Reactive oxygen species: Connecting eustress, hormesis, and allostasis in plants

Hormesis, priming, and allostasis, three fields related to stress tolerance and toxicity, have much to say about reactive oxygen species (ROS), oxidative stress, and antioxidants. Among them, hormesis is a phenomenon that received much attention in the last two decades, and is about how toxic substances and stressful conditions often have a biphasic dose-response curve, showing that these substances and conditions have a beneficial effect at low doses. Another field, priming studies, has also been popular lately. H2O2 priming studies show that H2O2, a ROS, confers cross-tolerance to plants when applied at low doses, a classic example of eustress, as argued here. Last of the three is allostasis. A concept arising from human and mammalian studies, it shows that the stress response of the organism is beneficial in the short term and damaging in the long term. The existence of allostasis in plants has not been explored so far. In this article, it is proposed that hormesis, eustress (and H2O2 priming as eustress), and allostasis are all connected via ROS and oxidative stress in plants. These connections are elaborated and a workable approach for antioxidant activity is presented, with allostasis complementing hormesis in the temporal dimension. This approach can be used to explain the observed antioxidant activity decrease under prolonged or heavy stress in some studies. Evidence for the debated existence of eustress is also presented, and it is argued as an example of allostasis.