Phased secondary small interfering RNAs (phasiRNAs) constitute a major category of small RNAs in plants, but most of their functions are still poorly defined. Some phasiRNAs, known as trans-acting siRNAs, are known to target complementary mRNAs for degradation and to function in development. However, the targets or biological roles of other phasiRNAs remain speculative. New insights into phasiRNA biogenesis, their conservation, and their variation across the flowering plants continue to emerge due to the increased availability of plant genomic sequences, deeper and more sophisticated sequencing approaches, and improvements in computational biology and biochemical/molecular/genetic analyses. In this review, we survey recent progress in phasiRNA biology, with a particular focus on two classes associated with male reproduction: 21-nucleotide (accumulate early in anther ontogeny) and 24-nucloetide (produced in somatic cells during meiosis) phasiRNAs. We describe phasiRNA biogenesis, function, and evolution and define the unanswered questions that represent topics for future research.

定相次级小干扰RNA（phasiRNA）是植物中小RNA的主要类别，但其大部分功能仍不清楚。已知一些phasiRNA，称为反式作用siRNA，可靶向互补mRNA进行降解并在发育中发挥作用。然而，其他 phasiRNA 的靶点或生物学作用仍然是推测性的。由于植物基因组序列可用性的增加、更深入和更复杂的测序方法以及计算生物学和生化/分子/遗传分析的改进，对 phasiRNA 生物发生、其保存及其在开花植物中的变异的新见解不断出现。在这篇综述中，我们调查了 phasiRNA 生物学的最新进展，特别关注与雄性生殖相关的两类：21-核苷酸（在花药个体发育早期积累）和 24-核苷酸（在减数分裂期间在体细胞中产生）phasiRNA。我们描述了 phasiRNA 的生物发生、功能和进化，并定义了代表未来研究主题的未解答问题。