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.既往多数以拟南芥（Arabidopsis）为模式材料的研究，再度引发了学界对根向水性（root hydrotropism）核心议题的关注与探讨，诸如水分感知位点、生长素（auxin）的参与调控等关键问题。本研究以玉米初生根为实验材料，对其向水性反应展开分析。通过采用非侵入式检测方法（non-invasive method），本研究发现根尖区域对水刺激最为敏感，可诱发根的弯曲反应；伸长区（elongation zone）的其余区域同样具备水分感知能力，但其敏感性相较于根尖更低。通过对向水性响应根中的激素进行定量检测，本研究首次提供了直接证据，证明吲哚-3-乙酸（indole-3-acetic acid, IAA）的重新分布发生于根弯曲进程之中：这种重新分布通过维持根干燥侧的IAA水平实现，而相较于水分充足的非向水性响应根，湿润侧的IAA水平显著降低。与根湿润侧较低的IAA含量结果相一致，本研究对弯曲进程中根伸长区开展的转录组分析（transcriptomic analysis）显示，相较于干燥侧，湿润侧的Aux/IAA转录因子上调程度更为显著。转录组分析同时揭示，涉及漆酶（laccase）与过氧化物酶（peroxidase）基因的木质素合成（lignin synthesis）及细胞壁交联（wall-crosslinking）过程，或许是调控向水性响应根的细胞壁延展性（cell wall extensibility）、进而影响细胞伸长的关键途径。对向水性弯曲过程中细胞伸长的空间分布分析表明，根两侧伸长区的细胞分裂以及细胞伸长峰值的差异性位移均受到调控，以此实现快速的根弯曲反应。基于上述研究结果，本研究提出了玉米初生根向水性响应的作用模型。