Priming and positioning of lateral roots in Arabidopsis. An approach for an integrating concept

Lateral root pattern formation is controlled mechanistically by separate processes: oscillating priming signals determine lateral root frequency and mechanical cues determine their spatial positioning along the primary root.Branching by de novo formation of lateral roots along the primary root of Arabidopsis seedlings follows a complex longitudinal and transverse pattern. How this pattern is generated is presently under debate. The 'bending hypothesis' proposes that lateral root primordia are initiated by a local accumulation of auxin at the convex side of bends resulting from deflections through obstacles, gravitropic bending, or other means. In contrast, the 'oscillation hypothesis' proposes the existence of an endogenous clock-type oscillator mechanism producing periodic pulses of gene expression in the root tip that determine the future sites of primordium initiation. Here we report physiological experiments dissecting periodic priming signals, pre-disposing the root to rhythmic lateral root formation, from bending-mediated signals responsible for the subsequent positioning of their initiation along the growing root. While the frequency of lateral roots can be promoted by auxin in the mature root, their positioning follows a pre-formed pattern determined by previous bending. Both types of signals turn out to be necessary, complementary components in an integrating concept of lateral root patterning.