These would be more versatile and less invasive than the current approach, and could also open new avenues of research on plantCwater relations at the microscale. The physiological mechanism for water perception proposed here provides clues toward identifying molecularCgenetic actors in this pathway. the hydropatterning competent zone and that such biophysical cues inform the patterning of lateral roots. Using diverse chemical and environmental treatments we experimentally demonstrate that growth is necessary for normal hydropatterning of lateral roots. Transcriptomic characterization of the local response of tissues to a moist surface or air revealed extensive regulation of signaling and physiological pathways, some of which we show are growth-dependent. Our work supports a sense-by-growth mechanism governing hydropatterning, by which water availability cues are rendered Tesaglitazar interpretable through growth-sustained water movement. Water deficit strongly limits plant growth and development. While a number of strategies that plants use to cope with this stressor have been identified (1), details of the signaling pathways necessary for perception of water deficit are still poorly defined. In systems such as traditional genetic approaches have been employed to elucidate water-perception pathways with considerable success (2). While similar approaches have succeeded in identifying candidate osmosensory proteins in plants (3C5), concerns regarding redundancy of signaling components and/or lethality associated with genetic knockouts suggest that alternative strategies may be necessary. In addition, many studies have focused primarily on understanding the function of signaling pathways that act at the single-cell level. Responses of plant roots to water availability, such as altered growth dynamics or tissue patterning, occur at the organ scale (1). These processes emerge from the actions of many cells and therefore may rely on the perception of environmental cues across the organ. Thus, an exploration of water perception using an organ-scale process as a model system may provide unique insight different from the scope of single-cell studies. To explore how environmental cues pattern physiological responses at the organ scale we characterized water perception in the context of root hydropatterning, an organ-scale developmental response to variation in external water availability (1, 6). During hydropatterning lateral roots become activated in regions of the primary root directly contacting sources of available water, such as agar media, and fail to be induced where water is less available, such as air (Fig. 1 and = 38 seedlings) and position of competent/fixed-zone boundary (red, = 47 seedlings). Shaded regions, SEM. Measurements are averages of three experimental replicates. ((maize) primary roots. This zone of competence closely correlated with the root growth zone, where cell expansion and water uptake occur. Mathematical modeling of water movement in this region suggested that a substantial growth-sustained difference in tissue water potential was present in the competent zone that distinguished tissues contacting external environments with high or low water availability. We show that tissue water potentials in the competent zone are strongly predictive of future patterns of lateral root emergence. These results implicate organ growth as an important contributing process in water perception in plant root tissues, representing a key advancement in our understanding of this phenomenon. Results The Competent Zone for Hydropatterning Coincides with the Growth Zone. Hydropatterning of lateral roots is readily studied in plant seedlings grown on the surface of an agar medium where one side of the root contacts the agar and the other side contacts the air in the headspace of the Petri dish. To determine which regions of Tesaglitazar root tissue are competent to respond to water availability during hydropatterning we applied an agar sheet to a previously air-exposed side of a primary root and tracked subsequent patterns of lateral root development (Fig. S1showed that oscillating changes in auxin signaling necessary for WBP4 lateral root patterning also occur at Tesaglitazar the end of the growth zone,.