PJB-2019-379
Gamma irradiation on nuleolar activity in root tip cells of tetraploid Triticum turgidum ssp. durum L.
Dr Eben Von Well
Abstract
The effect of gamma irradiation of kernels on nucleolar activity in root tip cells of tetraploid wheat T. turgidum ssp. durum L. cv. Orania (AABB) was investigated. The number of nucleoli present in nuclei in the different irradiation dosages were also used as an indicator of the cells entering mitosis. Five roots were collected every two and a half hours from 17.5 - 47.5 hours after imbibition and fixed in La Cour’s 2BD, stained with Feulgen and counter stained with Fast Green. The number of cells with a different number of nucleoli, the mean number of nucleoli in 500 interphase to early prophase cells, the mitotic index as well as the number of micronuclei containing nucleoli were scored. Nucleolar activity was highly significantly different for most treatment comparisons. Untreated material produced a maximum of four nucleoli formed by the nucleolar organizing regions on chromosomes 1B and 6B. In irradiated material an additional two nucleoli were noted, produced by reactivation of the dormant nucleolar organizing region on chromosome 1A. A drop in the number of nucleoli was detected at the higher dosages, indicating a decline in rRNA synthesis in these cells. The onset of mitosis was highly significantly retarded in all the dosages in comparison to the control as indicated by the dosage by time interaction on the total number of nucleoli and the mitotic index. This may be due to a checkpoint in the G2 phase for the repairing of damaged DNA before the cells enter mitosis. The appearance of a nucleolus in a micronucleus indicates that all the proteins that are needed for nucleolus formation, are situated in the NOR sites. A total number of five micronuclei containing a nucleolus indicates that NOR sites can be activated in micronuclei in the absence of suppressor genes.
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