Comparative analysis reveals gravity is involved in the MIZ1-regulated root hydrotropism.

Hydrotropism is the directed growth of roots toward the water found in the soil. However, mechanisms governing interactions between hydrotropism and gravitropism remain largely unclear. In this study, we found that air-system and agar-sorbitol-system only induced oblique water-potential gradients; agar-glycerol-system only induced vertical water-potential gradients; sand-system established both oblique and vertical the water-potential gradients. We employed obliquely-oriented and vertically-oriented experimental systems to study hydrotropism in Arabidopsis and tomato plants. We found that the gravitropism-deficient mutant aux1 showed enhanced hydrotropism in the oblique orientation but impaired root elongation towards the water in a vertical orientation. Miz1 mutant exhibited deficient hydrotropism in the oblique orientation but normal root elongation towards the water in a vertical orientation. We performed comparative analyses using different hydrotropic systems and found that gravity hindered the ability of roots to search for obliquely-oriented water. Importantly, in contrast to miz1, the miz1/aux1 double-mutant exhibited hydrotropic bending in the oblique orientation. Additionally, gravity facilitated roots' search for vertically-oriented water, because the miz1/aux1 double-mutant showed attenuated root elongation towards the water in a vertical orientation, in contrast to miz1. Our results suggest that gravitropism is required for MIZ1-regulated root hydrotropism in both the oblique orientation and vertical orientation, providing further insight into the role of gravity in root hydrotropism.