Studies On The Aerenchyma Formation And The Programmed Cell Death (PCD) Of Cortical Cells Induced By Waterlogging In Wheat Roots

Waterlogging is a serious abiotic stress to winter wheat grown in Yangtz River valley.. The rainy season from March to April overlaps with the booting stage of winter wheat in this aera, and waterlogging is caused by intermittent rainfall. The absorption function of roots is weaken under waterlogging, and this leads to serious damage on growth and yieid of wheat. The improving of wheat tolerance to waterlogging is the key method to ensure the stable and high production in Yangtz River valley. The research was carried out on wheat at booting stage under waterlogging. Using of multiple research methods to investigate the PCD mechanism of cortical cells and aerenchyma formation in wheat roots under waterlogging. The main results were:1. The structural changes of roots in two wheat cultivars, Huamai No.8 (waterlogging tolerant) and Huamai No.9 (less waterlogging tolerant), were studied. It was observed that evident aerenchyma formed in both of the two cultivars after 144 h of waterlogging. There were some tiny intercellular spaces produced firstly by cell saparation and some cortical cells became enlarged. Then lysigenious spaces were formed by cell death and dissolution of these enlarged cells. As the waterlogging processing, more and more lysigenious spaces were produced, this formed a root structure resembling a somewhat disorganized 'spoked wheel'.The porosity of adventitious roots under waterlogging were investigated. The porosity of roots in Huamai No.8 increased at all times with an evident extent, but it was not obvious in Huamai No.9. In addition, the roots of Huamai No.8 remained intact after 144 h of waterloging. The roots of Huamai No.9 lost its integrity and the stele splited away off the cortex, with some ruptured cells in the stele. The differences of changes of structure and porosity in roots provides a cytological base, which indicates Huamai No.8 is more waterlogging tolerant than Huamai No.9.2. Terminal deoxynucleotide mediated nick end labeling (TUNEL) was conducted on roots of Huamai No.8. Waterlogging promoted the DNA fragmentation in cortical cells, and the percentage of the TUNEL-positive nuclei in cortex increased obviously after 8 h of waterlogging. As the waterlogging processing, the number kept increasing and achieved to a maximum after 144 h.3. The ultrastructural changes of corticals cells were studied. It indicated that PCD was induced by waterloging and occurred as a series of cytological events, exhibiting a unique characteristic. In an early stage of PCD, the first detectable changes occurred in plasm membrane (PM) and cytoplasm, including the PM invagination, cytoplasmic vacuolation and the heavy staining granules in vacuolars. In a mid and late stage, the nuclear changes were observed, including the distorted shapes of nucleus, chromatin condensaion to the nucler periphery and the blur nucler periphery. Moreover, in this stage, concentric circles of ER membrane and double-membrane-bound bodies, which were similar to the autophagosomes, were also observed evidently. At the very late stage of PCD, the PM ruptured, the organelles and the cell wall degraded. Moreover, the cellular contents disappeared, with a space formed.4. The ultra-cytochemical localization of acid phosphatases (ACPases) was carried out. After waterlogging, significant increase of ACPases activity was observed in intercellular gap, cell wall, PM and nucleus, and the activity kept increasing from 12 h.-144 h. The ACPases activity was still exactly high on the debris of cellular contents and the cell wall in cells that undergoing degradation. It indicates that ACPases are present throughout the entire PCD process and play an important role in the degradation of organelles and other cellular compents.5. The ultra-cytochemical localization of Ca²⁺-ATPase was carried out. After 4 h of waterlogging, the ACPase activity increased significantly in intercellular gap, PM and ER. After 12 h of waterlogging, significant decrease of ACPase was observed in PM, but the activity was still very high. It is deduced Ca²⁺-ATPase is responsible for the change of Ca²⁺ concentration and Ca²⁺ homeostasis in cells undergoing PCD.