The tasiR-ARF pathway in plants: origin, functions, and interplay of miR-390, tasiRNAs and ARF3

Trans-acting small interfering RNA (tasiRNAs) are a special type of endogenous small RNAs (sRNAs) found only in plants. Their biogenesis requires an initial miRNA-mediated cleavage of RNA precursors transcribed from TAS genes. TasiRNAs act in trans to silence gene expression by cleaving mRNAs with sequences partially complementary to their own. While Arabidopsis thaliana contains several TAS genes not found in other plants, the miR390-TAS3-ARF pathway is highly conserved among land plant lineages. This pathway exerts its function by silencing a subgroup of Auxin Response Factor (ARF) genes; these tasiRNAs are termed tasiR-ARFs. Many downstream auxin signals are mediated by ARFs acting as transcription factors to confer sensitivity and robustness to the hormone responses in diverse development contexts. These pathway functions are critical for plant growth, developmental timing, and correct organ patterning, such as leaf morphology and polarity, lateral root architecture, and flowering, as well as coping with stress. The phenotypes caused by mutations affecting tasiR-ARF production vary across plant species, showing pleiotropic effects, suggesting a co-opted process where the tasiR-ARF pathway evolution occurred to serve different functions, depending on plant developmental cues. One way to unify the diverse roles of this pathway would be through auxin response integration, possibly by exploring the evolution of ARF3 transcription factors and downstream genes. In this review, we discuss versatility of the tasiR-ARF pathway in land plants according to known developmental and environmental responses where the phytohormone auxin plays an essential role.