HETEROCHROMATIN AND REPETITIVE DNA FREQUENCY VARIATION IN REGENERATEDPLANTS OF HELIANTHUS-ANNUUS L

Plant regeneration from cotyledons of seeds of a single progeny of a p ure line of Helianthus annuus was studied in respect of the nuclear DN A contents of control and regenerated plants. Control plants were divi ded into two groups: those developed from seeds at the periphery of th e inflorescence (showing a high basic 4C DNA content) and those from s eeds developed in the middle of the inflorescence (showing a low basic 4C DNA content). It was observed that plants from peripheral seeds ha ve a higher morphogenetic potential than those from central seeds. Cyt ophotometric analyses indicated that plants regenerated from cotyledon s of both peripheral and central seeds show the same basic 4C DNA amou nt, which is higher that that observed in vivo in peripheral seeds. Mo lecular analysis by slot blotting and hybridization with different DNA families showed that the difference in nuclear DNA content between pl ants from peripheral and central seeds in vivo are mainly related to d ifferences in the frequency of highly repeated, ''slow'' medium repeat ed (MR2), and ribosomal DNA families; by contrast, the increase in DNA amount in regenerated plants is mainly due to ''fast'' medium repeate d sequences (MR1). Moreover, the frequency of kinetically isolated ''u nique'' sequences was higher in peripheral seeds than in central ones and still higher in regenerated plants. Optical-density measurements o f interphase nuclei showed an increase of heterochromatin in regenerat ed plants, suggesting that, whatever DNA is amplified in these plants, it remains condensed and probably inactive.