Arabidopsis SSB1, a Mitochondrial Single-Stranded DNA-Binding Protein, is Involved in ABA Response and Mitochondrial RNA Splicing.

A mitochondrion is a semiautonomous organelle that provides energy for life activities and balances plant growth and stress responses. Abscisic acid (ABA) regulates multiple physiological processes, including seed maturation, seed dormancy, stomatal closure and various abiotic stress responses. However, the relationship between mitochondrial activity and the ABA response is unclear. In this study, an Arabidopsis mutant, ssb1-1, was isolated because of its hypersensitivity toward ABA. Assessment results showed that ABA negatively regulates the expression of Arabidopsis SSB1. Mutations in ABA-insensitive 4 (ABI4) and ABI5, genes of key transcription factors involved in ABA-dependent seed dormancy, attenuated the ABA sensitivity of ssb1-1 during germination, suggesting that Arabidopsis SSB1 may act as a regulator in ABA response. Inhibition of endogenous ABA biosynthesis reversed the NaCl-sensitive phenotype of the ssb1-1 mutant, indicating that enhanced ABA biosynthesis is critical for the salinity stress response of ssb1-1. Moreover, compared to that of the wild type, ssb1-1 accumulated more reactive oxygen species (ROS) and exhibited increased sensitivity to the application of exogenous H2O2 during seed germination. SSB1 is also required for mitochondrial RNA splicing, as indicated by the result showing that SSB1 loss of function led to a decreased splicing efficiency of nad1 intron1 and nad2 intron1. Taken together, our data reported here provide insights into a novel role of Arabidopsis SSB1 in ABA signaling and mitochondrial RNA splicing.