Root patterning: it takes two to tangle

The mechanisms that pattern lateral root primordia are essential for the elaboration of root system architecture , a trait of key importance for future crop breeding. But which are most important: periodic or local cues? In this issue of Journal of Experimental Botany (pages 1411–1420), Kircher and Schopfer manipulate growth direction to demonstrate the importance of both sources of patterning information. It may seem odd to use petri dishes to tease out the rules by which roots ramify underground – to aid the search for water and nutrients, circumnavigate soil particles and balance the forces on the aboveground plant for holdfast. But the simplicity of the 'free running' root-on-a-plate can reveal mechanisms of action that could easily go undetected when studying the system under more demanding conditions. Knowing how the basic mechanisms operate then helps us understand the underground plant system at a different level. This has led to precise descriptions of stages of lateral root primordium formation and outgrowth, the implication of many genes in this process, and to a recognition of the importance of overlying tissue in the emergence of lateral roots (see review by Peret et al., 2009). A particularly exciting question deals with lateral root pat-terning: where and when new lateral root primordia form. Together with the subsequent decision on how fast to grow out from these primordia, this process defines the basic features of root architecture. This question has been particularly puzzling because while there is some regularity to the spacing , the distance between individual lateral roots is not fixed. Indeed, it is far more variable than, say, the spacing of leaves about a stem. During the past decade two parallel lines of evidence have emerged from work on petri plates that emphasize either periodic or local cues for root primordium positioning (Laskowski, 2013; Van Norman et al., 2013). So, on the one hand there is evidence that cyclic activation of an auxin response reporter in elongating cells at the root tip marks these as founder cells for lateral root primordia (Fig. 1A) et al., 2015). On the other hand, it has been shown that local curvature of roots matters for the selection of sites of lateral root primordium formation (Fig. As frequently happens in science, particularly when interesting rival hypotheses emerge, both lines of evidence can be put diametrically against each other, holding that if either one of them turns out to be …