Degradation of detrimental microRNAs safeguards the fertilized egg cells to establish an ECS-dependent polytubey block in Arabidopsis

Fertilization of the egg cell by more than one sperm cell results in aneuploidy and genomic dysfunction. To avoid this, flowering plants have intricate polytubey block mechanisms that prevent additional pollen tubes from entering the ovule. In one mechanism for blocking polytubey, the fertilized egg cell secretes the aspartic endopeptidases EGG CELL-SPECIFIC1 (ECS1) and ECS2, which degrade pollen tube attractants. However, the mechanism underlying the egg-cell-specific expression of ECS1 and ECS2 was unknown. Here, we demonstrate that in Arabidopsis thaliana egg cells, SMALL RNA DEGRADING NUCLEASE (SDN) degrades microRNAs (miRNAs) targeting ECS genes, thereby allowing ECS1 and ECS2 to accumulate in egg cells to establish the polytubey block. Like Arabidopsis ecs1 ecs2 mutants, sdn1 sdn2 sdn3 mutants exhibited polytubey, as did plants specifically overexpressing miRNAs targeting ECS genes in egg cells. Therefore, the SDN-miRNA-ECS module allows ECS accumulation in egg cells to block polytubey.