Nucleo-cytoplasmic distribution of SAP18 reveals its dual function in splicing regulation and heat stress response in Arabidopsis.

The dynamic shuttling of proteins between the nucleus and cytoplasm orchestrates vital functions in eukaryotes. Here, we unveil multifaceted functions of Arabidopsis Sin3-associated protein 18 kDa (SAP18) in regulating development and heat stress tolerance. Proteomic analysis demonstrated that SAP18 is a core component of the nuclear Apoptosis- and Splicing-Associated Protein (ASAP) complex in Arabidopsis, contributing to the precise splicing of genes associated with leaf development. Genetic analysis further confirmed SAP18's critical role in different developmental processes as part of the ASAP complex, including leaf morphogenesis and flowering time. Interestingly, upon heat shock SAP18 translocates from the nucleus to cytoplasmic stress granules and processing bodies. The heat-sensitive phenotype of SAP18 loss-of-function mutant revealed its novel role in plant thermoprotection. Our findings significantly expand our understanding of SAP18 relevance for plant growth, linking nuclear splicing with cytoplasmic stress responses, and providing new perspectives for future exploration of plant thermotolerance mechanisms.