The SUN Domain Proteins OsSUN1 and OsSUN2 Play Critical but Partially Redundant Roles in Meiosis1[OPEN]

During meiosis, Sad1/UNC-84 (SUN) domain proteins play conserved roles in promoting telomere bouquet formation and homologous pairing across species. Arabidopsis (Arabidopsis thaliana) AtSUN1 and AtSUN2 have been shown to have overlapping functions in meiosis. However, the role of SUN proteins in rice (Oryza sativa) meiosis and the extent of functional redundancy between them remain elusive. Here, we generated single and double mutants ofOsSUN1andOsSUN2in rice using genome editing. TheOssun1 Ossun2double mutant showed severe defects in telomere clustering, homologous pairing, and crossover formation, suggesting that OsSUN1 and OsSUN2 are essential for rice meiosis. When introducing a mutant allele ofO. sativa SPORULATION11-1(OsSPO11-1), which encodes a topoisomerase initiating homologous recombination, into theOssun1 Ossun2mutant, we observed a combinedOsspo11-1- andOssun1 Ossun2-like phenotype, demonstrating that OsSUN1 and OsSUN2 promote bouquet formation independent of OsSPO11-1 but regulate pairing and crossover formation downstream of OsSPO11-1. Importantly, theOssun1single mutant had a normal phenotype, but meiosis was disrupted in theOssun2mutant, indicating that OsSUN1 and OsSUN2 are not completely redundant in rice. Further analyses revealed a genetic dosage-dependent effect and an evolutionary differentiation betweenOsSUN1andOsSUN2. These results suggested that OsSUN2 plays a more critical role than OsSUN1 in rice meiosis. Taken together, this work reveals the essential but partially redundant roles of OsSUN1 and OsSUN2 in rice meiosis and demonstrates that functional divergence of SUN proteins has taken place during evolution. Two SUN domain proteins have partially redundant functions in the regulation of telomere clustering, homologous pairing, and crossover formation during rice meiosis.