Exclusion of metals from the symplasm: A possible mechanism of metal tolerance in higher plants

Abstract

Hypotheses concerning the tolerance of plants to metal stress have been grouped into two classes, internal mechanisms where metals enter the symplasm but are subsequently rendered harmless, and exclusion mechanisms where tolerance is based on the plant's ability to prevent entry of metals into the symplam. While most authors accept that metal tolerance can be achieved through exclusion, the role of exclusion in tolerance has been downplayed in the literature. This may be due to the lack of a clear definition of exclusion as a phenomenon, and to the popularity of new emerging hypotheses such as complexation by chelatins or metallothionein‐like proteins. Despite these problems, data which support the role of exclusion in metal tolerance nave been published. Possible mechanisms might include: 1) immobilization of metals at the cell wall, 2) complexation of metals by chelates exuded from plant roots, 3) formation of a redox barrier at the plasma membrane, and 4) formation of a pH barrier at the plasma membrane. Evidence concerning these hypotheses is reviewed.