Relationship between cadmium, glutathione and cadmium-binding peptides (phytochelatins) in leaves of intact tobacco seedlings

Abstract

Most studies of Cd 2+ effects on plants have focused on formation of thiol-rich peptides, often called phytochelatins, in cultured cells or roots exposed to moderate-to-high levels of this metal. Less attention has been paid to effects on leaf tissue, lower level exposure, and on glutathione, the most prominent cellular thiol. Here, glutathione and phytochelatin levels were monitored during a 7 day time-course in leaves of intact tobacco seedlings exposed to non-growth-inhibiting, 5, 10, 20, or 40 μM Cd 2+. Increased Cd 2+ levels in the growth medium resulted in increased accumulation of Cd 2+ in leaves. Glutathione levels decreased only slightly during the first hours of Cd 2+-exposure and, by day 2, were completely recovered to control levels. Data presented support the contention that in leaves of tobacco seedlings exposed to low levels of Cd 2+, complexation with cytosolic glutathione and antiport-mediated transport of Cd 2+ ion and its subsequent sequestration into the vacuole may be sufficient mechanisms for accomodation of intracellularly accumulated cadmium. Phytochelatins were synthesized in a sequential order with smaller peptides being synthesized prior to larger peptides. Furthermore, above 5 μM Cd 2+ exposure there was a positive correlation between the number of γ-glutamylcysteine repeat units and the endogenous Cd 2+ concentration. The stoichiometry of SH in phytochelatins to Cd 2+ was not constant, but varied with Cd 2+ concentration and duration of Cd 2+ exposure. This is in contrast to certain observations made with putative γ-glutamylcysteine dipeptidyl transferase and suggests that the regulation of phytochelatin synthesis in vivo may differ from that in vitro.