Abstract: Hypocotyl is the part between the origin of radicle and cotyledon node of seedling, and it is the embryonic organ of higher plants. During seed germination, hypocotyls play an important role in the transport and transmission of nutrients and signal molecules between the aboveground and underground parts of plants. Therefore, the hypocotyl elongation process is important for plant survival and growth.In this study, a large number of Arabidopsis mutants were observed and screened, and a mutant u2bl with short hypocotyl phenotype was identified, and its mechanism of regulating the elongation of Arabidopsis hypocotyl was identified explored. Genotype identification and semi- quantitative experiments proved that u2bl-1 and u2bl-2 were mutants with T-DNA insertion and functional deletion. Secondly, phenotypic observation identified that mutant u2bl showed shorter hypocotyls under long-day (16 h light /8 h darkness), short-day (8 h light /16 h darkness) and dark conditions. Cytological microscopic observation showed that the number of hypocotyl cells of mutant u2bl was not significantly different from that of Col-0, and the length of hypocotyl cells was significantly shorter than that Col-0.The above experimental results show that the short hypocotyl phenotype of mutant u2bl was caused by the reduced hypocotyl cell length. Gibberellin (GA), a plant hormone, is the main effector that promotes hypocotyl elongation. To verify whether U2BL regulated hypocotyl elongation by involving in the GA pathway, u2bl mutants and Col-0 were treated with different concentrations of GA and paclobutrazol (PAC), an endogenous GA synthesis inhibitor. The results showed that after treatment with different concentrations of GA, the growth amplitude of hypocotyl length of u2bl mutant was significantly smaller than Col-0, which presented as a phenotype insensitive to exogenous GA. The results of treatment with different concentrations of PAC also showed that the inhibition of PAC on the hypocotyl elongation of mutant u2bl was not significant relative to Col-0.The above results indicated that U2BL affected the signal transduction of GA. U2BL was fused with the mCherry fluorescent tag and stably transferred to Col-0 to obtain transgenic plants. The seedlings, which grew for 7 days, were placed under a laser confocal microscope to observe the subcellular localization in the root tips.The results showed that U2BL was enriched in the nucleus . The RT-qPCR assay showed that in the u2bl mutants, the transcription levels of PRE1, SAUR16, YUC2, YUC8 and PIF4 genes were all significantly down-regulated. This indicated that U2BL indirectly regulated hypocotyl elongation by regulating the above genes.In summary, U2BL is involved in the GA signal transduction pathway and regulates hypocotyl elongation in Arabidopsis thaliana by promoting the expression of genes related to hypocotyl elongation.This study preliminarily expounded the molecular mechanism of U2BL regulating plant hypocotyl elongation, expanded the role of U2BL in the growth and development of Arabidopsis thaliana, and provided reference for further study on the interaction between U2BL and GA regulating plant growth and development.