BiAux modulates auxin co-receptors activity to form lateral roots

The root system plays an essential role in plant growth and adaptation to the surrounding environment. Lateral roots (LR) originate postembryonically from a group of pericycle founder cells (FC) that are specified by a root clock in an auxin dependent manner and extend the root´s surface. Auxin perception entails the physical interaction of TIR1 or AFBs proteins with Aux/IAA repressors to form a co-receptor system. Despite the apparent simplicity, how a specific TIR1/AFBs - Aux/IAA module is assembled is not completely understood. Here, we have identified and characterized a novel natural compound, called BiAux, that is a N-glucoside bisindole conjugate. This compound or rBiAux, an improved reduced form, precisely promote FC specification and lateral root emergence without affecting other auxin-related responses, including root growth. Genetic analyses demonstrated that rBiAux signaling requires the function of TIR1 and AFB2 receptors. Remarkable, rBiAux enhances TIR1 or AFB2 interaction with Aux/IAA28 and Aux/IAA18, two regulators of FCs and lateral root formation. Furthermore, rBiAux signaling seems to be dependent on ARF7, a transcription factor involved in FC specification and LR formation. Taken together, our data indicate that BiAux enhances the activity of auxin co-receptors involved in FC specification and lateral root formation. To understand teh role of BiAux, we have analyzed the effect of BiAux or IAA or BiAux+IAA in roots of Arabidopsis seedlings. RNAseq analyses of polyA mRNAs isolated from roots of wild type seedlings grown at 22ºC in half strength MS medium for 4 days and then treated with 5 µM BiAux or 10 nM IAA or 5 µM BiAux plus 10 nM IAA for 3 extra days.