Figure 1.

Growth and microtubule distribution in an early germling. (A) Time-lapse images (Fig 1.mov) of a binucleate GFP-α-tubulin–expressing germling growing and going through mitosis. Images are projections of Z-axis stacks acquired using a wide-field microscope. Note that microtubules are less brightly fluorescent in young germlings than in hyphal tip cells, presumably because the GFP has had longer to mature in the older hyphal tip cells. The time, in seconds, after the start of observation is indicated in the lower left corner of each panel. At the 4322-s time point, the germling is entering mitosis. Two small spindles have formed and only a few very faint cytoplasmic microtubule fragments remain (arrows). At the 4502-s time point, all cytoplasmic microtubules have disassembled. Cytoplasmic microtubules reassemble after mitosis (5582-s time point). (B) Profile of tip growth. Growth of the germling tip (in micrometers) is plotted with respect to time (in seconds). Each open diamond represents a single time point. The slope (growth rate) over two intervals was determined by regression analysis and is shown with black lines. Thus, for the interval from 1801 to 4502 s, the growth rate was 0.015 μm/min, and for the interval from 8180 to 14147 s, the growth rate was 0.194 μm/min. This germling went through mitosis during the observation period (gray region). The spindle length is shown with filled circles. Note that for this germling tip growth rates varied greatly over time. However, growth rates were generally faster after nuclear division when there were four nuclei in the germling instead of two.