Positioning of lateral roots
The placement of lateral roots along the primary root had been thought to be a random process dependent on external inductive signals. We identified a wave of periodic gene expression at the root tip that determines the positions along the root that are competent to respond to these inductive signals. To understand the regulation of these pre-branch sites we performed genome-wide RNA expression analysis and identified a large number of genes whose levels appeared to oscillate. To characterize the role of plant hormones in pre-branch site formation we modulated different biosynthetic pathways. Blocking carotenoid biosynthesis resulted in a near complete loss of prebranch sites. However, the two hormones known to be derived from carotenoids, ABA and strigolactone had no effect on pre-branch site formation. To identify the carotenoid cleavage product that regulates pre-branch site formation, we tested a series of inhibitors that suggest the active molecule is likely a beta-carotenoid cleavage product. Using one of these inhibitors to screen carotenoid derivatives, we identified an endogenous compound that regulates root growth rate. In crop species it modifies root system architecture.