Arsenic Transport, Metabolism and Toxicity in Plants

Arsenic is a toxic metalloid present in large areas in some parts of world including densely populated areas of Bangladesh and West Bengal, India. Being a carcinogenic metalloid, it affects the health of millions of people of affected areas through drinking water and food. Nonetheless, the spread of arsenic contamination reaches to non-affected areas also. Rice is the major crop of the affected areas and is thus the most important carrier of arsenic in grains and in various rice based products throughout the world. Arsenic exists in various inorganic and organic forms with arsenite and arsenate being the major inorganic forms of concern. This global issue has got attraction of a number of studies to understand the details of arsenic uptake, transport, metabolism and toxicity in plants. The transporters responsible for the uptake and root-to-shoot transport of inorganic arsenic have been identified. In addition, transporters responsible for sequestration of arsenic in vacuoles are also discovered. Inside the plants, arsenic induces the production of reactive oxygen species and causes oxidative stress leading to damage to proteins, carbohydrates, lipids and DNA and ultimately cell death. Various antioxidant enzymes and molecules are increased to counteract the oxidative stress. In addition, specific arsenic-binding ligands like phytochelatins are synthesized to chelate and sequester arsenic in vacuoles. This is achieved througth concerted modulation of synthesis and degration of thiols. A number of molecular changes including altered expression of microRNAs and transcription factors take place. The available knowledge about the arsenic metabolism and its toxicity paves the way to tackle the issue. This update discusses not only the present knowledge on this issue but also the lacunae, which need to be filled.