Cellular Totipotency

Abstract

All multicellular organisms, including higher plants and animals, are finally derived from a single-celled zygote through successive mitotic divisions and differentiation. In animals most of the cells (other than stem cells), as they differentiate into specific cell types, lose the potentiality to reconstitute a new organism. However, all living plant cells retain the potential to revert back to the meristematic state and form new plants on exposure to favorable conditions, irrespective of their specialization and ploidy level (haploid, diploid or triploid). Plant tissue culture has considerably enlarged the scope of regeneration of plants from highly differentiated and structurally and functionally specialized cells of leaves, roots, stem, floral parts, and endosperm. In vitro regeneration of plants is also possible from isolated gametic cells (microspores, unfertilized egg or synergids). The potentiality of differentiated and specialized cells to form complete plants like the zygote is referred to as Cellular Totipotency. The term was probably coined by T.H. Morgan (1901). However, it was the famous German plant physiologist, Göttlieb Haberlandt, who in his famous address to the German Academy in 1902 introduced the concept of cellular totipotency and suggested that the terminally differentiated plant cells, as long as they contain the entire complement of chromosomes, should be capable of regenerating whole plants. He made a novel approach to prove his hypothesis, in which single cells were isolated from highly differentiated tissues of the plant body and cultured in nutrient medium. His idea attracted the attention of many scientists to pursue this line of investigation, and in 1939 White reported reproducible differentiation of shoot buds in tissue cultures of tobacco. In 1965, Vasil and Hildebrandt, achieved regeneration of full plants from isolated single cells of tobacco, and in 1975 it became possible to regenerate complete plants from mesophyll protoplasts of tobacco (Takebe et al. 1971). The totipotency of plant cells is now well established. For long the main approach to induce regeneration from cultured tissues has been to manipulate the culture medium. However, during the past two decades tremendous success has been achieved in identification of genes involved in the differentiation of shoots and roots in Arabidopsis thaliana. Over expression of these genes transgenically enhanced the regeneration response and induced regeneration in hitherto recalcitrant species.