Mechanism of the Inhibition of Cell Division of Plant Cells by Ionizing Radiations

Abstract

Some plants are harmed by relatively low doses of ionizing radiations due to the selective radiosensitivity of certain vital cellular processes. For instance, the growth of barley (Hordeum vulgare L.) roots of germinating seeds is completely inhibited after a dose of 400 R of y radiation (1). Following irradiation, the root elongates normally to a finite length (about I cm) and then ceases to lengthen. This cessation of root growth is due to the stoppage of cell division in the root tip. The mechanism of this selective inhibition of cell division is not known. Chromosomal damage could be one possibility. However, some investigations that I have conducted suggest that there are other possibilities to consider. This letter summarizes these studies, and it is hoped that it will be of help to those interested in the problem. A little over 20 years ago, I reported that excised barley root tips from seedlings could be made to adhere to a negatively charged glass surface in a solution of several chemicals under red light (about 660 nm), and that subsequently they could be induced to detach from the glass surface under far-red light (about 730 nm) (2). The attachment is due to the generation of a positive electric charge in the outer membrane of root tip cells, and the detachment is due to its nullification. The entire photoprocess of attachmentdetachment can be completed in I to 2 min. This rapid photoreversibility can be sustained for several cycles. The photoreceptor for this process is a chromoprotein involved in growth processes named (3) with photoreversible properties. Following red irradiation, its maximum absorbancy at 660 nm shifts to 730 nm, and following far-red irradiation it shifts back to 660 nm. A few years later, I discovered that this photoreversible attachment of barley root tips is very sensitive to low doses of X rays. Loss of photoreversibility can be detected within a few minutes following exposure to a few hundred R of X rays (4). Since phytochrome is involved in growth processes, it is possible that the system damaged by ionizing radiation resulting in inhibition to cell division and the system involved in photoreversible attachment of root tips controlled by phytochrome could be one and the same. Moreover, the early detection of a radiation effect on root tip attachment suggests that the lesion could be the initial site of the effect of X irradiation. Immediately the possibility arises that the photoreceptor, phytochrome, could be dam ged by ionizing radiations. Any damage to it or its fun tion could result in deleterious consequences to growth. I have looked for changes in phytochrome absorbancy following X irradiation and have found none, indicating that the phytochrome molecule is probably not damaged. The inhibition to root growth through the selective stoppage of cell division can be seen in other circumstances. Notably, a deficiency of boron, an essential trace element in plant nutrition, results in termination of cell division in the root tip with little or no effect on root elongation. Similarly, low concentrations of chlorine selectively inhibit root growth through stoppage of cell division in root tips. Can it be possible that ionizing radiations, phytochrome, boron, and chlorine all act on the same system that controls cell division?