Nucleocytoplasmic Trafficking of the Arabidopsis WD40 Repeat Protein XIW1 Regulates ABI5 Stability and Abscisic Acid Responses

WD40 repeat-containing proteins (WD40 proteins) serve as versatile scaffolds for protein–protein interactions, modulating a variety of cellular processes such as plant stress and hormone responses. Here we report the identification of a WD40 protein, XIW1 (for XPO1-interacting WD40 protein 1), which positively regulates the abscisic acid (ABA) response in Arabidopsis. XIW1 is located in the cytoplasm and nucleus. We found that it interacts with the nuclear transport receptor XPO1 and is exported by XPO1 from the nucleus. Mutation of XIW1 reduces the induction of ABA-responsive genes and the accumulation of ABA Insensitive 5 (ABI5), causing mutant plants with ABA-insensitive phenotypes during seed germination and seedling growth, and decreased drought stress resistance. ABA treatment upregulates the expression of XIW1, and both ABA and abiotic stresses promote XIW1 accumulation in the nucleus, where it interacts with ABI5. Loss of XIW1 function results in rapid proteasomal degradation of ABI5. Taken together, these findings suggest that XIW1 is a nucleocytoplasmic shuttling protein and plays a positive role in ABA responses by interacting with and maintaining the stability of ABI5 in the nucleus. XIW1 was identified as a substrate for the nuclear transport receptor XPO1, and is exported from the nucleus in an XPO1-dependent manner. In the presence of abscisic acid, XIW1 accumulates in the nucleus, where it interacts with ABI5 and promotes ABI5 stability and ABA responses.