, including increased intron retentions. Therefore, lariat accumulation in the sic-1 allele may be responsible for its miRNA deficiencies and related morphological phenotypes, as well as the alterations in transcript splicing. We present a unified theory for sic mutant phenotypes based on SIC-DBR1 interaction and sic intron lariat accumulation.