OsBRK1-mediated phosphorylation of OsPFN2 regulates meiotic spindle-actin assembly and rice fertility

Formation of a meiotic spindle structure is crucial for chromosome segregation and fertility in plants. Previous studies have shown that actin decorates spindle microtubules in mammalian oocytes, forming spindle actin, which is indispensable for genome stability and gamete segregation. However, the regulatory mechanisms that underlie spindle-actin assembly remain unknown. Here, we report that dysfunction of OsPFN2, a rice profilin protein, disrupts meiotic spindle-actin assembly and spindle microtubule structure and causes errors in chromosome alignment and segregation in pollen mother cells, resulting in male sterility. Furthermore, our results demonstrate that OsPFN2 interacts with Rice Morphology Determinant (OsRMD), a rice formin protein whose depletion also affects spindle-actin assembly and the structure of meiotic spindle microtubules. Intriguingly, we identified an interaction between OsPFN2 and Bub1-Related Kinase 1 (OsBRK1) and demonstrated that OsBRK1 depletion enhances spindle-actin assembly. In addition, we found that OsBRK1 phosphorylates OsPFN2 and that the resulting phosphorylated OsPFN2 retains its capability to bind actin. However, these phospho-mimetic actin-OsPFN2 complexes are not used by OsRMD. Our findings thus reveal that the OsPFN2-OsRMD module controls the assembly of meiotic spindle actin and that OsBRK1 fine-tunes this process through phosphorylation of OsPFN2.