ABSTRACT
Although plants and animals share many structural and functional features as both represent groups of higher eukaryotic organisms, their patterns of body formation and development are rather different. The crucial stage of animal sex differentiation occurs during gastrulation when primordial germ cells are formed and later migrate to the gonads. In higher animals, usually as a result of the presence of specific sex chromosomes, either male or female gonads are established which immediately begin to influence the resulting gender phenotype. This germline represents the only cells from which genomes are transmitted to the sexual progeny. However, no germinal line is set aside during the embryogenesis in plants. Some cell lines, the meristems, maintain their ability to divide (similar to stem cells in animals) and give rise to both vegetative parts of plants (roots, stem, branches, leaves) and later complex reproductive organs (flowers). The majority of plant species are hermaphrodites, forming one type of bisexual flower; the rest are mostly monoecious (possessing male and female flowers on the same individual) or dioecious (male and female flowers are formed on different individuals). Flowers originate from apical or axillary shoot meristem buds, when subject to appropriate internal and environmental factors. This implies that any change in the genome of meristematic cells that occurs during the life of the plant can be transmitted to the sexual progeny. Moreover, plant cells possess an extremely high regeneration capacity, totipotency, whereby any cell of the plant body, both somatic and generative, can in principle give rise to a new individual. This indicates that the changes occurring during plant development, which lead to a high degree of differentiation, must be reversible. On the other hand, plants—probably due to their inability of locomotive movement—have evolved a high tolerance to both genetic and epigenetic changes. These include even such gross alterations as haploidy, polyploidy, aneuploidy and chromosome aberrations which are generally not tolerated by higher animals.
