Hydrotropism in the primary roots of maize

Recent studies mainly in Arabidopsis have renewed interest and discussion in some of the key issues in root hydrotropism, such as the site of water sensing and the involvement of auxin. Here we examined hydrotropism in maize primary roots. By using a non-invasive method, we found that the very tip of the root is the most sensitive to the hydrostimulant, triggering root bending. Other regions in the elongation zone are also able to sense water, but with lower sensitivity than the tip. By quantifying the hormones in the hydrotropic roots, we provided the first direct evidence that an indole-3-acetic acid (IAA) redistribution occurred proceeding root bending. The redistribution was achieved by maintaining the IAA level in the dry side, but was significantly reduced in the wet side compared to the well-watered non-hydrotropic roots. Consistent with the lower IAA content in the wet side of the root, our transcriptomic analysis for the elongation zone of the roots proceeding bending showed a stronger upregulation of Aux/IAA transcription factors than the dry side. Transcriptomic analysis also revealed that lignin synthesis and wall-crosslinking involving laccase and peroxidase genes may be a key process in regulation of cell wall extensibility and thus cell elongation in hydrotropic roots. Spatial analysis of cell elongation during hydrotropic bending suggests that cell division and differential shift of the peak of cell elongation along the elongation zone on two sides of the root are regulated to achieve fast bending. Based on our findings, we proposed a model of hydrotropic response in maize primary roots.