Nothing lives forever, and cells are no different. Plants and animals have evolved elegant mechanisms for eliminating and modifying cells - by cell suicide! Cell suicide is referred to as programmed cell death, because the suicide process is genetically programmed and follows a predictable set of events. Programmed cell death is essential during plant development and in response to environmental ques and pathogen attack. We are only beginning to understand how programmed cell death works in plants, but you can watch exclusive movies of suicidal plant cells right here on our web page!
Our lab studies plant tracheary element cells which undergo programmed cell death as a normal part of development. As a tracheary element differentiates, it constructs a rigid secondary cell wall and synthesizes a compliment of hydrolytic enzymes. After secondary cell wall synthesis is complete, the cell commits suicide. As you will see, the death of the cell is not a gradual process, but instead involves a rapid implosion of the large central vacuole and the immediate cessation of cytoplasmic streaming. Autolysis of the cell follows, and the cytoplasm is completely hydrolyzed, (CLICK HERE TO SEE AUTOLYSING CELL) leaving a hollow cell corpse composed of secondary cell wall. Tracheary element corpses join end-on-end to form water- conducting vessels in plants (the xylem). Water flows from the roots to leaves through the center of the cell corpses. Details of tracheary element programmed cell death can be found in Groover et al. (1997), "Programmed cell death of plant tracheary elements differentiating in vitro," (Protoplasma 196: 197-211).
The movies that follow illustrate the rapid cell death of tracheary elements. The movies were made by isolating mesophyll cells from leaves of zinnia seedlings, then inducing them to differentiate as tracheary elements in culture with plant growth regulators. The cells were placed in a microchamber with a cover slip bottom and observed using an inverted microscope. A video camera was mounted in the camera port of the microscope, and the differentiation of the cells was recorded on a timelapse VCR so that you could watch them too! In each movie, real time is recorded in the upper left-hand corner of the screen.
PLEASE NOTE... The first two movie files are large, and may not be directly accomodated by the temporary file space allocated to your browser. You can get around this problem by copying the movie file to your computer. In Netscape, this is done by right clicking the hypertext corresponding to the movie file you want to view, then selecting "save link as." Save the file, then open it by selecting "Open file" from the "File" pulldown menu.
Movie 1. The beginning of this movie shows a cell before secondary cell wall synthesis. You can see cytoplasmic streaming, indicating the cell is alive and happy. Then the movie jumps forward in time, and you can see that the cell has synthesized a secondary cell wall (stripes). The cell will suddenly stop cytoplasmic streaming, and at the same time you can see that there is a sudden implosion. The implosion is the large central vacuole collapsing. The cell is now, for all practical purposes, dead. We don't show you the last part of differentiation, the degradation of the cell contents by hydrolytic enzymes released from the vacuole, because it takes several hours. The final product of differentiation is a hollow cell corpse consisting of secondary cell wall.
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TO PLAY MOVIE 1 (Quicktime v. 4)
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TO PLAY MOVIE 1 (MPEG)
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TO PLAY MOVIE 2 (Quicktime v. 4)
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TO PLAY MOVIE 2 (MPEG)
