Study Of The Molecular Machenism Of Rice Shoot Apex Organogenesis

Shoot apex has important role in plant development. The organogenesis of lateral organs is determined by shoot apex. There are many factors are involved in regulating the development of shoot apex of dicots, such as gibberellin (GA), auxin and some transcription factors. But in monocots, especially in rice, the factors regulating shoot apex are not known. Therefore, studying the factors involved in rice shoot apex and learning the relationship between them are very important to know the molecular mechanism of shoot apex development.In this study, we focused on YABBY, WOX (WUSCHEL-LIKE HOMEOBOX) and HD-ZIP I (HOMEODOMAIN ZIP I) transcription factor families which are specific to plant kingdom. We cloned these genes, studied their functions in rice shoot apex development. The main results in this study are as following:1. OsYAB1 belongs to rice YABBY family. We studied the expression profile of OsYAB1. The results showed that OsYAB1 was expressed in organs or tissues with rapidly division and elongating. This model is very similar to that of GA3ox2 and GA20ox2.2. Over-expression of OsYAB1 in transgenic rice resulted in a semi-dwarf phenotype because of decreased GA₁ level. In these dwarf plants the transcripts of the biosynthetic gene GA3ox2 were decreased, while that of the deactivation gene GA2ox3 was increased. Co-suppression of OsYAB1 in transgenic plants induced expression of GA3ox2 and GA20ox2, but did not affect that of GA2ox3. In these co-suppression plants, the development of leaf and some floral organs (stamen and carpel) were affected.3. Experiments with transgenic plants expressing inducible OsYAB1 construct, yeast one-hybrid and protein-DNA interaction assays revealed a direct involvement of the protein in the regulation of GA3ox2 expression.4. The expression of OsYAB1 could be either transiently induced by GA (GA₃ treatment) or repressed by a GA inhibitor (paclubtrazol). These data together suggest that OsYAB1 is a regulator of GA homeostasis in rice.5. OsHOX4 is a member of HD-ZIP I transcription factor family. The expression pattern of OsHOX4 was very similar to OsYAB1 in some tissues such as leaf primordia, young leaves and floral primordia etc. 6. Over-expression of OsHOX4 in transgenic rice resulted in a semi-dwarf phenotype. But unlike the plants overexpressing OsYAB1, in these dwarf plants the GA₁ level was elevated and GA₃ could not suppress the dwarf phenotype. This indicated that OsHOX4 has a negative role in GA signal transduction. In addition, the expression of GA2ox3 increased in these dwarf plants.7. The expression level of OsYAB1 was elevated in OsHOX4 over-expression plants and OsHOX4 protein could bind to the promoter of OsYAB1. These results indicated that OsHOX4 protein is a transcriptional activator of OsYAB1 and OsHOX4 could activate the expression of GA2ox3 through OsYAB1.8. OsYAB3 and OsWOX3 belong to rice YABBY and WOX family respectively. The expression of both genes was excluded from the shoot apical meristem (SAM), but showed a similar pattern in leaf primordia and young leaves. In reproductive organs, the expression pattern of both genes also overlaps, except that OsYAB3 was expressed in carpel primordia where OsWOX3 transcripts were not detected.9. Twisted and knotted-like leaves were produced in OsYAB3 RNAi or OsWOX3 over-expression plants, while no phenotypic changes were observed in OsYAB3 over-expression or OsWOX3 RNAi plants.10. Ectopic expression of classâ… KNOTTED LIKE HOMEOBOX1(KNOX1) genes in leaves was induced by either OsYAB3 RNAi or OsWOX3 over-expression.11. OsWOX3 over-expression repressed OsYAB3. Data from inducible OsWOX3 expression and DNA-protein interaction assays indicated that OsWOX3 acted as a transcriptional repressor of OsYAB3.12. Functions of OsYAB1, OsHOX4, OsWOX3, OsYAB3, the relationship between these genes and their role in regulating rice shoot apex were discussed in this study.