Nuclear Chromatin Structure in Relation to Cell Differentiation and Cell Activation in the Cap and Quiescent Centre ofZea maysL.

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Abstract
Barlow, P. W. 1985. Nuclear chromatin structure in relation to cell differentiation and cell activation in the cap and quiescent centre of Zea mays L —J. exp. Bot. 36: 1492–1503. Nuclear chromatin structure has been analysed by electron microscopy of thin sections of cells in four zones of the root cap—meristem, central, slime-secreting and outermost cells—and also in the quiescent centre of the root before and after decapping. The chromatin pattern has been related to the DNA and RNA synthetic activity of the nuclei. During cap cell maturation there was a progressive condensation of the chromatin and this was accompanied by some reduction of RNA synthesis. The degree of condensation was estimated from the area and number of pieces of electron dense chromatin which increased and decreased, respectively, during cap maturation. The volume fraction of condensed chromatin was also estimated but, in the cap, was not found to be a good indicator of nuclear activity. The outermost cells of the cap showed the greatest degree of chromatin condensation but were still active in RNA synthesis. Microdensitometry of their nuclear DNA contents gave an indication of loss of DNA in some of the nuclei. Decapping activated DNA and RNA synthesis in the quiescent centre and also stimulated a decondensation of chromatin: the number of condensed pieces of chromatin increased, and their size and volume fraction both decreased 4 h after decapping. The number of pores per unit length of nuclear envelope profile was also estimated. In the cap this number increased during cap maturation; in the activated quiescent centre the number remained constant except for a small rise 4 h after decapping