Computer-Assisted Identification of Protoplasts Responsible for Rare Division Events Reveals Guard-Cell Totipotency

With the use of a computer-controlled microscope system to assist in the positioning and rapid relocation of large numbers of cultured cells, we were able to identify those protoplasts with the capacity to divide within a highly recalcitrant culture in which only a tiny fraction of the total population proceeds to produce viable microcalli. In the cultures used, comprising Beta vulgaris L. (sugar beet) leaf protoplasts, it was confirmed that these cells can be recognized solely on the basis of morphological characters. Therefore, a direct link exists between competence for cell division in vitro and cell type. Divergent callus morphologies and totipotent potential could also be ascribed to distinct protoplast types and hence to cells with a specific origin. The progenitors of the totipotent protoplasts in these cultures have been confirmed as being stomatal guard cells. Consequently, in plants even the most highly adapted living cells clearly retain and can reactivate all of the functional genetic information necessary to recreate the whole organism; an extreme degree of cytodifferentiation is, therefore, no hindrance to expressing totipotent potential. In addition to the considerable practical value of these findings, their implications concerning our understanding of both the control of gene expression and plant cell differentiation and its reversibility are of fundamental significance.