Peroxiredoxin antioxidants in seed physiology

Peroxiredoxins are thiol-requiring antioxidants found in organisms ranging from bacteria to humans. They can be divided into two subgroups with either one or two conserved cysteine residues. In plants, 1-Cys peroxiredoxins have been identified in a number of grasses and cereals, and in the dicotyledonous species Arabidopsis thaliana. In contrast to other antioxidants, the 1-Cys peroxiredoxin genes are expressed solely in seeds, and only in the parts of the seeds surviving desiccation, i.e. the embryo and the aleurone layer. The expression pattern is characteristic of late embryogenesis-abundant genes. The PER1 protein of barley is present in high concentrations in the nucleus at the onset of desiccation. 1-Cys genes are expressed in a dormancy-related manner in mature seeds, in that transcript levels are high in imbibed dormant seeds, but disappear upon germination of their non-dormant counterparts. 1-Cys transcript levels can be up-regulated by ABA and osmotic stresses and suppressed by gibberellic acid. Two hypotheses have been put forward on the function of 1-Cys peroxiredoxins in seed physiology. First, these proteins might protect macromolecules of embryo and aleurone cells against damaging reactive oxygen species during seed desiccation and early imbibition. And second, seed peroxiredoxins might play a role in the maintenance of dormancy. These hypotheses are discussed, taking into account present knowledge of the biochemistry and molecular biology of peroxiredoxins.