An Arabidopsis thaliana mutant, cryophyte, was isolated and found to have an enhanced cold stress-induction of the master regulator of cold tolerance, C-repeat binding factor 2 (CBF2), and its downstream target genes. The mutant is more tolerant to chilling and freezing stresses but is more sensitive to heat stress. Under warm but not cold growth temperatures, the mutant has a reduced stature and flowers earlier. Under long day conditions, flowering of the mutant is insensitive to vernalization. The mutant is also hypersensitive to the phytohormone abscisic acid. The mutation was found in a DEAD box RNA helicase gene that is identical to the previously identified low expression of osmotically responsive genes 4 (LOS4) locus, which was defined by the los4-1 mutation that reduces cold regulation of CBFs and their target genes and renders Arabidopsis plants chilling sensitive. We show evidence suggesting that the CRYOPHYTE/LOS4 protein may be enriched in the nuclear rim. In situ poly(A) hybridization indicates that the export of poly(A)⁺ RNAs is blocked in the cryophyte/los4-2 mutant at warm or high temperatures but not at low temperatures, whereas the los4-1 mutation weakens mRNA export at both low and warm temperatures. These results demonstrate an important role of the CRYOPHYTE/LOS4 RNA helicase in mRNA export, plant development, and stress responses.