Hormesis in Plants: The Role of Oxidative Stress, Auxins and Photosynthesis in Corn Treated with Cd or Pb

Abstract

Hormesis, which describes the stimulatory effect of low doses of toxic substances on growth, is a well-known phenomenon in the plant and animal kingdoms. However, the mechanisms that are involved in this phenomenon are still poorly understood. We performed preliminary studies on corn coleoptile sections, which showed a positive correlation between the stimulation of growth by Cd or Pb and an increase in the auxin and H 2 O 2 content in the coleoptile sections. Subsequently, we grew corn seedlings in hydroponic culture and tested a wide range of Cd or Pb concentrations in order to determine hormetic growth stimulation. In these seedlings the gas exchange and the chlorophyll a fluorescence, as well as the content of chlorophyll, flavonol, auxin and hydrogen peroxide, were measured. We found that during the hormetic stimulation of growth, the response of the photosynthetic apparatus to Cd and Pb differed significantly. While the application of Cd mostly caused a decrease in various photosynthetic parameters, the application of Pb stimulated some of them. Nevertheless, we discovered that the common features of the hormetic stimulation of shoot growth by heavy metals are an increase in the auxin and flavonol content and the maintenance of hydrogen peroxide at the same level as the control plants.

Rapid Hormetic Responses of Photosystem II Photochemistry to Cadmium Exposure

“…Dose-response studies are proposing hormesis as a central dose-response phenomenon for a variety of stressors [8,[35][36][37][38][39][40]. Hormesis is a widespread phenomenon usual in nature, independent of the kind of stressor, the physiological process or the organism it occurs [8,[35][36][37][38].…”

“…Dose-response studies are proposing hormesis as a central dose-response phenomenon for a variety of stressors [8,[35][36][37][38][39][40]. Hormesis is a widespread phenomenon usual in nature, independent of the kind of stressor, the physiological process or the organism it occurs [8,[35][36][37][38]. It is described as the stimulatory effect of low doses or short exposure times, described by a biphasic dose response with a low dose stimulation and a high dose inhibition [41][42][43] representing an ''over-compensation'' response to a disruption in homeostasis [44].…”

“…Since any substantial effect on plant growth after exposure to Cd can be detected only if photosynthesis is disturbed [47], a hormetic response to Cd is frequently coupled with changes in the mechanisms of photosynthesis or the photosynthetic apparatus [8]. The toxic effects of Cd provoke oxidative stress in plants and are related with the production of ROS [4,38,48,49] that can be regulated by NPQ in such a way so that plants can cope with the stress [21,24,28,29]. Therefore, it was hypothesized that after exposure of clary sage to Cd, the photosynthetic machinery could display an hormetic response to Cd, if plant tolerance mechanisms were activated and photosystem II photochemistry could be regulated by the photoprotective mechanism of NPQ in a such way that PSII functionality to be enhanced without any chloroplast ultrastructure perturbations.…”

Response of Corchorus olitorius Leafy Vegetable to Cadmium in the Soil

“…Cadmium (Cd) is a divalent cation and a metallic trace element that is non-essential to living organisms [ 6 , 7 ], but can be easily absorbed by roots and transported to the aboveground parts of plants [ 8 ]. Cd is a non-biodegradable and persistent heavy metal that can either occur naturally through volcanic eruption or deposited into the soil as a result of many anthropogenic activities [ 6 ].…”

“…Hormesis or hormetic effect is a bi-phasic concentration-response phenomenon that is characterized by a stimulatory effect of low doses and inhibitory effect of high doses of toxic substances, such as heavy metals [ 6 , 8 , 11 ]. This phenomenon is represented as a U- or J-shaped or inverted U- or J-shaped dose-response curves depending on the variable under evaluation [ 6 , 11 , 12 ].…”