| Summary: | During in vitro maize plant regeneration somatic cells change their normal fate and undergo restructuring to generate pluripotent cells able to originate new plants. Auxins are essential to achieve such plasticity. Their physiological effects are mediated by auxin response factors (ARFs) that bind auxin responsive elements within gene promoters. Small trans-acting (ta)-siRNAs, originated from miR390-guided <i>TAS3</i> primary transcript cleavage, target ARF3/4 class (tasiR-ARFs). Here we found that <i>TAS3b</i> precursor as well as derived tasiR-ARFbD5 and tasiR-ARFbD6 display significantly lower levels in non-embryogenic callus (NEC), while <i>TAS3g</i>, miR390 and tasiR-ARFg are more abundant in the same tissue. However, Argonaute (AGO7) and leafbladeless 1 (LBLl) required for tasiR-ARF biogenesis showed significantly higher transcript levels in EC suggesting limited tasiR-ARF biogenesis in NEC. The five maize ARFs targeted by tasiR-ARFs were also significantly enriched in EC and accompanied by higher auxin accumulation with punctuate patterns in this tissue. At hormone half-reduction and photoperiod implementation, plant regeneration initiated from EC with transient <i>TAS3g</i>, miR390 and tasiR-ARFg increase. Upon complete hormone depletion, <i>TAS3b</i> became abundant and derived tasiR-ARFs gradually increased at further regeneration stages. <i>ZmARF</i> transcripts targeted by tasiR-ARFs, as well as <i>AGO7</i> and <i>LBL1</i> showed significantly lower levels during regeneration than in EC. These results indicate a dynamic tasiR-ARF mediated regulation throughout maize in vitro plant regeneration.
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