Ivna Štolfa

Heavy Metal-Induced Oxidative Stress in Plants: Response of the Antioxidative System

Heavy metals (HMs) are among the most important environmental pollutants, particularly in areas with strong anthropogenic pressure. For plants, high levels of HMs are extremely toxic since they may act in several different modes: by the direct inhibition of plant growth and biosynthetic pathways or through the production of reactive oxygen species (ROS). Certain metals generate ROS due to their involvement in redox reactions like Fenton and/or Haber–Weiss reactions, while metals without redox capacity enhance ROS production by reducing the antioxidant glutathione pool, activating calcium-dependent systems and influencing iron-mediated processes. ROS production affects lipids, proteins, and DNA and consequently leads to cell death. In response, plants are equipped with complex enzymatic and nonenzymatic mechanisms involved in antioxidative defense to neutralize HM toxicity, and the main components of these mechanisms will be reviewed in this chapter.

Physiology and biochemistry of leaf bleaching in prematurely aging maple (Acer saccharinum L.) trees. II. Functional and molecular adjustment of PSII

Physiology and biochemistry of leaf bleaching in prematurely aging maple (Acer saccharinum L.) trees. II. Functional and molecular adjustment of PSII In the present study we aimed to investigate physiological and molecular mechanisms of photosynthetic performance decline in prematurely aged bleached leaves of silver maple (Acer saccharinum L.) trees. We used in vivo chlorophyll a fluorescence measurement to analyze changes in PSII photochemistry, relative abundance of photosynthetic proteins (D1, LHCII, Cytf and Rubisco LSU), relations between chlorophylls and their precursor protochlorophyllide as well as elemental composition of the leaves. Decreases in Al, Cr, Cu, Fe, K, Zn and an increase in S concentrations were found in bleached leaves in comparison to healthy green ones. The bleached leaves were visually expressing symptoms characteristic of Fe deficiency. Further, they had considerably decreased chlorophyll contents and protochlorophyllide contents, overall photosynthetic activity and relative abundance of major photosynthetic proteins. All the results indicate that modifications in the molecular organization of photosynthetic electron-transport chain components in bleached leaves led to functional adaptation of the PSII achieved by modifications of some reaction centres (RCs), turning them from active to dissipative. This provided efficient adaptation of bleached leaves to high-light induced oxidative damage during summer.

Glutathione and Related Enzymes in Response to Abiotic Stress

In variable environmental conditions, plants cannot survive without low-weight, non-enzymatic glutathione molecules. It has a role not only in plant growth and development, but also, in plant defense mechanisms. Variety of abiotic factors, such as extreme temperatures, water deficiency, high salt, and chemical pollutants, accelerate the generation of reactive oxygen species and methylglyoxal, which change glutathione redox state and, in turn, modulate gene and protein expression to increase plant acclimation to abiotic stress. Predicted climate change in future and consequently rising level of abiotic stresses will likely negatively affect plant growth and productivity worldwide. Therefore, enhancement of the functions of glutathione and its related enzymes as well as altering their levels in transgenic plants may be a useful strategy for improving the stress resistance in plants. In this chapter, we outline the key recent advances in this field of research and discuss glutathione significance in abiotic stress-exposed plants.

Physiology and biochemistry of leaf bleaching in prematurely aging maple (Acer saccharinum L.) trees: I. Hydrogen peroxide level, antioxidative responses and photosynthetic pigments

Physiology and biochemistry of leaf bleaching in prematurely aging maple (Acer saccharinum L.) trees: I. Hydrogen peroxide level, antioxidative responses and photosynthetic pigments Essential signaling processes such as changes in calcium mobilization, protein phosphorylation and gene expression are known to be modulated by hydrogen peroxide (H2O2). A lot of silver maple trees in the city of Osijek (Croatia) were noticed to have bleached leaves by early summer as well as during the whole vegetation season. In this study we aimed to investigate the processes that regulate H2O2 levels in healthy (green) and prematurely aged (bleached) leaves. For that purpose, photosynthetic performance and antioxidative response of green and bleached silver maple leaves were studied. Bleached leaves had higher hydrogen peroxide level, a three-fold level of total soluble proteins as well as a lower level of ascorbic acid. Concentrations of chlorophyll a, chlorophyll b, total chlorophylls and total carotenoids as well as maximum quantum yield of photosystem II were lower in bleached leaves. This indicated their impaired photosynthetic performance. Further more, bleached leaves were characterized by lower specific activities of the main antioxidative enzymes, which influenced their reactive oxygen species scavenging capability. The higher level of H2O2 content in bleached leaves as the consequence of reduced antioxidative enzyme specific activities as well as ascorbic acid level could be the reason for the down-regulation of photosynthetic performance and premature aging of those leaves.

Involvement of peroxidases in structural changes of barley stem

The aim of this work was to investigate anatomical characteristics of barley (Hordeum vulgare) internodes and to reveal the connection with activity of guaiacol-peroxidases, their isoenzyme profiles, lignin and cellulose content. Three lower internodes were sampled at elongation stage. Internodes differed in their anatomical features by showing that lignin deposition follows developmental pattern. Epidermis and hypodermis in the first internode were characterized by continuous layer of the lignified cells while in the second and third internode thin walled parenchyma cells could be observed in the hypodermis. These observations were in accordance with increment of peroxidase activity from the first to the third internode. Two acidic and two basic isoforms of peroxidases were present. The third internode had higher total soluble protein content and lower dry weight as well as Klason lignin content in comparison to the first and second internode. Studying lignification process in barley stem by using anatomical and physiological approach will provide valuable information regarding developmental changes connected with lignin deposition in the stem.

Oxidative stress in leaves of two olive cultivars under freezing conditions.

Olive is one of the most important cultivated Mediterranean plants. In order to determine the differences in frost resistance of two, two-year-old olive cultivars (Olea europaea cv. Leccino and cv. Oblica) growing on different types of nutrient substrates (soil and coconut fibres), the trees were exposed to low temperature (-5 °C) in the dark. It was shown that low temperature caused an increase in H2O2 concentration, level of lipid peroxidation and carbonyl protein content in both cultivars and on both nutrient substrates, respectively. The CAT and APX activities significantly varied depending on the cultivar, the nutrient substrate type and the time of exposure to low temperature. Cv. Oblica and cv. Leccino growing on coconut fibres showed a better antioxidative response to low temperature probably due to the higher nitrogen and phosphorus concentration established in this type of nutrient substrate. That positive antioxidative response determined on coconut fibres was more pronounced in leaves of cv. Leccino.