At high concentrations, reactive oxygen species (ROS), trigger genetically programmed cell suicide. However, they also have an important function as secondary messengers in signal transduction cascades, which regulate many physiological and developmental processes in plants. Several forms of ROS are present in plants, including the superoxide radical (O2–), singlet oxygen (1O2), hydroxyl radical (OH*) and hydrogen peroxide (H2O2. H2O2 has weak toxicity compared with other ROS species but it can trigger highly reactive hydroxyl radicals, and the relatively longer life span of H2O2 makes it harmful to organelles.
Plants have a complex antioxidant system for maintaining the homeostasis of ROS. In general, this system can be divided into enzymatic and non-enzymatic ROS-scavenging mechanisms. Three types of antioxidative enzymes, superoxide dismutase (SOD), catalase (CAT) and peroxidase, play a major role in keeping superoxide radicals and H2O2 at steady-state levels. There are two classes of peroxidases in plants: class I and class III. Class III peroxidases are encoded by a large multi-gene family – altogether, 73 members have been identified in Arabidopsis thaliana. Although they play a role in antioxidants by removing ROS, they also produce ROS.
PAs are major end-products of the flavonoid biosynthesis pathway and mainly accumulate in the seed coat of Arabidopsis. PA-deficient mutants exhibit yellow or pale brown seed coat colour due to reduced flavonoid pigmentation and accumulation. New research published in Annals of Botany demonstrates that increased activity of class III peroxidases in the seed coat of PA-deficient mutant seeds may compensate for the loss of PAs and that this may be an adaptive mechanism substituting for the antioxidant function of PAs that is required during seed development and later in seed germination.
Class III peroxidases are activated in proanthocyanidin-deficient Arabidopsis thaliana seeds. (2013) Annals of Botany 111 (5): 839-847.
It has previously been shown that proanthocyanidins (PAs) in the seed coat of Arabidopsis thaliana have the ability to scavenge superoxide radicals (O2–). However, the physiological processess in PA-deficit seeds are not clear. It is hypothesized that there exist alternative ways in PA-deficient seeds to cope with oxidative stress. The content of hydrogen peroxide (H2O2) and its relevance to the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidases was investigated in both wild-type and PA-deficit mutant seeds. A biochemical staining approach was used to detect tissue localizations of peroxidase activities in PA-deficit mutant seeds. PA-deficient mutants possess significantly lower levels of H2O2 than the wild-type, despite their higher accumulation of superoxide radicals. Screening of the key antioxidant enzymes revealed that peroxidase activity was significantly over-activated in mutant seeds. This high peroxidase activity was mainly confined to the seed coat zone. Interestingly, neither ascorbate peroxidase nor glutathione peroxidase, just the guaiacol peroxidases (class III peroxidases), was specifically activated in the seed coat. However, no significant difference in peroxidase activity was observed in embryos of either mutants or the wild-type, although gene expressions of several candidate peroxidases were down-regulated in the embryos of PA-deficient seeds. The results suggest that enhanced class III peroxidase activity in the seed coat of PA-deficient mutants is an adaptive strategy for seed development and survival.