CELLULAR & MOLECULAR BIOLOGY LETTERS Volume 8, (2003) pp 97 103 http://www.cmbl.org.pl Received 14 October 2002 Accepted 24 January 2003 Short Communication THE EFFECT OF SELENIUM ON THE ACCUMULATION OF SOME METALS IN Zea mays L. PLANTS TREATED WITH INDOLE-3-ACETIC ACID KRYSTYNA PAZURKIEWICZ-KOCOT 1, WITOLD GALAS 2 and ANDRZEJ KITA 2 1 Department of Plant Physiology, Faculty of Biology and Environmental Protection, Silesian University, Jagiellońska 28, 40-032 Katowice, Poland, 2 Department of Analytical Chemistry, Institute of Chemistry, Silesian University, Szkolna 7, 40-032 Katowice, Poland Abstract: In this study, we examined the relationship between the accumulation of NaHSeO 3, the plant hormone (IAA), and some nutrient elements (K +, Na +, Ca 2+ ) in the tissues of the roots, mesocotyls and leaves of Zea mays L. plants. Our experiments were carried out with eight- to nine-day old maize plants (Zea mays L. var K33xF2) grown on Hoagland's medium containing the standard macro- and microelements, IAA and NaHSeO 3. The accumulation of selenium, potassium, sodium and calcium in the seedlings was measured by emission spectroscopy using a spectrometer with excitation by the argon inductively coupled plasma technique (ICP-AES). We observed that when selenite and phytohormone (IAA) are present in the external medium of growing plants, they change the uptake and accumulation of some cations (K +, Na +, Ca 2+ ) in the leaf, mesocotyl and root tissues. The change of transport of some nutrient elements is probably one of the first observed symptoms of selenium s effects on plants. Key Words: Accumulation, Selenium, K +, Na +, Ca 2+, IAA, ICP-AES Method, Zea mays L. INTRODUCTION Selenium (Se) is a trace element with some important functions in living organisms, in particular in animals. Selenium is known to be essential for animals [1, 2]. Although its role in the animal organism is known in detail, further investigation is required to elucidate its role in plants [3, 4]. It is known that selenium occurs as selenate, selenite, selenide, elemental Se and organic Abbreviations used: IAA indole-3-acetic acid; ICP-AES inductively coupled plasma atomic emission spectrometry
98 CELL. MOL. BIOL. LETT. Vol. 8. No. 1. 2003 selenium in the soil, and that the uptake of selenium by plants is governed by many soil and plant factors [5]. One of the most important factors determining the uptake of this element is the form and concentration of selenium in the soil [3, 4]. There are five fractions of selenium in soils inorganic Se (selenate and selenite), organic Se (bound to humic acid and to fulvic acid) and organic Se (which has low molecular weight). Analyses of plants from seleniferous soils indicate that a small number of tolerant species, invariably associated with highly seleniferous regions, accumulates several thousand μg Se/g plant mass in their roots or leaves [3]. Other plants growing on the same soils absorbed much lower concentrations of this element. These differences prompted a division of plants into three groups primary selenium indicators or selenium accumulators (plants that accumulate several thousand μg Se/g), secondary selenium indicators or selenium absorbers (species of plants that absorb up to 1000 μg Se/g), and non-accumulators (plants that generally do not take up more than 25 μg Se/g). In the non-accumulating species of plants, there is a level of selenium that is tolerated, and a level at which it becomes toxic, and there is probably a dichotomous role for selenium in these plants. At high concentrations (the upper limit of selenium tolerance), selenium becomes toxic to these plants [3]. High selenium levels depress growth, and decrease the levels of organogenesis, protein synthesis and nucleic acid synthesis. Selenium affects metal distribution and sometimes increases the excretion of toxic elements [6]. In this study, we investigated the relationship between the accumulation of selenium (selenite form NaHSeO 3 ) and some nutrient elements (K +, Na +, Ca 2+ ) in the root, mesocotyl and leaf tissues of maize seedlings. We also investigated the effects of selenite ions on the accumulation of potassium, sodium and calcium ions in Zea mays L. plants treated with IAA. MATERIALS AND METHODS The experiments were carried out with eight- to nine-day old maize plants (Zea mays L. var K33xF2) grown on Hoagland's medium at 25 o C. Seeds of maize were cultivated in darkness. Then, individual seedlings were transferred to an aerated solution containing the standard macro- and microelements, and cultivated in a 12 hour light, 12 hour dark regime. The concentration of the phytohormone (IAA) in the external medium was 10-4 mol dm -3, and the concentration of selenite was 10-6 mol (NaHSeO 3 ) dm -3. The ph of the medium was 6.5. The accumulation of elements in the roots, mesocotyls and leaves of maize was measured via emission spectroscopy using a sequential spectrometer with excitation by the argon inductively coupled plasma technique (ICP-AES). This technique enables elemental analysis. The spectrometer was used with following parameters: frequency 27.12 MHz; power 1.1 kw; sample rate 1.0 cm 3 min -1 ; analytical lines (integration time) Se-196.026 nm, Ca-317.933 nm, Na-589.592 nm, K-766.490 nm. Standard solutions of the investigated elements at 1 mg cm -3 were used as a reference. The values in the
CELLULAR & MOLECULAR BIOLOGY LETTERS 99 tables represent the averages obtained from 5 measurements, relative error 6%- 8%. RESULTS AND DISCUSSION One of the most important factors determining the uptake and accumulation of the nutrient elements in plant tissues is the interaction between the ions of some elements and the effect of some physiologically important substances [7-10]. Selenium, in the form accumulated in plants [11] strongly interacts with some ions and substances [12, 13] and sometimes reduces their toxicity [6], especially in animal tissues [6]. However, the effect of selenium on plant cells and its role in plants is not well known. Primary indicators of Se generally grow in seleniferous regions. Secondary indicators are not limited to seleniferous regions; they are known to grow on soils that contain little or no selenium at all. The non-accumulating species, display retarded growth in the presence of selenium, and these plants are poisoned by selenium [3]. The toxicity of selenate (SeO 2 4 ) and selenite (SeO 2 3 ) to most plants can be connected with three factors: firstly, selenate and selenite are readily absorbed from the soil by roots and translocated to other parts of the plants; secondly, metabolic reactions convert these anions to the organic forms of selenium; and thirdly, organic selenium metabolites, which act as analogues of essential sulfur compounds, interfere with cellular biochemical reactions. The incorporation into proteins of the amino acid analogues selenocysteine and selenomethioine in place of the equivalent sulfur amino acids is the cause of selenium toxicity, and the exclusion of selenium from the proteins of accumulators is the basis of selenium tolerance. The accumulation of selenium is associated with a nutritional requirement for selenium [3]. Research during recent years has established that selenium accumulation is the result of evolutionary modifications in the enzyme pathways responsible for sulfur metabolism [3, 4]. Plants and other organisms utilize the following three elements for the regulation of cell membrane potential and turgor potassium, sodium and calcium. The uptake of K +, Na + and Ca 2+ occurs through ion channels in the plasma membrane of the cells. It is generally known that some plant hormones, such as IAA, induce the activation of the plasma membrane ATPases causing a hyperpolarization of the cell membrane. It is known that the electrical signaling in plants is an integral part of the generation of action potentials, which is based on changes in ion flux, including the efflux of chloride and potassium and the influx of calcium. Finally, K +, Na + and Ca 2+ may regulate a wide variety of physiological responses in plants, including elongation growth, respiration, water uptake, phloem unloading, activation of proteinase inhibitor genes and gas exchange [7, 8, 10].
100 CELL. MOL. BIOL. LETT. Vol. 8. No. 1. 2003 The correlation between the concentration of selenium and IAA in the external medium and the accumulation of K +, Na + and Ca 2+ ions is summarized in Figs. 2, 3 and 4. The dependence between selenium content in plants and selenium concentration in the external medium is shown in Fig. 1. Our results indicate a high degree of accumulation of this element in the tissues of plants, and show that selenium affects the uptake and accumulation of the nutrient elements in plants. The toxic effect of selenium on plants is reflected mainly in the form of growth changes and its influence on some metabolic processes [4]. One of the effects of the extensive absorption of selenium by plants is the uptake disturbance of the essential macro- and microelements in plants [6]. Some authors have already given evidence of a decrease in the accumulation of the necessary microelements Mn 2+, Zn 2+ and Cu 2+, and some heavy metals like Cd 2+. The results presented in this paper indicate a strong accumulation of selenite ions in plant tissues, especially in the roots of plants (Fig. 1). In addition, selenite ions affect the uptake, distribution and accumulation of nutrient elements in plant tissues and induce concentration changes of some of them in the plants. Our results indicate a dependence between the uptake and accumulation of K +, Na + and Ca 2+ in the leaves, mesocotyls and roots and the presence of IAA, NaHSeO 3 and IAA+NaHSeO 3 in the external medium of growing plants (Figs. 2, 3 and 4). Fig. 1. Selenium content in maize plants (Zea mays L.) treated with NaHSeO 3 (10-6 Compared to the control plants, an increase in the K + ion content was found in the leaves and mesocotyls of plants, while in the roots, we observed a certain decrease in the content of this ion (Fig. 2).
CELLULAR & MOLECULAR BIOLOGY LETTERS 101 Fig. 2. Accumulation of K + in maize plants (Zea mays L.) treated with NaHSeO 3 (10-6 Fig. 3. Accumulation of Na + in maize plants (Zea mays L.) treated with NaHSeO 3 (10-6 Our results indicate a high degree of accumulation of Na + in the root and mesocotyl tissues of plants grown in a medium containing IAA and IAA+NaHSeO 3. A decrease in Na + ion content was displayed by the leaves of plants growing in a medium with NaHSeO 3 (Fig. 3). For the other plants, the observed fluctuations in the Na + ions contents were at the level of experimental error. The strongest effect of calcium accumulation was observed for the roots and mesocotyls of plants treated with NaHSeO 3 and IAA+ NaHSeO 3 (Fig. 4). It is possible that the primary influence of selenite on the plant cells depends on its interaction with the plasmalemma. The accumulation effect of the ions can be
102 CELL. MOL. BIOL. LETT. Vol. 8. No. 1. 2003 explained as a primary influence of the selenite on the protoplasmic membranes and finally on metabolic cell processes. It is possible that the selenite ions change the permeability coefficient of plasmatic membranes for some ions, and therefore, affect ion transport in plant cells. The change of transport abilities of some ions is probably one of the first observed symptoms of selenium s effects on plants. Fig. 4. Accumulation of Ca 2+ in maize plants (Zea mays L.) treated with NaHSeO 3 (10-6 REFERENCES 1. Levander, O.A. Selenium: biochemical actions, interactions, and some human, health implications. Clinical, biochemical and nutritional aspects of trace elements. Elsevier, New York (1982) 345-368. 2. Wachowicz, B., Żbikowska, H.M. and Nowak, P. Selenium compounds in the environment; their effect on human health. Cell. Mol. Biol. Lett. 6 (2001) 375-381. 3. Brown, T.A. and Shrift, A. Selenium toxicity and tolerance in higher plants. Biol. Rev. 57 (1982) 59-84. 4. Giessel-Nielsen, G., Gupta, U.C., Lamand, M. and Westermarck, T. Selenium in soils and plants. Adv. Agronomy 37 (1984) 397-460. 5. Yamada, H., Kang, Y., Aso, T., Uesugi, H., Fujimura, T. and Yonebayashi, K. Chemical forms and stability of selenium in soil. Soil Sci. Plant Nutr. 44 (1998) 385-391. 6. Landberg, T. and Greger, H. Influence of selenium on uptake and toxicity of copper and cadmium in pea and wheat. Physiol. Plant. 90 (1994) 637-644. 7. Canny, M.J. Translocation of nutrients and hormones. Advanced Plant Physiology. Pitman Publishing, London, UK (1984) 277-296.
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