Study On The Formation Of Aerenchyma And The Development Of The Sclerenchyma On The Edge Of Exodermis In Rice Roots

This paper makes a systematic anatomical study on the formation of aerenchyma and the sclerenchyma on the edge of exodermis (ROL barrier) in order to reveal the mechanism and regulatory factors of rice roots development with Yangdao 6 as material. Some fruits were acquired after corresponding experiments as followed:1. The whole-root had the typical characteristics of apical growth, reflecting the cytogenesis, differentiation, maturation, death and other development of root cells in different parts from the apical to the base. Seminal root and adventitious root had similar structures, but there were some differences. Aerenchyma started to form at 1.5 cm behind the root tip. The first cells to collapse were located in the middle of the cortex and the radial walls from the collapsing cells aggregate together, leaving a large gas space or lacuna between them. Lateral root primordia developed from specific cells of pericycle and inner cortex cells by the periclinal division. The development of a barrier to ROL attributed to lignin and suberin deposition. Both exodermal cell layers and outermost cortical layers next to the sclerenchyma of ROL barrier were also thickened and lignified. There were significant differences in morphology and structure of rice roots under different media.2. The pattern of aerenchyma formation could originate by the combination of cortical cell autolysis and enlarged intercellular space. The process occurred in some stages: cortical cells stopped expansion and the intercellular space became larger at the initial stage. Cortical cells to collapse and autolysis were found with the disintegration of cell contents. After the evacuation of cell contents, cortical cells became shrinking and invaginated. Finally, cell wall degraded and aggregated together, which results in gas spaces formation in a radial direction.3. Aerenchyma formation appeared to be a form of nonapoptotic programmed cell death in the middle of root cortex. Orderly ultrastructural events were observed under transmission electron microscope, which displayed mainly: cortical cells exhibited chromatin condensation to the nuclear periphery after high vacuolization. A large vacuole occupied the centre of the cell accompanying the deformation of cell nuclear. Soon after, the tonoplast collapsed together with plasma membrane invagination and cell contents disintegration. 4. To properly explore the effects of ethylene on the aerenchyma formation of rice roots, exogenous ethephon were applied to investigate the induction of ethylene on the constitutive aerenchyma formation. The number of air spaces formed by disintegrated cells in middle cortex increased significantly with the increasing concentrations of ethephon from 0.4 to 400 mg·L-1, while the activity of superoxide dismutase (SOD) and catalase (CAT) enzyme showed downward trends. The root porosity were significantly higher in the treatment with 100 mg·L-1 ethephon. Furthermore, earlier cortical cell death was observed under ethephon treatment, most of nucleus of cells at 0.4 cm from the root apex disintegrated with many Golgi apparatus, mitochondria and membrane-bound vesicles around the cell wall. In addition, there was no degraded DNA fragment in the control, whereas the faintly detectable band was found 4 hours later in the treatment and then the smaller molecules bands increased in intensity with time. Thus, the ethephon induced the death of cortical cells in large quantities simultaneously.5. RT-PCR was used to detect the differences of the XET gene expression in different parts of rice roots and the effect of the exogenous ethylene on XET gene expression. The results demonstrated that the accumulation of XET gene was supposed to be associated with the aerenchyma development, and the application of exogenous ethephon might induce the accumulation of endogenous ethylene, leading to a very large increase of XET gene expression and aerenchyma formation in treated roots.6. ROL barriers on the edge of the exodermis plays great role in root morphogenesis, material transport and stress adaption. The lowland rice (Yangdao 6) and upland rice(Zhonghan 3) were tested by comparing the morphologic differences of ROL barrier under water stress of different consistency PEG solutions. The cell wall of sclerenchyma in ROL barrier was significantly thickened in conventional lowland rice (Yangdao 6) with the increase of PEG concentration, but upland rice (Zhonghan 3) was not sensitive to low PEG concentration. The thickening of ROL barrier obviously became stronger at 50 g·L-1 and higher concentrations. Besides, suberisation of cell walls increased significantly with the time extension. Thus, the conclusion suggested that the roots of rice had the buffer capacity under drought stress to resist the water loss and protect the roots from drought injury effectively.