Coilin is a marker protein for subnuclear organelles known as Cajal bodies, 
which are sites of various RNA metabolic processes including the biogenesis of 
spliceosomal small nuclear ribonucleoprotein particles. Through 
self-associations and interactions with other proteins and RNA, coilin provides 
a structural scaffold for Cajal body formation. However, despite a conspicuous 
presence in Cajal bodies, most coilin is dispersed in the nucleoplasm and 
expressed in cell types that lack these organelles. The molecular function of 
coilin, particularly of the substantial nucleoplasmic fraction, remains 
uncertain. We identified coilin loss-of-function mutations in a genetic screen 
for mutants showing either reduced or enhanced expression of an alternatively 
spliced GFP reporter gene in Arabidopsis thaliana The coilin mutants feature 
enhanced GFP fluorescence and diminished Cajal bodies compared with wild-type 
plants. The amount of GFP protein is several-fold higher in the coilin mutants 
owing to elevated GFP transcript levels and more efficient splicing to produce a 
translatable GFP mRNA. Genome-wide RNA-sequencing data from two distinct coilin 
mutants revealed a small, shared subset of differentially expressed genes, many 
encoding stress-related proteins, and, unexpectedly, a trend toward increased 
splicing efficiency. These results suggest that coilin attenuates splicing and 
modulates transcription of a select group of genes. The transcriptional and 
splicing changes observed in coilin mutants are not accompanied by gross 
phenotypic abnormalities or dramatically altered stress responses, supporting a 
role for coilin in fine tuning gene expression. Our GFP reporter gene provides a 
sensitive monitor of coilin activity that will facilitate further investigations 
into the functions of this enigmatic protein.