The switch in cell-identity acquisition leading to either adventitious rooting or xylogenesis is controlled by SHR and SCR, and involves AUX1, in Arabidopsis thaliana hypocotyls and stem thin cell layers.

Insert Adventitious roots (ARs) contribute to propagation of crops/forest species. SHORT ROOT (SHR) and SCARECROW (SCR) transcription factors affect primary (PR)/lateral (LR) root formation and radial patterning. In Arabidopsis PR, SHR is expressed in the stele, its protein moves into the adjacent layer to control SCR transcription and endodermis specification, and SHR and SCR jointly regulate microRNAs involved in vascular differentiation. There is also evidence of SHR- and SCR-like genes involvement in AR formation in cultured woody cuttings. Arabidopsis stem thin cell layers (TCLs) are without vasculature and need auxin and cytokinin for AR formation. AR-induction occurs in TCL endodermis and seedling hypocotyl pericycle, however AtSHR and AtSCR involvement in adventitious-rooting-identity acquisition is unknown in both cases. AUX1 and LAX3 are auxin-influx carriers. LAX3 contributes to AR-tip auxin-maximum in planta and TCLs, and AUX1 is involved in organ identification in vitro. The interaction of the carriers with SHR and/or SCR in AR-identity acquisition is unknown. Xylogenesis in vitro and in planta is auxin-induced. Arabidopsis shows xylogenesis in addition to AR-formation under the same exogenous hormones in planta and TCLs, but how xylogenic identity is acquired is unknown. The research was aimed to determine SHR, SCR, AUX1 and LAX3 involvement in Arabidopsis AR- and xylogenic-identity-acquisition, and the relationship between the morphogenic programmes. To the aim gene expression and auxin localization were monitored, and seedlings/TCLs of single/double mutants of all the genes, grown with the same auxin and cytokinin concentrations, were analyzed. Results show that AR formation is inhibited from the first divisions in all null mutants, except lax3, showing ARprimordia in both seedlings and TCLs. All mutants also exhibited enhanced xylogenesis in planta and TCLs, however lax3 response was lower than in the other mutants. In conclusion, the same genes, under the same hormonal input, affect AR-induction and xylogenesis at the same time but in an opposite way, highlighting that the programmes are subjected to a reciprocal fine-tuning involving SHR, SCR, and AUX1 in planta and in vitro. The results open the way to understand the genetic basis of AR-recalcitrance in woody species in which the AR-formation-block is associated to enhanced xylogenesis.