LINDELL BEACH, B.C. – Scientists have discovered that plants protect themselves against stem cell DNA damage brought on by environmental stress by causing the cell to self-destruct rather than passing on the defective DNA.Stem cells at the tips of a plant’s roots and shoots are central to the production of plant tissue.Researchers Nick Fulcher and Robert Sablowski of the Norwich Biosciences Institutes’ John Innes Centre in the United Kingdom made the discovery while experimenting on arabidopsis, or rock cress, to find out if plant stem cells contain mechanisms to protect themselves from DNA damage.”Cell death is a well characterized response to DNA damage in animal stem cells,” Fulcher said.”However, plants were thought to lack this mechanism mainly due to the absence of key genetic cell death activators.”Their experiments showed otherwise. Fulcher said investigations into DNA damage in corn as early as the 1960s suggested a unique system to protect genomic integrity in the root tip.Stem cells in the root surround a group of slowly dividing cells he called the quiescent centre, and it was thought that these cells would repair DNA lesions more efficiently than surrounding meristem cells, which are undifferentiated cells found in areas of the plant where growth takes place.Because of this perceived enhanced tolerance to environmental stress, it was thought stem cells could activate cell division when DNA was damaged and create a new population of stem cells while excluding those that were damaged.In the experiments, arabidopsis seedlings were treated with chemicals to induce double stranded DNA breaks.However, instead of seeing any repair to the lesions, “we observed that stem cells appeared to selectively undergo cell death whereas surrounding meristem cells survived. This result was therefore very surprising.”The research suggests cell death works to protect the stem cells from being genetically compromised.However, Fulcher acknowledged that some DNA damage likely escapes the mechanism and there is a continuous background level of genetic damage in all organisms that results from mistakes in DNA replication or normal cellular metabolic processes.Some small amounts of unrepaired or inaccurately repaired damage could be passed into future generations of the plant under normal conditions.Cell death is almost always triggered by an external or environmental influence, and Fulcher suggested that a low tolerance threshold likely exists in stem cells until rising levels of damage trigger the cell death mechanism.It explains how plants evolve over time through natural selection and adaptation to environmental conditions, he added.”For example, X-rays are expected to damage the DNA uniformly throughout the plant, but (instead,) they caused preferential death of the stem cells. We believe that the sensitivity of the stem cells is a consequence of the particular set of genes in these cells.”Research at other labs has found cell death responses to pollutants such as ozone, while other studies have discovered defence mechanisms to protect against fungal and herbivorous pathogens to restrict the spread of infection.However, these responses are in leaf tissue and not stem cells.”We are currently running experiments using heavy metals such as aluminum and cadmium to try and place stem cell death into a naturally occurring environmental context,” Fulcher said.”Heavy metal toxicity in soil is known to induce DNA damage in plants and affect growth, which limits agricultural productivity around the world. It is possible that the cell death response carries a short-term growth penalty but has nevertheless been selected in plant evolution because of a long-term survival advantage. If this is the case, the response might actually be disadvantageous for crop growth.”He said it is possible that mutant plants lacking the ability to perceive DNA damage and activate cell death could grow better in extreme environmental conditions.Researchers have identified two such mutant plants after growing them with genotoxic chemicals, one of which grew better on aluminum containing growth media compared with wild type plants.Future research will study mutant plants that lack the cell death phenomenon.The aim is to investigate their levels of tolerance to environmental extremes and show the conditions in which they can successfully grow compared to wild plants that would not cope in similar conditions.Drought, high salinity and the accumulation of hazardous chemicals in soils are side-effects of a changing climate. An understanding of how plants cope with these stresses could help the development of future agricultural crops.
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