No evidence for cerium dioxide nanoparticle translocation in maize plants.

The rapidly increasing production of engineered nanoparticles has raised questions regarding their environmental impact and their mobility to overcome biological important barriers. Nanoparticles were found to cross different mammalian barriers, which is summarized under the term translocation. The present work investigates the uptake and translocation of cerium dioxide nanoparticles into maize plants as one of the major agricultural crops. Nanoparticles were exposed either as aerosol or as suspension. Our study demonstrates that 50 µg of cerium/g of leaves was either adsorbed or incorporated into maize leaves. This amount could not be removed by a washing step and did not depend on closed or open stomata investigated under dark and light exposure conditions. However, no translocation into newly grown leaves was found when cultivating the maize plants after airborne particle exposure. The use of inductively coupled mass spectrometer allowed detection limits of less than 1 ng of cerium/g of leaf. Exposure of plants to well-characterized nanoparticle suspensions in the irrigation water resulted also in no detectable translocation. These findings may indicate that the biological barriers of plants are more resistant against nanoparticle translocation than mammalian barriers.

Chlorophyll breakdown in senescent leaves identification of the biochemical lesion in a stay-green genotype of Festuca pratensis Huds.

Chlorophyll breakdown in senescent leaves proceeds in essentially three steps: dephytylation by the action of chlorophyllase; conversion of chlorophyllide to phaeophorbide by Mg-dechelatase; and oxygenolytic cleavage of the chlorine-macrocycle by a newly discovered dioxygenase. The metabolic lesion responsible for high retention of chlorophyll during foliar senescence in a mutant genotype of meadow fescue (Festuca pratensis Huds.) was located in the third step of the breakdown pathway. Senescent leaves of both the normally yellowing reference genotype, c.v Rossa, and the non-yellowing mutant Bf993 were shown to be competent with regard to chlorophyllase and Mg-dechelatase. On the other hand, thylakoids isolated from senescent leaves of cv. Rossa were able to carry out oxygenolysis of phaeophorbide into a colourless fluorescent catabolite in vitro, whereas Bf993 thylakoids were deficient in this activity. It is concluded that the Sid locus, a mutant allele of which is responsible for the stay-green character, encodes or regulates the gene for, phaeophorbide a dioxygenase.

AtOSA1, a member of the Abc1-like family, as a new factor in cadmium and oxidative stress response.

The analysis of gene expression in Arabidopsis (Arabidopsis thaliana) using cDNA microarrays and reverse transcription-polymerase chain reaction showed that AtOSA1 (A. thaliana oxidative stress-related Abc1-like protein) transcript levels are influenced by Cd2+ treatment. The comparison of protein sequences revealed that AtOSA1 belongs to the family of Abc1 proteins. Up to now, Abc1-like proteins have been identified in prokaryotes and in the mitochondria of eukaryotes. AtOSA1 is the first member of this family to be localized in the chloroplasts. However, despite sharing homology to the mitochondrial ABC1 of Saccharomyces cerevisiae, AtOSA1 was not able to complement yeast strains deleted in the endogenous ABC1 gene, thereby suggesting different function between AtOSA1 and the yeast ABC1. The atosa1-1 and atosa1-2 T-DNA insertion mutants were more affected than wild-type plants by Cd2+ and revealed an increased sensitivity toward oxidative stress (hydrogen peroxide) and high light. The mutants exhibited higher superoxide dismutase activities and differences in the expression of genes involved in the antioxidant pathway. In addition to the conserved Abc1 region in the AtOSA1 protein sequence, putative kinase domains were found. Protein kinase assays in gelo using myelin basic protein as a kinase substrate revealed that chloroplast envelope membrane fractions from the AtOSA1 mutant lacked a 70-kD phosphorylated protein compared to the wild type. Our data suggest that the chloroplast AtOSA1 protein is a new factor playing a role in the balance of oxidative stress.