Changes in DNA content of nuclei in rust uredospore germlings during the start of differentiation

Digital video microscopy in conjunction with the DNA-binding fluorescent probe 4′,6-diamidino-2-phenylindole was used to determine the relative DNA content of single nuclei in germ tubes of uredospores of the bean rust fungus (Uromyces phaseoli), a parasite of bean plants (Phaseolus vulgaris). The uredospores develop a series of infection structures in response to contact stimuli, and the first structure to appear, the appressorium, occupies the stomatal opening. This study was made to determine the time after the start of germination on an inductive surface that DNA replication and mitosis occur. It was found that the start of DNA replication detected by increased nuclear fluorescence power of some individual nuclei in the population was nearly coincident with mitosis between 2.0 and 2.5 h after the start of germination and that the appressorium was completed about 30 min later. The fluoresence power of nuclei was about the same before DNA replication began as at the end of mitosis; hence the germ tube nuclei were in the G1 phase of the cell cycle before mitosis.