Analysis of chromatin and DNA during chromosome endoreduplication in the endosperm ofTriticum durum Desf.

Summary Chromatin structure was studied in nuclei of the endosperm of durum wheat (Triticum durum Desf., cv. Creso), where a large number of cells undergo chromosome endoreduplication during caryopsis development. Optical density profiles of interphase nuclei at different ploidy levels after Feulgen staining were determined cytophotometrically. It was observed that, within each development stage, polyploid nuclei (6–12C and 12–24C) show more condensed chromatin than euploid nuclei (3–6C): this should indicate that endoreduplication is accompanied by some reduction of nuclear activity. Within the same ploidy level, 3–6C and 6–12C nuclei become increasingly condensed with development (except for the last stage), while 12-24C nuclei are identical at all stages. DNA methylation at different stages of caryopsis development was then analyzed in genomic DNA, highly repeated sequences and ribosomal DNA, by digestion with cytosine-methylation-sensitive restriction enzymes. We observed that (i), depending on the enzyme, DNA from caryopses may show higher mean length than DNA from shoot apices and variations occur during endosperm development; (ii) highly repeated DNA sequences also show some variation in base methylation between apices and endosperms and among endosperm development stages, even though to a lesser extent than genomic DNA; (iii) rDNA shows variations only between endosperm and apices while no variation was observed among endosperm development stages in relation to chromosome endoreduplication. Our data may be explained by assuming the occurrence, during endosperm development, of processes of chromatin condensation possibly involved in silencing the activity of extra copies of DNA resulting from chromosome endoreduplication. At least in part, DNA methylation is involved in the process of chromatin condensation. rDNA shows no variation during endosperm development: this suggests that rDNA copies are actively transcribed in both triploid and endoreduplicated nuclei.