We have developed experimental systems to study hydrotropism in seedling roots of Arabidopsis thaliana (L.) Heynh. Arabidopsis roots showed a strong curvature in response to a moisture gradient, established by applying 1% agar and a saturated solution of KCl or K(2)CO(3) in a closed chamber. In this system, the hydrotropic response overcame the gravitropic response. Hydrotropic curvature commenced within 30 min and reached 80-100 degrees within 24 h of hydrostimulation. When 1% agar and agar containing 1 MPa sorbitol were placed side-by-side in humid air, a water potential gradient formed at the border between the two media. Although the gradient changed with time, it still elicited a hydrotropic response in Arabidopsis roots. The roots curved away from 0.5-1.5 MPa of sorbitol agar. Various Arabidopsis mutants were tested for their hydrotropic response. Roots of aba1-1 and abi2-1 mutants were less sensitive to hydrotropic stimulation. Addition of abscisic acid restored the normal hydrotropic response in aba1-1 roots. In comparison, mutants that exhibit a reduced response to gravity and auxin, axr1-3 and axr2-1, showed a hydrotropic response greater than that of the wild type. Wavy mutants, wav2-1 and wav3-1, showed increased sensitivity to the induction of hydrotropism by the moisture gradient. These results suggest that auxin plays divergent roles in hydrotropism and gravitropism, and that abscisic acid plays a positive role in hydrotropism. Furthermore, hydrotropism and the wavy response may share part of a common molecular pathway controlling the directional growth of roots.
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