Figure 3.

Hydrotropic response of heterozygous nhr1 and wild-type roots in a system with a gradient in air moisture. In this system, an air moisture gradient was created around the roots between the oasis (water) and the cuvette (saturated solution of CaCal₂) for testing their positive hydrotropic response (A–F). The additive effect of gravitropism and hydrotropism was tested in a control system where both the oasis and the cuvette contained water (G–L). At time 0 (A, D, G, and J), roots were placed horizontally with a distance of 2 to 3 mm from the tips growing in the air to the water source. In the system with air moisture gradient, wild-type roots were hydrotropically stimulated and, as a consequence, showed negative gravitropic response 6 and 24 h after the beginning of the experiment (B and C). Heterozygous nhr1 roots were not hydrotropically stimulated 6 and 24 h after the beginning of the experiment (E and F). These roots showed either a right or a left-handed twist in their growth direction. Root growth direction of heterozygous nhr1 ended behind the microscope slide in a loop-like structure 24 h after the beginning of the experiment (F). This root growth behavior hampered the measurements of root curvature angles. In the moisture air system, 6 (H) and 24 (I) h after the beginning of the experiment, root growth of wild-type seedlings was primarily directed toward the water source instead to the gravity vector. After 6 (K) and 24 (L) h, roots of heterozygous nhr1 seedlings grew mostly in the direction of gravity. The figures are representative of five independent experiments (n = 24).