Reproductive phasiRNAs in monocots: Emerging regulators of anther development and male fertility

In many grass species, reproductive phasiRNAs are highly expressed during various stages of anther development. The distinct spatiotemporal patterns of the 21- and 24-nt phasiRNAs are vital for proper anther and pollen maturation. Perturbations of phasiRNA cause male sterility, underscoring the role of reproductive phasiRNAs in male fertility. Recent studies have identified key molecular components involved in phasiRNA biogenesis, such as PHAS precursors, miRNA triggers, RDR6, SGS3, Dicer-like proteins, and Argonautes. In this review, we summarize our current understanding of reproductive phasiRNA production, transcriptional regulation, molecular functions, and accumulation patterns. The miR2118- and miR2275-triggered pathways are conserved across many flowering plants and provide a framework for understanding reproductive phasiRNA biogenesis. A newly identified class of premeiotic 24-nt phasiRNAs appears to arise through distinct mechanisms, and their biological functions remain unclear. Additionally, we examine evidence linking phasiRNAs to environmental stress responses and fertility maintenance. Despite significant advances in understanding the reproductive phasiRNA pathway, major questions regarding their targets, mechanism of action, and regulatory roles remain. Understanding how phasiRNAs are involved in reproductive development and fertility in flowering plants will help address these knowledge gaps.