Ultrastructural distribution of DNA within plant meristematic cell nucleoli during activation and the subsequent inactivation by a cold stress

We have investigated the precise location of DNA within the meristematic ce ll nucleolus of Zea mays root cells and Pisum sativum cotyledonary buds, in the course of their activation and induced inactivation following a subseq uent treatment at low temperature. For this purpose, we combined the acetyl ation method, providing an excellent distinction between the various nucleo lar components, with the in situ terminal deoxynucleotidyl transferase-immu nogold technique, a highly sensitive method for detecting DNA at the ultras tructural level. In addition to the presence of DNA in the condensed chroma tin associated with the nucleolus, we demonstrated that a significant label was detected in the nucleolus of quiescent cells in both plant models. Evi dent labels were also found in the dense fibrillar component of actived nuc leoli. Whereas in inactivated nucleoli no significant label was observed wi thin the dense fibrillar component, an intense label was seen over the larg e heterogeneous fibrillar centres only during inactivation. The granular co mponent was never significantly labelled. These results appear to indicate that the DNA present in the dense fibrillar component of activated nucleoli withdraws from this structure during its inactivation and becomes incorpor ated in the large fibrillar centres. These observations suggest that in pla nt cells inactivation of rRNA genes is clearly accompanied by changes in th e conformation of ribosomal chromatin. (C) 1998 Academic Press.