This is a simple model for 'quorum' or collision sensing in Arabidopsis. (A) A growing lateral root from a young seedling encounters an established plant (red box). (B) Within a certain distance, ROS produced by the tip of the growing seedling contributes to oxidation of cell wall associated phenols to yield BQs (red arrow). DMBQ is shown here as an example. These BQs accumulate, contribute to the autocatalytic production of new ROS intermediates, amplify the signal (green arrow). (C) Evidence of an established root system arrests root elongation in a quorum-like process, regulating global root system architecture.