Kinetin as trigger for mitosis in mature endomitotic plant cells

Abstract

Pea root callus tissue cultivated for many months on a nutrient medium containing yeast extract and the auxin, 2,4-dichlorophenoxyacetic acid, is predominantly composed of dividing cells of the tetraploid chromosome number even though the initial tissues cultured from the root were known to consist in part of cells of the diploid chromosome number. In experiments designed to determine the basis for what appeared to be cell selection by the nutrient medium, 1-mm thick segments of 60 hr germinating seedling roots of the garden pea, Pisum sativum, excised 10–11 mm behind the root tip, were cultivated on a synthetic nutrient medium in which the yeast extract was replaced by a mixture of vitamins, amino acids, amides and urea. On such a medium, callus tissues developed, but only diploid cell mitoses were observed. Upon the addition of 1 ppm kinetin (6-furfurylaminopurine) to the synthetic medium, the number of cells in mitosis approximately doubled and a very high percentage of dividing cells possessed the tetraploid chromosome number. Lower concentrations of kinetin were proportionately less effective in stimulating mitosis and in bringing tetraploid cells into mitosis. From these studies it has been concluded that kinetin specifically triggers mitosis in mature pea root tissue cells which have undergone DNA- and chromosome-doubling by endomitotic reduplication before kinetin treatment. This conclusion is based on the following lines of evidence: ( a) After 2 days of kinetin treatment, the tetraploid cells which enter mitosis showed diplochromosomes, indicating that chromosome doubling had been endomitotic; ( b) Tetraploid mitoses were largely localized in mature cortical cells of the root while most diploid mitoses occurred in tissues of the central cylinder; ( c) One-mm root tips cultured for up to 7 days in 1 ppm kinetin showed no tetraploid mitoses, which is expected since few or no endomitotically doubled cells are likely to occur in the apical meristem of the root. Certain experiments in which adenine sulfate or adenosine was fed the mature pea root segments together with kinetin suggest that an interaction between adenine and kinetin does occur in which adenine reverses in part the effect of kinetin on induction of mitoses in mature endomitotic cells.