Pre-mRNA splicing factor required for splicing and for the turnover of unstable transcripts. May be a U5 snRNP-associated protein involved in the formation of U4/U6-U5 tri-snRNP. Involved in responses to abiotic stresses. Involved in microRNAs (miRNAs) biogenesis by functioning in primary miRNAs (pri-miRNAs) splicing. Required for DNA methylation and transcriptional silencing through the RNA-directed DNA methylation (RdDM) pathway. Component of a pre-mRNA splicing complex (PubMed:23877244). Interacts with ZOP1 (PubMed:23524848). Interacts with PRP31 (PubMed:25684655). Ubiquitous. Up-regulated by cold stress, but not by abscisic acid or salt treatment. Lethal when homozygous. The sta1-1 mutation causes the stabilization of the normally unstable STA1 transcript (PubMed:16751345) and increases the expression levels of miRNA target genes (PubMed:23268445).