| The mictrostructal and ultrastructal features in interaction between Wheat and Puccinia recondita f.sp tritici (cv.lorvinlO x race 162; cv.lorvinlOx race 165) were systematically studied by microscopy ,electron microscopy, agarose gel and in situ end labeling which provided the basis for further resistance research. The features of cell death in incompatible combination (cv.lorvin x race 162) indicated that HR is a form of programmed cell death(PCD).The results are given as following:1. Microscopic observation showed that there was no significant difference in the early stage (8-12h after inoculation, a.i.) of the interaction, The fact that some chloroplasts began to transport and deform and collapse subsequently since 12h a.i. indicated the morphological change in incompatible combination. At the later stage (72h a.i). the organelles in mesophyll cell besides fungus infection became shorter and shorter and the nuclei disintegrated completely eventually. All these symptoms indicated the cells were dying or ending of life. But there was no great change in the host cell in compatible combination with fungus hyphae extension though very few chloroplasts began to transport and deform2. Ultrastructural observation in incompatible combination showed that the nuclei gradually became irregular with highly condensed chromatin and stayed at the edge of cells, the chloroplast became disordered since 12h to 24h a.i. The nuclear membrane underwent disruption and the nucleus eventually degenerated, the chloroplasts collapsed and some mitochondrion shrank and became disorganized which were the last organelles disappeared since 48h to 72h a.i. Endoplasmic reticulum and vacuoles did play a vital55role in the further degeneration of other organelles just similar to the lysosome acting in the animal cells.Ultrastructure of the host cell in compatible combination began to change at 24h a.i., the host cell became plasmolyzed, and some of the plasmalemma showed slightly sunken. At the stage of 48a.i., some kind material(s) with electron non-transparency were deposited on the membrane . The chloroplasts were swollen or malformed. The thylakoid array inside chloroplasts became lossen or disordered, but the starch grains in it were still increasing. At the stage of 72h a.i., their nuclei started to change their shapes or malformed, the nuclear membrane appeared blurred in boundary, the crista of most mitochondria appeared indistinctly.and whole the cells started to decline.3. The occurring of DNA ladder in the course of cell death was the typically biochemical characteristicsof PCD. When the DNA sample from the leaves of the incompatible combination 24h, 48h or 72h a.i. were run agarose gel electroforesis, the DNA ladder was observed clearly on the electroforesis diagrams, however, no DNA ladder was observed in the DNA samples from 162 uredospores, uninoculated plantsor the plants of the compatible combination.4. The mesophyll DNA end labeling in situ (TUNEL) showed the positive reaction in leaves of incompatible combinationsince since 12h a i. The results of the labeling in situ indicated the nuclear DNA has degraded and at least the HR has been elicited since 12ha.i.To sum up all above mentionedresults, it is concluded that HR occurred in wheat-leaf rust incompatible combination (cv.LorvinlO x race 162) is a typical form of programmed cell death.
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