The importin α proteins IMPA1, IMPA2, and IMPA4 play redundant roles in suppressing autoimmunity in Arabidopsis thaliana

Proteins in the importin α (IMPA) family play pivotal roles in intracellular nucleocytoplasmic transport. Arabidopsis thaliana possesses nine IMPA members, with diverse tissue-specific expression patterns. Among these nine IMPAs, IMPA1, IMPA2, and IMPA4 cluster together phylogenetically, suggesting potential functional redundancy. To explore this redundancy, we analyzed single and multiple T-DNA mutants for these genes and discovered severe growth defects in the impa1 impa2 impa4 triple knockout mutant but not in the single or double mutants. Complementation with IMPA1, IMPA2, or IMPA4 fused to green fluorescent protein (GFP) rescued the growth defects observed in the impa1 impa2 impa4 mutant, indicating the functional redundancy of these three IMPAs. The IMPA-GFP fusion proteins were localized in the nucleus and nuclear envelope, suggesting their involvement in nucleocytoplasmic transport processes. Comparative transcriptomics revealed that salicylic acid (SA)-responsive genes were significantly upregulated in the impa1 impa2 impa4 triple mutant. Consistent with this observation, impa1 impa2 impa4 mutant plants accumulated SA and reactive oxygen species to high levels compared with wild-type plants. We also found enhanced resistance to the anthracnose pathogen Colletotrichum higginsianum in the impa1 impa2 impa4 mutants, suggesting that defense responses were constitutively activated in the impa1 impa2 impa4 mutant. Our findings shed light on the redundant roles of IMPA1, IMPA2, and IMPA4 in suppressing the autoimmune responses and suggest avenues of research to clarify their potentially unique roles.