Responsiveness of yeast cells to auxin in relation to cell age

Using Urografin density gradient centrifugation, we fractionatedcells in a stationary culture of a homothallic strain, D6-0,of Saccharomyces cerevisiae and obtained two groups; a largecell fraction (L fraction) and a small cell fraction (S fraction).Indole-3-acetic acid (20 mg/liter) expands cells of the S fractionbut not those of the L fraction. Differences in the physiologicalnature between the two fractions were studied in relation totheir responsiveness to auxin. In general, L cells contain largeramounts of DNA, RNA and protein per cell than do S cells. Whencells of the two fractions were transferred to a nutrient medium,S fraction showed a longer lag period than L fraction beforebuds appeared on cells. Preceding cell division, cells of Sfraction enlarged significantly, while cells of L. fractionenlarged only slightly. In the 4 hr following inoculation, DNAcontent per culture doubled in the L fraction while the increasewas very small in the S fraction. Electron microscopy revealedthat cell walls were thicker in the S fraction than in the Lfraction. Development of mitochondria and other intracellularmembraneous structures was poorer in cells of the S fractionthan in those of the L fraction. The relationship between thecell generation cycle and responsiveness to auxin is discussed. (Received March 11, 1971; )