ENDOREDUPLICATION – A MECHANISM TO REGULATE DNA SYNTHESIS DURING SEED DEVELOPMENT AND GERMINATION
Rewers, M. and Sliwinska, E.
Department of Plant Genetics, Physiology and Biotechnology, UTP University of Science and Technology, Bydgoszcz, Poland
Contact: Elwira Sliwinska, email@example.com
Endoreduplication is an alternative form of the cell cycle in somatic tissues, in which repeated rounds (endocycles) of nuclear DNA replication occur without subsequent mitosis. Increase in DNA content by endoreduplication leads to endopolyploidy of some cells (somatic polyploidy; DNA content >4C). It affects cell growth and differentiation as well as transcriptional and translational activity. Endopolyploid cells occur in seed tissues undergoing differentiation and expansion, and in specific cell types, such as those of the endosperm, pericarp, suspensor, and cotyledons, making them polysomatic (i.e. containing cells of different ploidy). Our long-term studies, which cover over 30 species, revealed that the intensity of endoreduplication in developing and germinating seeds is species- and organ-specific and usually correlates negatively with genome size. For example, during development of seeds of Phaseolus vulgaris in the axis the only endopolyploid cells are those with an 8C DNA content and their proportion only slightly changes with seed maturation, while in the cotyledons the intensity of endoreduplication greatly increases: from 30% at 20 DAF (8C-32C) to 40% at 60 DAF (8C-128C). In contrast, in developing seeds of Medicago sativa in both the embryo axis and cotyledons, endopolyploidy up to 8C occurs and its intensity changes little. Generally, higher endopolyploidy occurs in haustorial and non-persistent cotyledons than in persistent ones. During germination it either does not change or it decreases in the cotyledons, while it increases in the axis. This increase occurs in different regions of the axis, depending on the type of seedling establishment; it is the highest in the transition zone of epigeal species and in the hypocotyl in hypogeal species. In conclusion, in polysomatic species (the majority of angiosperms) endoreduplication seems to be a common strategy during seed development to establish large storage cells in cotyledons, and during Phase II of germination, when the mitosis still does not occur, to drive axis elongation. However, there are also non-polysomatic species, e.g. Helianthus annuus, in which endoreduplication does not occur in any organ or developmental stage. Thus cell division may be also a mechanism of development of cotyledon storage tissues and be involved in the completion of germination.