Interactive effects of temperature and heavy metals (Cd, Pb) on the elongation growth in maize coleoptiles

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

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 H₂O₂ 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.

Artificial Light at Night Alleviates the Negative Effect of Pb on Freshwater Ecosystems

Artificial light at night (ALAN) is an increasing phenomenon worldwide that can cause a series of biological and ecological effects, yet little is known about its potential interaction with other stressors in aquatic ecosystems. Here, we tested whether the impact of lead (Pb) on litter decomposition was altered by ALAN exposure using an indoor microcosm experiment. The results showed that ALAN exposure alone significantly increased leaf litter decomposition, decreased the lignin content of leaf litter, and altered fungal community composition and structure. The decomposition rate was 51% higher in Pb with ALAN exposure treatments than in Pb without ALAN treatments, resulting in increased microbial biomass, β-glucosidase (β-G) activity, and the enhanced correlation between β-G and litter decomposition rate. These results indicate that the negative effect of Pb on leaf litter decomposition in aquatic ecosystems may be alleviated by ALAN. In addition, ALAN exposure also alters the correlation among fungi associated with leaf litter decomposition. In summary, this study expands our understanding of Pb toxicity on litter decomposition in freshwater ecosystems and highlights the importance of considering ALAN when assessing environmental metal pollutions.

Single and combined effects of Cd and Pb on the growth, medium pH, membrane potential and metal contents in maize (Zea mays L.) coleoptile segments

The mechanisms of the toxic effects of Cd and Pb on plant cell growth are still poorly understood. In particular, little is known about their interactive effects, which usually occur in the environment. Moreover, the data that do exist in the literature are controversial. This study describes experiments that were performed with maize (Zea mays) coleoptile segments, which is a classical model system for studies of plant cell elongation growth. Cadmium and lead, which were added at 0.1 mM, reduced the endogenous and IAA-induced elongation growth of maize coleoptile cells. When both metals were added together or in sequence, their effect on IAA-induced growth was more toxic. The medium pH changes, which were measured simultaneously with growth, indicated that while Pb stopped IAA-induced proton extrusion, Cd only partially diminished it. Although Cd was generally more accumulated than Pb in the maize coleoptile segments, when IAA was added together with Pb, it significantly increased the accumulation of the metal. The short-term electrophysiological experiments showed that the addition of Cd caused the depolarisation of the membrane potential (E_m), whereas Pb caused membrane hyperpolarisation. In the long-term electrophysiological experiments, it was found that the Cd-induced E_m changes are complex. In conclusion, these results suggest that the effects of Cd and Pb as well as their combination on the elongation growth of maize coleoptile cells and the accumulation of the metals result, among others, from different ionic mechanisms by which each metal change the membrane potential of the cells.

Effects of juglone and lawsone on oxidative stress in maize coleoptile cells treated with IAA

Naphthoquinones are secondary metabolites widely distributed in nature and produced by bacteria, fungi and higher plants. Their biological activity may result from induction of oxidative stress, caused by redox cycling or direct interaction with cellular macromolecules, in which quinones act as electrophiles. The redox homeostasis is known as one of factors involved in auxin-mediated plant growth regulation. To date, however, little is known about the crosstalk between reactive oxygen species (ROS) produced by quinones and the plant growth hormone auxin (IAA). In this study, redox cycling properties of two naphthoquinones, juglone (5-hydroxy-1,4-naphthoquinone) and lawsone (2-hydroxy-1,4-naphthoquinone), were compared in experiments performed on maize coleoptile segments incubated with or without the addition of IAA. It was found that lawsone was much more effective than juglone in increasing both H₂O₂ production and the activity of antioxidative enzymes (SOD, POX and CAT) in coleoptile cells, regardless of the presence of IAA. An increase in the activity of Cu/Zn-SOD isoenzymes induced by both naphthoquinones suggests that juglone- and lawsone-generated H₂O₂ was primarily produced in the cytosolic and cell wall spaces. The cell potential to neutralize hydrogen peroxide, determined by POX and CAT activity, pointed to activity of catalase as the main enzymatic mechanism responsible for degradation of H₂O₂ Therefore, we assumed that generation of H₂O₂, induced more efficiently by LW than JG, was the major factor accounting for differences in the toxicity of naphthoquinones in maize coleoptiles. The role of auxin in the process appeared negligible. Moreover, the results suggested that oxidative stress imposed by JG and LW was one of mechanisms of allelopathic action of the studied quinones in plants.