Abstract
Sulphur(S)-deficiency is emerging as a major problem for agricultural productivity. Cadmium (Cd) exerts its phytotoxicity against defence, growth and development. S-rich compounds (glutathione, phytochelatins, etc.) limit the impacts of Cd-toxicity. We investigated what happens during S-deficiency and Cd exposure (dual stress) in mustard. Major findings were: S-deficiency increases the susceptibility of plants to Cd-generated oxidative damage and modulates the AsA-GSH antioxidant cycle; SOD is not the first line of defence against metal stress and S-rich metabolites play a prime role; S-deprived plants are more prone to Cd and oxidative stress; and great loss is incurred to defence modules and growth under dual stress, restricting the efficiency of phytoremediation.
Soils in many parts of the world are contaminated with heavy metals, leading to multiple, deleterious effects on plants and threats to world food production efficiency. Cadmium (Cd) is one such metal, being toxic at relatively low concentrations as it is readily absorbed and translocated in plants. Sulfur-rich compounds are critical to the impact of Cd toxicity, enabling plants to increase their cellular defence and/or sequester Cd into vacuoles mediated by phytochelatins (PCs). The influence of sulfur on Cd-induced stress was studied in the hyperaccumulator plant Indian mustard (
Brassica juncea)
using two sulfur concentrations (+S, 300 µM
S
O
4
2
−
and S-deficient −S,
30
μ
M
S
O
4
2
−
) with and without the addition of Cd (100 µM CdCl
2
) at two different time intervals (7 and 14 days after treatment). Compared with control plants (+S/−Cd), levels of oxidative stress were higher in S-deficient (−S/−Cd) plants, and greatest in S-deficient Cd-treated (−S/+Cd) plants. However, additional S (+S/+Cd) helped plants cope with oxidative stress. Superoxide dismutase emerged as a key player against Cd stress under both −S and +S conditions. The activity of ascorbate peroxidase, glutathione reductase and catalase declined in Cd-treated and S-deficient plants, but was up-regulated in the presence of sulfur. Sulfur deficiency mediated a decrease in ascorbate and glutathione (GSH) content but changes in ascorbate (reduced : oxidized) and GSH (reduced : oxidized) ratios were alleviated by sulfur. Our data clearly indicate that a sulfur pool is needed for synthesis of GSH, non-protein thiols and PCs and is also important for growth. Sulfur-based defence mechanisms and the cellular antioxidant pathway, which are critical for tolerance and growth, collapsed as a result of a decline in the sulfur pool.