Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Heavy Metal Stress
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notype. Increased activities of antioxidative enzymes in Cd-treated plants and their
involvement in mechanisms of metal tolerance were also reported in pea (Pisum sativum
L. cv. Azad) (Dixit et al., 2001).
4. METAL TRAFFICKING
Transition metals such as Fe, Cu, Mn, and Zn are essential minerals for normal plant
growth and development, although they can be toxic when present at excess levels.
Thus, for healthy plant growth, a range of transition metals must be acquired from the
soil and distributed within the plant, but their concentrations must be carefully regulated
within different cells and organelles. Membrane transport systems are likely to
play a central role in these processes. The application of powerful genetic and molecular
techniques has now identified a range of gene families that are likely to be involved
in transition metal transport. These include heavy metal ATPases (HMAs), Natural
resistance associated macrophage proteins (Nramps), the cation diffusion facilitator
(CDF) family, the ZIP family, and cation antiporters (Table 1). An overview of the broad
range of potential transport systems likely involved in uptake,distribution, and homeostasis
of transition metals in plants has been published by Hall and Williams (2003).
4.1. Intracellular Sequestration
As excess metal ions accumulate within the plant cell, they have to be removed from the
cytosol. This is usually done by either efflux or compartmentalization. The main storage
organelle for toxic compounds in plant cells is the vacuole. The central vacuole is
the largest compartment of a mature cell, and may occupy more than 80% of the total cell
volume. In addition to the large central vacuole, several small vacuoles may exist within
a plant cell. In fact, two vacuolar proton pumps, an ATPase and Ppas, energize vacuolar
uptake of most solutes. Uptake can be catalyzed by either channels or transporters. For
most ions, more than one transporter/channel exists at the vacuolar membrane (Martinoia
et al., 2000) (Fig. 1).
Vacuolar compartmentalization or cell wall binding in leaves could play major
roles in hyperaccumulation of heavy metals. Cosio et al. (2004) investigated the role of
leaf cells in allocating metals in such hyperaccumulating plants as T. caerulescens
“Ganges” and A. halerii, both hyperaccumulators of Zn and Cd, and to a lower extent
in T. caerulescens “Prayon”, an accumulator of Cd. Their work demonstrated that
cellular uptake of Cd and Zn in leaf cells of hyperaccumulating plants involved a carriermediated
mechanism. Moreover, differences in metal uptake among T. caerulescens
“Ganges”, T. caerulescens “Prayon”, and A. halleri could not be explained by differences
in transport capacities of tonoplasts in leaves. Therefore, they concluded that
regulation mechanisms must be present before the plasma membrane of a leaf cell that
directed metals to their final location within a cell. Moreover, as the accumulation
capacity of protoplasts was modified after a plant’s pre-exposure to Cd, this indicated