Using Microdensitometry and Image Analysis to Study Nuclear Cycle Events in a Plant Pathogenic Fungus

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Abstract Determining the length of the various stages of the nuclear cycle and correlating nuclear cycle events with particular aspects of cell morphology generally require the use of autoradiography. The most useful label is tritiated thymidine, because it is incorporated only into DNA. Unfortunately fungi, unlike most other organisms, do not take incorporate thymidine from medium. In the past, microspectrophotometry has been used to study fungal nuclei, but this technique measures nuclei one at a time. Recent advances in image capture and analysis software have made it possible to use the technique of photomicrodensitometry to quantify DNA in nuclei stained with fluorochromes that bind quantitatively to DNA. We are using this method to study nuclear cycle events in cells of the pathogenic fungus Ustilago maydis. The basidiomycete fungus Ustilago maydis causes corn smut disease. This fungus is dimorphic—in addition to growing by filaments in corn plants, it can exist saprobically in a yeast-like budding stage.