Studies On Programmed Cell Death In Wheat Roots Caused By Waterlogging And The Powdery Mildew Fungus Blumeria Graminis F.sp. Tritici

Waterlogging is an abiotic stress in wheat cultivation in Yangtze River valley. It seriously affects the growth and yield of wheat in this area. To date, few studies have been made on programmed cell death (PCD) in aerenchyma formation of wheat roots. This study was carried out on wheat (Triticum aestivum L. cv. Huamai 8) seedlings subjected to waterlogging. DNA agarose gel electrophoresis, TUNEL assay and transmission electron microscopy were used to investigate the PCD aspects of root cells; fluorescence microscopy was used to detect the accumulation of reactive oxygen species (ROS) and Ca²⁺ in wheat roots. We aimed to investigate whether aerenchyma formation in wheat roots was a PCD process of root cortical cells and to confirm whether ROS and Ca²⁺ were related to aerenchyma formation. The main results were as follows:1. Many wheat root cortical cells underwent a PCD process under waterlogging. The main aspects were shown as follows: genomic DNA fragmented and showed DNA laddering; chromatin condensed; nuclear envelope degraded; mitochondrial cristae swelled and degraded; electron density of cell wall reduced.2. PCD in wheat root under waterlogging was strictly controlled. The PCD didn't occur in the stele but only in the cortex. 1.5-48 h after waterlogging, many cortical cells underwent a PCD process and the percentage of TUNEL-positive nuclei was 18%-33%. Few cells underwent a PCD process after aerenchyma formation.3. ROS and Ca²⁺ accumulated before DNA fragmentation and obvious dynamic changes of ROS and Ca²⁺ were observed during aerenchyma formation. The fluorescence due to ROS or Ca²⁺ appeared in endodermis at the beginning, then in exodermis, and in the whole root at last. ROS and Ca²⁺ might be related to the PCD process of root cortical cells under waterlogging.4. Waterlogging affected wheat root growth obviously. The growth of primary roots was reduced during the whole waterlogging period. The generation of adventitious roots was reduced at the beginning of waterlogging but increased thereafter. In addition, waterlogging not only induced aerenchyma formation but also led to cell wall thickening in wheat roots. Wheat powdery mildew, caused by pathogen Blumeria graminis f.sp. tritici, is an important disease worldwide. To date, little is known about systemic programmed cell death caused by powdery mildew fungus in wheat. This study was carried out on powdery-mildew-fungus-infected wheat (Triticum aestivum L. cv. Huamai 8) seedlings and leaf-cutting ones. DNA agarose gel electrophoresis, TUNEL assay and transmission electron microscopy were used to investigate the PCD aspects of root cells; fluorescence microscopy was used to detect the accumulation of ROS and Ca²⁺ in wheat roots. We aimed to investigate whether powdery mildew fungus and leaf-cutting induced PCD in wheat roots and to confirm whether ROS and Ca²⁺ were related to the cell death. The main results were as follows:1. After infection, many wheat root cells underwent a PCD process. The main aspects were shown as follows: genomic DNA fragmented and showed DNA laddering; chromatin condensed and formed peripheral conglomeration in nuclei; perinuclear spaces partly dilated; nuclear envelope degraded and so on. For no evidence of infection was observed in wheat roots, the root cell death induced by the pathogen was a systemic PCD process.2. Compared to leaf-cutting plant, powdery-mildew-fungus-infected ones exhibited narrower leaves, thinner stalks, thickened root cell walls, higher fraction of TUNEL-positive nuclei, more intensive fluorescence due to ROS and Ca²⁺. Lack of assimilates was the most possible reason for PCD in leaf-cutting plants, but the systemic PCD in powdery-mildew-fungus-infected ones might be mediated by long-distance signals.3. Obvious changes of ROS and Ca²⁺ were observed in both PM and LC, ROS and Ca²⁺ might be related to the PCD in both PM and LC.4. PCD in wheat roots caused by powdery mildew fungus Blumeria graminis f.sp. tritici was different from that caused by waterlogging, such as the different positions, the different rates of TUNEL-positive nuclei and the different functions...