A Molecular framework for the differential responses of primary and lateral roots to auxin in Arabidopsis thaliana

The Arabidopsis thaliana root system, which comprises primary, lateral, and adventitious roots, responds differentially to environmental and hormonal signals. For example, exposure to high concentrations of the plant hormone auxin promotes lateral root (LR) initiation and emergence; however, high levels of auxin inhibit primary root (PR) growth. Here, we show that transacting small interfering RNA-dependent post-transcriptional gene silencing of AUXIN RESPONSE FACTOR 3 ( tasiRNA-ARF PTGS) likely plays a role in promoting the differential responses of PR and LR to auxin. In the presence of 1-naphthaleneacetic acid (NAA), a synthetic auxin, the expression domain of ARF3 (a negative regulator in auxin signaling) expanded into the meristem zone (MZ) in PR. Moreover, we found a remarkable increase in the expression of a cleavage-resistant version of ARF3 ( pARF3::ARF3(m)-GUS ) in PR even without application of exogenous auxin, suggesting the involvement of posttranscriptional regulation. In addition, the expression levels of microRNA390 ( miR390 ), which triggers biogenesis of tasiRNA-ARF , were substantially reduced in PR, but increased in LR. Taken together, our findings suggest that high auxin levels differentially regulate the growth and development of PR and LR via the tasiRNA-ARF pathway.