Regulation of Assimilatory Sulfate Reduction by Cadmium in Zea mays L.

Abstract

Plants cultivated with Cd can produce large amounts of phytochelatins. Since these compounds contain much cysteine, these plants should have an increased rate of assimilatory sulfate reduction, the biosynthetic pathway leading to cysteine. To test this prediction, the effect of Cd on growth, sulfate assimilation in vivo and extractable activity of two enzymes of sulfate reduction, ATP-sulfurylase (EC 2.7.7.4) and adenosine 5'-phosphosulfate sulfotransferase were measured in maize (Zea mays L.) seedlings. For comparison, nitrate reductase activity was determined. In 9-day-old cultures, the increase in fresh and dry weight was significantly inhibited by 50 micromolar and more Cd in the roots and by 100 and 200 micromolar in the shoots. Seedlings cultivated with 50 micromolar Cd for 5 days incorporated more label from 35SO42-into higher molecular weight compounds than did controls, indicating that the predicted increase in the rate of assimilatory sulfate reduction took place. Consistent with this finding, an increased level of the extractable activity of both ATP-sulfurylase and adenosine 5'-phosphosulfate sulfotransferase was measured in the roots of these plants at 50 micromolar Cd and at higher concentrations. This effect was reversible after removal of Cd from the nutrient solution. In the leaves, a significant positive effect of Cd was detected at 5 micromolar for ATP-sulfurylase and at 5 and 20 micromolar for adenosine 5'-phosphosulfate sulfotransferase. At higher Cd concentrations, both enzyme activities were at levels below the control. Nitrate reductase (EC 1.6.6.1) activity decreased at 50 micromolar or more Cd in the roots and was similarly affected as ATP-sulfurylase activity in the primary leaves.In 1957 a cysteine-rich, Cd-binding protein was isolated from equine kidney (10) and subsequently called metallothionein. In higher plants the inducible Cd-binding compounds showed properties distinctly different from metallothioneins: in Ouchterlony two-dimensional immunodiffusion tests, no cross-reaction could be observed (7), and attempts to establish the primary structure by Edman degradation failed (5). Similarities were a high cysteine content, A254:A280> 1, and comparable circular dichroism spectra (5), indicating the lack of aromatic residues and the possible binding of heavy metals by mercaptide complexes.

Cadmium-Induced Sulfate Uptake in Maize Roots

“…1). For lengthier treatments, whereas Cd led to a progressive chlorosis and apical necrosis in leaves, it did not cause any evident damage to the root system, which is different from that reported by Nussbaum et al (1988) in the same species, though at higher Cd 2ϩ concentrations.…”

“…3). Such an effect, which may seem in contrast with the higher sulfate uptake, could be explained as a consequence of the Cd-induced activation of the sulfate-reductive assimilation pathway (Nussbaum et al, 1988;Heiss et al, 1999).…”

Cysteine, γ-Glutamylcysteine, and Glutathione Levels in Maize Seedlings

“…3) suggests that GSH synthesis in vivo is reduced, which may be the consequence of preferential inhibition of GSH synthetase by Cd (7). This effect occurs despite the possibility of enhanced sulfur reduction in Cd-treated maize roots (15) and the formation of PCs that might chelate Cd (4,27).…”

“…Endosperms, scutella, and roots were homogenized with 4.0 mL of extractant, shoots with 8.0 mL. Homogenates were centrifuged at 13,000g in the cold for 15 …”

“…The lack of recovery of GSH to control levels is puzzling for several reasons. Presumably, GSH is translocated to the root (19), the Cd-treated roots have increased activities of the enzymes involved in sulfate assimilation and GSH synthesis (15,22), and PCs accumulate in excess of that required to chelate Cd (27).…”