Mechanisms of arsenic hyperaccumulation by plants

Abstract

Arsenic (As) which is a heavy metal is ubiquitously present in soil as well as in water. As has been ranked as a potent carcinogen and is found to be very harmful to all the living beings ranging from bacteria to plant to animals as well as humans. All the organisms possess various defense mechanisms to combat such types of stresses. However, if it remains detoxified in plants, it may lead to oxidative stress, misfolded proteins thus disrupting the functioning of the proteins, mutations in the genetic material which ultimately results in the inhibition of the growth, disruption of photosynthe-sis, and loss in crop yield. Plants are sessile creatures of nature so they are more vulnerable to any type of stress. However, they possess a very strong defense system that fights against these stresses. There are various mechanisms responsible for defense against As stress such as phytochelatin (PC)-dependent defense in which As forms complex with PCs and these complexes are sequestered inside the vacuole. The antioxidant defense system is a very basic and strong player in this defense system. One of the interesting parts of this system is the hyperaccumulation of As. However, hyperaccumulation is not common to all the plants. This is a trait of some specific plant species which had gained a very high capacity of accumulation of As in the aboveground part without suffering phytotoxic effects during evolution. Hyperaccumulator plants differ from normal or non-accumulator plants in various ways. Among them, very fast translocation of As from root to aboveground part, much higher detoxification ability, and higher sequestration capacity of As in aboveground part are the main mechanism which differentiates hyperaccumulator plants to non-accumulator plants. In particular, a determinant role in driving the uptake, translocation to leaves, and, finally, sequestration in vacuoles is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of arsenical compound resistance 3 (ACR3). In this chapter, we will discuss mainly the As toxicity in the plants along with the mechanisms that are involved in hyperaccumulator plants, detoxification of As in plants, as well as the tolerance of As in plants.