Lateral root branching in higher plants is promoted in regions locally contacting a source of water, and suppressed in regions exposed to low water availability. We found that developmental competence to respond to this environmental signal is limited to growing tissues. We profiled gene expression in regions of the maize primary root exposed to low and high water availability both within and outside of the zone of competence. Overall design: Maize primary roots were grown along an agar surface. Roots were manually dissected longitudinally to separate the size exposed to air (Air) from the side contacting agar (Contact). These tissue sections were then cut radially into segments 0-5 mm from the root tip for the developmentally competent zone (Competent) and 5-15 mm from the root tip for the developmentally fixed zone (Fixed). Matching tissue types from 2 primary roots were pooled for each biological replicate. RNA was profiled from 3 biological replicates.