Phased Secondary Small Interfering RNAs (phasiRNAs) in Plants: Their Biogenesis, Genic Sources, and Roles in Stress Responses, Development, and Reproduction.

PhasiRNAs (phased, secondary, small interfering RNAs) constitute a major category of small RNAs (sRNAs) in plants, but most of their functions are still poorly defined. Some phasiRNAs, known as trans-acting small interfering RNAs (tasiRNAs), are known to target complementary mRNAs for degradation and to function in development. Yet for other phasiRNAs, either the targets or biological roles remain speculative. New insights into phasiRNA biogenesis, their conservation, and variation across the flowering plants continue to emerge via 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, in particular focusing on two classes associated with male reproduction: 21-nt (accumulated early in anther ontogeny) and 24-nt (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.