Origin And Development Of The Root Cap In Rice

Root is essential for nutrients and other messages exchange that occurred between plant and soil. Well-developed crop root systems are bases for maintaining high yields. The tip of the root is covered by a thimble-shaped root cap that could direct root growth via perception and transduction for various environmental stimuli. Until now, however, little was known about how the root cap of rice develops and functions to regulate the adaptive behavior of the root. To address this, we examined the formation of rice root cap during embryogenesis; characterized the anatomy and structure of the rice radicle root cap; investigated the factors that involved in the root cap regeneration; detected the rice lines with extremely differences in root cap configuration; then analyzed the functions and inside molecular mechanisms of the cap configuration. The main results are summarized as follow:1. Using semi-thin sections, we found that:the signs that embryonic root cap development were first observed at 3.5 DAA, and the root cap junction could be unambiguously recognized at 4 days. Following the division of root cap stem cells below the cap junction, starch granules appeared at 7 DAA in the lower 10 layers of columella root cap cells; whereas starch granules in lateral root cap cells could only be readily seen at 8 DAA. Starch granule formation in the columella and lateral root cap cells indicates that these cells are fully differentiated and thus the root cap might be fully functional. Rice seeds could germinate at 8 DAA but not at 7 DAA, suggesting that seed germination requires a functional root cap. The root cap is fully developed at 9 DAA, which is consisted of 3 layers stem cells,13 layers columella cells with starch granules and surrounded lateral root cap cells.2. We also studied the root cap development during embryogenesis in Arabidopsis, results showed that:the origin of embryonic root cap was first detected at 4 DAA, and starch granules appeared at 7 DAA in the central part of columella root cap cells. The root cap is fully developed at 9 DAA and Arabidopsis seeds could germinate at 9 DAA but not at 8 DAA. These results collectively showed that rice and Arabidopsis go through with similarly root cap formation process during embryogenesis.3. Semi-thin sections of radicle root tip showed that:the radicle root cap contains 13-14 layers of columella root cap cells with approximately 50 cells in each layer; and the columella root cap cells are surrounded by 3-5 layers of lateral root cap cells. Lugol’s staining and EdU staining showed that:between the columella root cap and root cap junction is the columella root cap meristem, with 8 columns and 1-2 layers of cells; The cells in the upper tier are stem cells of the columella root cap, which, could produce the columella root cap stem cell daughter cell at the lower tier through an anticlinal division and eventually form the entire column of columella root cap cells. Cell lineage tracing showed that:columella stem cells could generate columella stem cells, columella stem cell daughter cells and then columella cells via anticlinal division; and lateral root cap cells generated from lateral root stem cells, which different from the columella root cap stem cells; lateral root cap stem cells could produce lateral root cap stem cells and their daughter cells by both periclinal and anticlinal divisions; these daughter cells could undergo anticlinal divisions and eventually differentiate into starch granule-containing lateral root cap cell.4. After studied the root cap regeneration, we found that:dynamic changes of auxin levels in root tip involved in the root cap regeneration; and shoot-derived auxin has a role in root cap development. In the absence of a functional stem cell niche, shoot-derived auxin is absolutely needed to for root cap regeneration. In all, root cap regeneration in rice requires the presence of QC or shoot-derived auxin. Transcriptomics profile showed that plant hormones, such as auxin and cytokinin, involved in the root cap regeneration.5. We examined the root tip angles amoung 7 rice lines, results showed that: extremely tip angle differences exist between Teqing and Zhenshan 97. In addition, we found that the cell length in Teqing is more longer than that in Zhenshan 97, which may contribute to the tip angle differences between them. After the statistical analysis of root tip angles in Teqing/Zhenshan97 RIL population and QTL mappings, we detected 4 QTLs, and found that the QTL located in the Chr4 (we named it Qta4) were repeatedly detected in the 3 independly experiments.6. After treatment by various compounds, we found that the inhibitors of ethylene signal transduction or ethylene biosynthesis could induce more border cells in root tip that has small tip angles, indicated that the genes controlled the root cap configuration and the ethylene signal involved in the border cells formation. After the analysis of GAL4/UAS::GUS enhancer trap lines that specially expressed in part of the root cap cells, we found that the inhibitors of ethylene enhanced the renewed ability of root cap cells, and then promote the border cells formation.