Dedifferentiation and Differentiation of Cells in Crown Gall ofVicia Faba

Open Access
Publisher Website
Google Scholar
Abstract
The anatomy and cytology of the crown gall in Vicia faba L. have been studied during its development. The galls have been fixed at the following dates after inoculation with Agrobacterium tumefaciens: 2, 4, 6, 9, 16, 19, 23, 29, 35, 69, and 81 days. As controls normal stems as well as 23-day-old galls induced by indole-3-acetic acid have been examined. The first visible reaction of the cells to the bacterial inoculation is the inflation and increased stainability of the nuclei, and an increase in the amount of cytoplasm. After two days the first cells divide; these are diploid. During the development of the gall the diploid cells continue to divide, forming meristemlike structures. These small cells are also the first to differentiate into scalariform or reticulate tracheal cells, which later form the woody cores of the fully-developed crown galls. The next cells to divide are the tetraploid cells. These form a cortex-like layer around the dividing diploid cells. More highly polyploid—at least up to 32-ploid—cells are also seen at various mitotic stages. A proportion of the polyploid divisions exhibit diplochromosomes, quadruplochromosomes or even larger bundles of chromatids. The number of these bundles in one cell never seems to exceed the 2n chromosome number in Vida faba, i. e. 12. Such divisions obviously represent the first mitosis after one or several endomitotic cycles. The conclusion seems warranted that when, after endomitosis, a cell has begun to divide mitotically, it continues to do so without undergoing any further endomitoses. This is corroborated by the fact that even highly polyploid divisions with normal chromosome structure are observed. The very largest polyploid cells in the stem of Vicia faba do not divide in response to the crown gall stimulus, but begin to enlarge. Their nuclei grow greatly in size, obviously through endomitosis. In older galls (68 and 81 days) all other cell types except these giant cells are seen to differentiate into lignified structures. Mitotic irregularities are infrequent in crown gall. As a rule, they are the result of the difficulties which the diplochromosomes or larger chromosome bundles encounter in their separation In mitosis. In a number of cells, fragments—both centric and acentric—are found. Since structural changes in chromosomes also occur in normal cells of Vicia faba, it is questionable whether these fragments are referable to the crown gall effect. In the light of the evidence concerning the cytological effects of growth substances in experiments, the well-established ability of crown gall tissue to produce growth substances in excess seems to be sufficient explanation for all the cytological processes characteristic of this tumour.