Disintegration of the prolamellar body structure at high concentrations of Hg2+

The effects of high concentrations of Hg2+ (10(-2) M and 10(-3) M) were investigated on the ultrastructure and on the light-induced transformation of isolated prolamellar bodies (PLBs) of dark-grown wheat leaves. Our earlier work on wheat leaf homogenates (Solymosi et al. [2004], Plant Biology 6, 358-368) showed that, depending on the concentration, Hg2+ reacts with protochlorophyllide, NADPH and the NADPH:protochlorophyllide oxidoreductase (POR, EC 1.3.1.33) enzyme and induces disaggregation of the macrodomain structure of this latter. Spectroscopic analyses confirmed that 15 min incubation with 10(-2) M Hg2+ at 4 degrees C completely inhibited the activity of POR also in isolated PLBs. Ultrastructural investigations revealed the loosening of the PLB structure in the Hg2+-treated sample, i.e., intensive vesicle formation on the surface of the PLB membranes. The hexagonal geometry of the inner lattice was not disturbed, however, the unit cell size significantly increased. The disruption of the PLB membranes upon irradiation was studied after 40 min incubation with 10(-3) M Hg2+ at 4 degrees C and a subsequent irradiation for 40 min at 20 degrees C. Equinnolar concentrations (10(-3) M) of NADPH and Hg2+ were added to the samples 10 min prior or after the addition of Hg2+. Our results suggest that Hg2+ accelerates the disruption of the PLB membranes and that NADPH can only partially prevent this process. These membrane transformations were similar to those observed in the initial steps of the Shibata shift of control samples.