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Background and Aims Root hydrotropism is a response to water-potential gradients that makes roots bend towards areas of higher water potential. The gene MIZU-KUSSEI1 (MIZ1) that is essential for hydrotropism in Arabidopsis roots has previously been identified. However, the role of root hydrotropism in plant growth and survival under natural conditions has not yet been proven. This study assessed how hydrotropic response contributes to drought avoidance in nature. Methods An experimental system was established for the study of Arabidopsis hydrotropism in soil. Characteristics of hydrotropism were analysed by comparing the responses of the miz1 mutant, transgenic plants overexpressing MIZ1 (MIZ1 OE) and wild-type plants. Key Results Wild-type plants developed root systems in regions with higher water potential, whereas the roots of miz1 mutant plants did not show a similar response. This pattern of root distribution induced by hydrotropism was more pronounced in MIZ1 OE plants than in wild-type plants. In addition, shoot biomass and the number of plants that survived under drought conditions were much greater in MIZ1 OE plants. Conclusions These results show that hydrotropism plays an important role in root system development in soil and contributes to drought avoidance, which results in a greater yield and plant survival under water-limited conditions. The results also show that MIZ1 overexpression can be used for improving plant productivity in arid areas.

It has been proposed that hydrotropism interacts with gravitropism in seedling roots; that is, roots which are highly gravitropic show less hydrotropism (Takahashi and Suge, 1991 Physiologia Plantarum82: 24-31; Takahashi and Scott, 1993 Plant, Cell and Environment16: 99-103). Here, we examine varietal differences in the hydrotropic response and its interaction with gravitropism in wheat roots. Primary seminal roots of wheat (Triticum aestivum L.) were hydrotropically stimulated by different moisture gradients established by placing wet cheesecloth and saturated solutions of different salts in closed chambers. From equations obtained by relative humidity (RH) at different distances from the wet cheesecloth, moisture gradients at the root-tip level were estimated to be 0·03 to 1·84% RH mm-1, depending upon the salt introduced into the chamber. The roots showed positive hydrotropism in response to 0·67% RH mm-1, and the response apparently increased as the gradient was strengthened. When the primary seminal roots of 12 cultivars were exposed to a moisture gradient of 1·84% RH mm-1, hydrotropic response significantly differed depending upon the cultivar tested. Among the cultivars, the roots of Norin 11, Norin 15, Norin 117, and Norin 125 responded hydrotropically more strongly than the others. These roots, with the exception of Norin 11, showed a less vigorous response to gravity compared to the remaining cultivars. However, the roots of Norin 20, Norin 38, and Norin 107 were relatively unresponsive to both a moisture gradient and to gravity. Thus, the primary seminal roots of wheat respond hydrotropically, and the responsiveness differs among cultivars. However, the varietal difference in hydrotropic response cannot be explained solely by converse differences in responsiveness to gravity.

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