Genome plasticity during the acquisition of embryogenic competence

Abstract

Hypocotyl explants from carrot and other species experience concomitant segregation 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 culture timepoints: hypocotyls, cell lines, and somatic embryo stages. We carried on to study the relationship between this DNA loss and sequence complexity modulation. Results from probing sequences that correspond to different 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 reference tissues. Modulation trends also showed that DNA levels of most sequences recover at the torpedo-plantlet stage, which again correlates DNA modulation and the acquisition of embryogenic competence. These results suggest that similar DNA variations may occur in plants in vivo during meiosis, possibly so that meiotic division may be properly completed.

Recalcitrance in clonal propagation, in particular of conifers

“…Stress, as is discussed elsewhere in this paper, is of wide-ranging interest as a major factor in embryogenesis, as for example, in androgenesis. It has been postulated that in some cases cell de-differentiation and SE may have been initiated by a process that shows similarities to the early meiotic stages (Nuti Ronchi et al 1992;Geri et al 1999;Chen et al 2001). This somatic meiotic-like reduction also appears to be associated with stress (Geri et al 1999).…”

“…It has been postulated that in some cases cell de-differentiation and SE may have been initiated by a process that shows similarities to the early meiotic stages (Nuti Ronchi et al 1992;Geri et al 1999;Chen et al 2001). This somatic meiotic-like reduction also appears to be associated with stress (Geri et al 1999). Brassinosteroids play an important role in embryogenesis by acting as key signaling molecules that influence position-dependent cell fate (Clouse 2000), and brassinosteroid-induced genes play a major role in meiosis (Yin et al 2002).…”

Developmental and environmental variation in genomes

“…In some cases, the cultured cells contained only 10% of the gene copies observed in the reference tissues. However, DNA levels of most sequences were recovered at the torpedo-embryonic stage (Geri et al, 1999). It is unclear how the quantity of unique sequences can change.…”

Meiosis-like reduction during somatic embryogenesis of Arabidopsis thaliana

“…Although somatic chromosome reduction events were observed in somatic cells in those reports, the roles of these events were not discussed. NutiRonchi et al (1992a, b) and Geri et al (1999) found, in the in vitro culture system of carrot somatic embryogenesis, a reduction in DNA levels, and they observed somatic chromosome reductional grouping and somatic meiosis, which might be related to the reduction of DNA levels. So they advanced a hypothesis that the reduction of somatic chromosomes may be a prerequisite for somatic embryogenesis.…”