NUCLEOPORIN1 mediates proteasome-based degradation of ABI5 to regulate Arabidopsis seedling establishment

NUCLEOPORIN1 (NUP1/NUP136), a member of the Nuclear Pore Complex (NPC), is located on the inner side of the nuclear membrane. It is highly expressed in seeds; however, its role in seed germination and seedling establishment has not yet been explored. Here, we identified an abscisic acid (ABA) hypersensitive phenotype of nup1 during seedling establishment in two nup1 mutant alleles. ABA treatment drastically changes the expression pattern of thousands of genes in nup1, including the major transcription factors (TFs) involved in germination, ABI3, ABI4, and ABI5. Double mutant analysis of NUP1 and these ABA-related genes showed that mutations in ABI5 can rescue the phenotype of nup1, suggesting that abi5-8 is epistatic to nup1-1 in seedling establishment. ABI5, a key negative regulator of germination, is abundant in dry seeds and rapidly degraded during germination. However, its spatiotemporal regulation and interaction with other molecular players during the degradation remain to be fully elucidated. We found that NUP1 is physically associated with ABI5 and the 26S proteasome. Mutation in NUP1 delayed ABI5 degradation through its post-translational retention in the nucleolus under ABA stress. Taken together, our findings suggest that NUP1 anchors the proteasome to NPC and modulates seedling establishment through proteasome-mediated degradation of ABI5 in the vicinity of NPC in the nucleoplasm.