DNA content, interphase AgNOR-area, number of (3)HrDNA hybridization signals and the methylation level in coding rDNA sequence in different organs of Lupinus luteus L

Nuclear DNA and AgNOR-area quantity have been measured cytophotometrically, the number of silver grains after in situ H-3 rDNA/DNA hybridization has been counted, and the level of rDNA methylation has been estimated in root meristems, differentiated root zones, hypocotyls, cotyledons, and leaves of 7-day-old seedlings of Lupinus luteus. DNA content increases by endoreplication, reaching the average DNA C-value of 4.2, in the meristem, 6.6C in hypocotyl, 15.9C in cotyledon, and 3.3C in leaf. The quantity of AgNOR-area, the highest in root meristem, does not follow DNA C-value. Number of grains after in situ H-3 rDNA/DNA hybridization corresponds to DNA amount, but their distribution differs, showing in many cases perinucleolar localization in organs displaying low AgNOR-area quantity. Southern hybridization has not demonstrated detectable changes in level rDNA methylation. We suggest that the low level of rDNA expression in non-meristematic tissues could be related to rDNA inactivation caused by its translocation into nucleolus-associated heterochromatin.