DNA amplification, chromatin variations, and polytene chromosomes in differentiating cells of common bread wheat in vitro and roots of regenerated plants in vivo

Abstract

Polytene chromosomes and giant nuclei were observed in differentiating cells of embryogenic calli and root tips of wheat plants (Triticum aestivum cv. Mustang) regenerated from long-term cell suspension cultures and selected cell lines after high-temperature stress. Many giant nuclei almost completely occupied the cellular space of the large parenchyma cells. The amount of DNA from the giant nuclei was as high as 174 C, equivalent to 3.03 × 104 pg of DNA, as revealed by Feulgen cytophotometry. Following cell differentiation, the giant nuclei that were endoaneupolyploid and polytene developed from interphase nuclei (aneuploid) and micronuclei. Polytene chromosomes from the B genome were tentatively identified by their banding patterns, as shown by the HCl–KOH–Giemsa banding method. Variations of polytene chromosomes were cable-like, dispersed, heterochromatinizated, or granulated at the different developmental stages. The number of granular chromatin particles in the cells was 85 to more than 300 when disintegration of the giant nuclei occurred. Some were exclusively heterochromatic, and many others consisted of both euchromatin and heterochromatin. The amount of DNA for these granular chromatin particles was estimated from 0.17 to 5.54 pg. Many minutes and double minutes existed in various stages of dividing and differentiating cells and accompanied the generation and disintegration of the giant nuclei. The DNA sizes of these minutes were estimated as 2.56 × 105 – 1.05 × 107 bp. These results suggested that DNA amplification, polytene chromosomes, minutes, and double minutes were induced in polyploid species with large chromosomes and maintained in embryogenic calli in vitro and apical segments of root tips of regenerated plants in vivo.Key words: giant nucleus, endoaneupolyploidy, polyteny, minutes and double minutes, cell differentiation, Triticum aestivum.