The UAP2-BRR2b Module Regulates Low-Temperature Hypocotyl Elongation by Modulating Pre-mRNA Splicing in Arabidopsis

Plants adapt to low temperatures through transcriptomic reprogramming, yet how the core spliceosome maintains splicing fidelity during cold-regulated development remains poorly understood. Here, we investigated the role of the U4/U6.U5 tri-snRNP-associated protein 2 (AtUAP2) in Arabidopsis using genetic, transcriptomic, and biochemical approaches. Under normal conditions (22 °C), uap2, brr2b, and uap2 brr2b mutants showed no obvious developmental defects. Upon low-temperature treatment, however, uap2 displayed a severe impairment in cold-dependent hypocotyl elongation. RNA-sequencing revealed that UAP2 deficiency induced widespread pre-mRNA mis-splicing under cold stress. Ectopic expression of correctly spliced AtGRP7 and AtLOS2 cDNAs in uap2 partially restored hypocotyl growth under cold conditions, supporting their roles as key downstream effectors. Mechanistically, protein interaction assays demonstrated that UAP2 physically binds to the U5 RNA helicase BRR2b. While brr2b mutants exhibited a hypocotyl growth phenotype similar to uap2, the uap2 brr2b double mutant displayed an enhanced growth defect under low temperatures. Overall, our findings suggest that the UAP2-BRR2b complex operates as a functional module required to sustain spliceosomal efficiency under low temperature, providing insights into the potential connection between core splicing machinery dynamics and cold-induced hypocotyl growth responses in Arabidopsis.