Genome plasticity during the acquisition of embryogenic competence

Hypocotyl explants from carrot and other species experience concomitant seg regation events and differentiation of homeotic structures during the first 20 days of culture on 2,4-dichlorophenoxyacetic acid (2,4-D). In addition to these cyto-morphological changes, significant amounts of nuclear DNA are lost, the molecular details of which we investigate in this paper. We have developed a slot-blot analysis assay to study the DNA content of a series of carrot samples; besides the leaves, this survey ranged over different cu lture timepoints: hypocotyls, cell lines, and somatic embryo stages. We car ried on to study the relationship between this DNA loss and sequence comple xity modulation. Results from probing sequences that correspond to differen t degrees of complexity, such as medium repetitive and unique sequences as well as sequences belonging to both classes (ribosomal cistrons, ubiquitin, actin, and chalcone synthase), consistently manifested a reduction in DNA levels during the acquisition of embryogenic competence. In some cases, the cultured cells would contain only 10% of the gene copies observed in the r eference tissues. Modulation trends also showed that DNA levels of most seq uences recover at the torpedo-plantlet stage, which again correlates DNA mo dulation and the acquisition of embryogenic competence. These results sugge st that similar DNA variations may occur in plants in vivo during meiosis, possibly so that meiotic division may be properly completed.