Plants respond to water-limiting conditions in a number of ways but little is known about how the xylem (the water-transporting vascular tissue of the plant) is itself affected. Now, Annelie Carlsbecker and colleagues reveal that vascular acclimation to water deficit in Arabidopsis involves abscisic acid (ABA) signalling via the microRNA miR165. They first show that xylem development is defective in mutants with impaired ABA biosynthesis and, conversely, that elevated ABA levels induce extra xylem strands. Following on from this, they report that ABA signalling regulates xylem patterning in a non-cell autonomous manner; the reduction of ABA signalling specifically in the endodermis is sufficient to disrupt patterning in the nearby xylem. The researchers also show that miR165, which is known to emanate from the endodermis to pattern the vasculature, mediates the response to ABA by regulating the expression of HD-ZIP III transcription factors that control xylem cell fate. Finally, the researchers show that reduced water availability also gives rise to extra xylem strands, and that this response is dependent on the miR165-mediated regulation of HD-ZIP III transcription factors. Together, these findings highlight how a core developmental pathway is re-employed to modify plant morphology under conditions of environmental stress.