Abstract: Plant miRNAs exhibit a dynamic and complex evolutionary landscape. Despite their rapid turnover, miRNAs play crucial roles in regulating key biological processes, underscoring their functional significance even when evolutionarily transient. This review explores the phenomenon of miRNA turnover through a comprehensive survey of miRNA conservation across land plants. We discuss how de novo miRNAs overcome challenges such as transcriptional activation, structural requirements for biogenesis, and functional integration into gene regulatory networks. Furthermore, we review the mechanisms of miRNA origination, including inverted duplication of target genes, derivation from retrotransposons or DNA transposons, spontaneous evolution, and a newly proposed mechanism through template switching mutations. Duplication of existing miRNAs plays a significant role in miRNA family expansion, driving the functional diversification and strengthening regulatory networks. While the phenomenon of miRNA loss has been preliminarily explored, its mechanisms remain insufficiently understood. To address this, we proposed three detailed steps to advance research into miRNA loss. This review provides an integrated perspective on the gain, expansion, and loss of plant miRNAs, offering insights into their evolutionary and biological significance.

Key words: expansion, gain, loss, microRNAs, plants.