MDF Regulates a Network of Auxin-Dependent and Auxin-Independent Pathways of Adventitious Root Regeneration in Arabidopsis.

Plants exhibit strong plasticity in growth and development, seen clearly in lateral and adventitious root development from differentiated tissues in response to environmental stresses. Previous studies have demonstrated the role of both auxin-dependent and auxin-independent signaling pathways in regulating the de novo formation of adventitious roots (ARs) from differentiated tissues, such as leaf petiole in Arabidopsis. One important question is how the auxin-dependent and auxin-independent pathways are coordinated. To investigate this question, we used a combined approach of inducible gene expression, mutant, and signaling reporter gene analysis during AR regeneration in the Arabidopsis petiole to understand regulatory relationships. Auxin signaling components AXR1 and AXR3 are each required for both AR and subsequent lateral root (LR) initiation, as is the ethylene signaling repressor POLARIS, but not EIN2. The PIN trafficking SNARE protein VAMP714 is required for LR rather than AR formation, through effects on auxin-induced gene expression. We identify the RNA splicing regulator MDF and the transcription factor RAP2.7 as new positive regulators of both the auxin-independent and auxin-dependent pathways, and show that MDF regulates RAP2.7, WOX5, and NAC1 while RAP2.7 regulates WOX5 but not NAC1 or YUC1. NAC1 is required for de novo root formation in a pathway independent of YUC1, WOX5, or RAP2.7. We propose a model in which MDF represents a point of molecular crosstalk between auxin-dependent and auxin-independent regeneration processes.