Nuclear-Targeted BRCA1 C-Terminus Domain of Physcomitrium NBS1 Effectively Enhances Oxidative Defence and Promotes...

The Nijmegen breakage syndrome 1 (NBS1) protein constitutes a core subunit of the MRE11-RAD50-NBS1 (MRN) complex, serving as a critical DNA damage sensor in the plant DNA damage response (DDR) pathway. We previously showed that in the moss Physcomitrium patens, NBS1 contributes to cellular ROS detoxification, with the BRCA1 C-terminus (BRCT) domain emerging as a determinant of this function. Despite its significance, the capacity of the BRCT domain to independently promote oxidative stress tolerance remains elusive. In this study, we sought to determine whether the BRCT domain of PpNBS1, functioning independently, is sufficient to regulate ROS homeostasis. Structural modelling indicated that the PpNBS1 BRCT domain adopts a conserved α-helical globular conformation typical of functional BRCT motifs, thereby affirming its regulatory potential. Transgenic rice plants overexpressing the isolated BRCT domain displayed limited protection against oxidative stress, attributable to cytoplasmic retention of the protein and modest inhibition of ROS accumulation. In marked contrast, fusing a nuclear localisation signal to the BRCT domain substantially improved stress tolerance, as demonstrated by superior growth performance, preserved membrane integrity, reduced lipid peroxidation and maintained chlorophyll levels under oxidative and cold stress. Nuclear-targeted BRCT expression effectively mitigated ROS accumulation, accompanied by coordinated transcriptional upregulation of antioxidant genes, culminating in elevated enzymatic activities and enhanced radical scavenging capacity. Collectively, our findings demonstrate that the antioxidant defence efficacy of the PpNBS1 BRCT domain is critically dependent on nuclear localisation. This study highlights the importance of spatial regulation in integrating genome surveillance components with oxidative stress resilience pathways in plants.