Root hydrotropism is the directional growth of roots toward a source of moisture in response to a water potential gradient. Root hydrotropism is therefore one of the most important factors for efficient water acquisition under water-limited conditions. However, the mechanisms underlying hydrotropism are not well understood. A previous study of the hydrotropism of primary roots in Arabidopsis thaliana identified the MIZU-KUSSEI (MIZ) 1 and MIZ2 genes as essential for this process. However, despite the important role of lateral roots in water acquisition under water deficient conditions, the mechanism of hydrotropism in lateral roots has not been investigated to date. The present study established an experimental system for the induction of lateral root hydrotropism in Arabidopsis and analyzed its characteristics in comparison with those of primary roots. Lateral roots were found to exhibit hydrotropism irrespective of their length. Expression of a pMIZ1::. GUS fusion gene at the tip of lateral roots and assessment of lateral root hydrotropism in the ahydrotropic mutant miz1, which did not display hydrotropism, demonstrated that the lateral roots of Arabidopsis have the capacity to respond to water potential gradients, and that this activity is regulated by MIZ1.
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