Published works on the effect of Se in plants response to heavy metal stress all confirm involvement of similar mechanisms for ameliorating effect of Se under other stresses. Selenium supplementation considerably reversed the Cd-induced decrease in fresh mass as well as the changes in lipid unsaturation and peroxidation. Moreover, the presence of Se in medium prevented changes in the DNA methylation pattern triggered in rape seedlings by high Cd concentrations (Filek et al. 2008). Two possible mechanisms for the action of Se were considered, removal of Cd from metabolically active cellular sites, and reduction of oxygen radicals (Filek et al. 2008).
Certain plant species are known to accumulate Se to levels far beyond those observed in other species. The possible functional significance of Se accumulation in these species has been studied in some crops e.g. Brassica juncea (2004) and natural vegetation species e.g. Stanleya pinnata and Astragalus bisulcatus (Quinn et al. 2010). It was shown that Se protects plants from fungal infection and herbivory (Quinn et al. 2010) and feeding by aphids (Hanson et al. 2004).
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Effects of cobalt supplementation on plants stress responses
Cobalt has long been known to be a micronutrient for animals and humans, where it is a constituent of vitamin B12. However, a physiological function for this element in higher plants has so far not been established. Vitamin B12 is synthesized by soil bacteria, intestinal microbes, and algae, but not in animals and plants. The only physiological role so far definitely attributed to Co in higher plants has been in N fixation by leguminous plants (Marschner 1995). Since Co is essential for mammals, fertilization of crops with Co will have the additional beneficial effect of enhancing its nutritional quality. Similar to other heavy metals, Co causes toxicity to plants at high concentration, and most of the recent literature focuses on the mechanisms through which plants can cope with Co stress (Micó et al. 2008). At low levels however, Co can have a number of beneficial effects, particularly in leguminous plants.
Co is a component of cobalamin (vitamin B12), which is required for the activity of several enzymes in N-fixing microorganisms include Rhizobium such as methionine synthase, ribonucleotide reductase and methylmalonyl-CoA mutase (Marschner 1995). Its importance in N fixation by symbiosis in Leguminosae (Fabaceae) has been established. Soybeans grown with only atmospheric N and no mineral N have rapid N fixation and growth with Co suppementation, but have minimal growth without Co additions (Ahmed and Evans 1960). In pea plants (Pisum sativum L.), the application of Co to the soil increased growth, nodule number and weight, plant nutrient levels, as well as seedpod yield and seed quality (Gad 2006). These effects could most likely be ascribed to the essentiality of Co for symbiotic Rhizobia that live in the nodules of these leguminous plants.
Activation of antioxidant enzymes
The high O2 consumption in nodules for provision of energy also creates a great potential for production of ROS. This is true in particular for leg-hemoglobin which is also subjected to auto-oxidation in which O2•− and H2O2 are released. For protection against this toxicity, legume nodules need an efficient defense mechanism (Marschner 1995). Addition of Co to legume plants caused activation of CAT, in parallel with growth improvement and increase in the nodulation and leg-hemoglobin concentration. Activation of CAT was not observed at higher Co concentration that resulted growth impairment (Jayakumar et al. 2008).
Resistance to pathogens
Co stimulated isoquinoline accumulation (an alkaloid) in medicinal plants, through up-regulation of the biosynthesis of aromatic amino acid precursors of alkaloids (Palit, Sharma and Talukder 1994). This last effect may suggest that Co could indirectly induce biotic stress resistance, but this hypothesis has not been addressed yet. In hyperaccumulators of Co, the high tissue Co levels may also offer direct protection from herbivory or pathogens, as was shown for other hyperaccumulated elements. Alkaloid accumulation in medicinal plants such as Datura innoxia Mill., Atropa caucasica, A. belladonna L. and Glaucium flavum Crantz (Talukder and Sharma 2007) is regulated by Co. It also increased rutin (11.6%) and cyanide (67%) levels in different species of buckwheat (Fagopyrum sagittatum Gilib., F. tataricum Gaertn., and F. emargitatum) (Talukder and Sharma 2007). Cobalt acts as a chelator of salicylidine-o-aminothiophenol and salicylidine-o aminopyridine and exerts biocidal activity against the molds Aspergillus nidulans Winter and A. niger Tiegh and the yeast Candida albicans. Antifungal activities of Co(II) with acetone salicyloyl hydrazone and ethyl methyl ketone salicyloyl hydrazone against A. niger and A. flavus have been established (Johari, Nagar and Sharma 1987).
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Delay of senescence
Another beneficial effect reported for Co is retardation of leaf senescence via inhibition of ethylene biosynthesis. The Co(II) ion is an inhibitor of the ethylene biosynthesis pathway, blocking the conversion of 1-amino-cyclopropane-l-carboxylic acid (ACC) (Branden et al. 1987). Senescence in lettuce leaf in the dark is retarded by Co, which acts by arresting the decline of Chl, protein, RNA and, to a lesser extent, DNA. The activities of RNAase and protease, and tissue permeability were decreased, while the activity of CAT increased. Cobalt delays ageing and is used for keeping leaves and fruit fresh in vetch (Vicia spp.) and apple respectively (Talukder and Sharma 2007). Cobalt inhibits IAA-induced ethylene production in winter wheat and beans, in kiwifruit (Actinidia chinensis Planch) (Talukder and Sharma 2007) and in wheat seedlings under water stress (Gaal, Ariunaa and Gyuris 1988).
Cobalt chloride markedly increases elongation of etiolated pea stems when supplied with indole acetic acid (IAA) and sucrose, but elongation is inhibited by Co acetate. Cobalt in the form of vitamin B12 is necessary for the growth of excised tumor tissue from spruce (Picea glaucaVoss.) cultured in vitro. It increases the apparent rate of synthesis of peroxides and prevents the peroxidative destruction of IAA. It counteracts the inhibition by dinitrophenol (DNP) in oxidative phosphorylation and reduces activity of ATPase and is known to be an activator of plant enzymes such as carboxylases and peptidases (Ahmed and Evans 1960). Cobalt has also been noted to cause repression of developmental distortion such as leaf malformation and accumulation of low-molecular-weight polypeptides in velvet plant (Gynura aurantiaca DC) and prevention of 3,6-dichloro-o-anisic acid-induced Chl degradation in tobacco leaves (Talukder and Sharma 2007).
Prevention of auxin-induced stomatal opening in detached leaf epidermis has been observed (Merritt, Kemper and Tallman 2001). However, this effect has so far not been studied in intact plants and may cause reduction of water loss and improvement of drought tolerance in plants. Presowing treatment of seeds with Co nitrate increased drought resistance of horse chestnut (Aesculus hippocastanum L.) (Tarabrin and Teteneva 1979).
Effects of aluminum supplementation on plants stress responses
It is well known that high Al concentration in soil solution is the most important factor in restricting plant growth on acid soils (Kochian, Hoekenga and Piñeros 2004). No conclusive evidence suggests that Al is an essential nutrient for plants (Marschner 1995).
Relative to Al accumulation, there appears to be two groups of plant species: Al excluders and Al accumulators. Most plant species, particularly crop plants, are Al excluders. Aluminum accumulators are plants with 1000 mg Al kg-1 or greater in leaves (Miyasaka, Hue and Dunn 2007). Aluminum accumulation found frequently among perennial, woody species in tropical rain forests. Tea (Camellia sinensis Kuntze) is one crop plant considered to be an Al accumulator, with Al concentrations of 30,700 mg kg-1 in mature leaves and 600 mg kg-1 in young leaves (Miyasaka, Hue and Dunn 2007). Another well-known Al-accumulating plant is hydrangea (Hydrangea macrophylla Ser.), which has blue-colored sepals when the plant is grown in acidic soils and red-colored sepals when grown in alkaline soils. The blue color of hydrangea sepals is due to Al complexing with the anthocyanin, delphinidin 3-glucoside, and the copigment, 3-caffeoylquinic acid (Watanabe and Osaki 2002).
Low levels of Al (up to 10 µM) sometimes stimulate root growth of non-accumulators such as turnip and soybean (Miyasaka, Hue and Dunn 2007). In Al accumulators such as tea plants, however, root and shoot growth and leaf numbers and area, respond positively to Al supplementation up to 125 µM Free Al3+ activity (300 µM Al concentration). Root axis were growing in length for longer time before elongation ceased, lignification was delayed and relative growth rate of root axis was about two times higher than control plants (Hajiboland et al., 2011). Application of Al on the leaves of tea plant grown in alkaline soil caused the plants to recover from chlorosis. In addition, when seedlings of Miconia albicans an Al accumulating species growing in the calcareous soil showed chorotic leaves, the symptom was completely recovered from after a portion of their root systems were exposed to Al solution (Watanabe and Osaki 2002). These results suggest some physiological role of Al in Al accumulator species. Early explanations for this enhancement in growth include increased Fe solubility and availability, prevention of internal Fe deficiency through displacement of Fe from inactive sites in calcicolous plants, prevention of P toxicity or promotion of P uptake, prevention of Ca depletion, alteration of growth regulators and protection against Cu/Mn toxicity (Foy, Chaney and White 1978). However, hypotheses such as those listed above have only been shown to apply in certain cases. It has been reported that, activity of H+ATPase in plasmamembrane-enriched fraction which had been treated with Al showed a 77% increase compared with that in the control (Matsumoto et al. 1986). During growth the activity of H+ATPase play a critical role for cell wall expansion mediated by auxin. Although studies on the effect of Al on H+ATPase activity were performed mainly on Al excluder species, this mechanism explains well the considerable stimulatory effect of Al on the elongation of root axis observed in Al accumulator species such as tea.
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Alleviation of H+ toxicity
There is evidence that the nature of beneficial effects of Al occur through the alleviation of H+ toxicity by Al3+. Alleviation of H+ toxicity is a general phenomenon achieved by cations (not solely Al3+), and the effectiveness was dependent upon the charge (Cat3+>Cat2+>cat1+). However, ameliorative effect of Al3+ on H+ toxicity was reported mainly in some crop species such as wheat and maize (Kinraide 1993) and this mechanism is insufficient to explain all the phenomena of Al-induced growth enhancement particularly in plants native to low pH soils. In a study on some Al-accumulators adapted to low pH soils and grow poorly in the absence of Al, other mechanisms such as improved nutrient uptake particularly P has been proposed as mechanism for Al-induced growth improvement.
Alleviation of boron deficiency
Low B content because of high leaching losses and high Al3+ content are characteristics of acid soils. Inside the plant, Al is likely to be present as Al(OH)3, which is structurally similar to B(OH)3. In the case of sensitive species, Al is assumed to exert its toxic effects in the apoplast through interaction with the negative binding sites of the cell walls, primarily pectin. For B, the predominant function is in the formation of primary cell walls, where it cross-links the pectic polysaccharides (Hu and Brown 1994). Interaction of Al and B was studied mainly in Al sensitive species. Based on the similarities of the molecules and of the symptoms characteristics for Al toxic and B-deficient plants, it has been proposed that Al may exert its toxic effect by inducing B deficiency (Poschenrieder, Llugany and Barcelo 1995). In tea plants, B deficiency and Al supplementation had marked influence on phenolics metabolism and fractionation in the young and old leaves and roots. A high CO2 assimilation rate, greater B root-shoot transport and increase in the cell wall bound B fraction are mechanisms for Al-mediated growth amelioration of B-deficient plants. Under these conditions, shoot Al allocated mainly to the old leaves and less Al was re-translocated into young leaves, where most Al was found in the cell wall-bound fraction (Hajiboland and Bastani, 2011).
Adaptation to P deficiency
Al application enhances growth that is accompanied by increased nutrient concentrations, especially P concentrations, in the tissues. In plants adapted to low pH soils in the tropical and temperate regions growth is stimulated by Al application which is assumed to be caused by the stimulation of N, P and K uptake although the increase of P content is partly due to Al-P precipitation on the root surface and /or in the Donnan free space. In these species pH decreased around the rhizophere that may solubilize Al-P precipitates coating the surface of roots (Osaki, Watanabe and Tadano 1997). In tea plants addition of Al and P, increased P absorption and translocation as well as root and shoot growth (Konishi, Miyamoto and Taki 1985). Similarly, the Al-accumulating shrub, Melastoma malabathricum L., exhibited increased growth of leaf, stem, and roots as well as increased P accumulation when Al was added to culture solutions (Osaki, Watanabe and Tadano 1997). An increased root length by Al supplementation in tea and other Al accumulator plants native to low pH soils could be an adaptation for these species grow on acid soils with low P availability. Phosphorus acquisition by plants is largely dependent upon spatial availability of P by roots. On the other hand, greater root length provides more water absorption area and increases considerably drought resistance.
Activation of antioxidant enzymes
In the roots of intact plants as well as cultured cells of tea plants higher activity of antioxidant enzymes was observed in the presence of Al (Ghanati, Morita and Yokota 2005). These results indicate that Al-induced increase in the activities of antioxidant enzymes, resulting in increased membrane integrity and delayed lignification and aging, is a possible reason for the stimulatory effects of Al on the growth of tea plants irrespective to the interaction with other micronutrients (Ghanati Morita and Yokota 2005). In the study on tea plants grown from seeds we observed increase in the activity of antioxidant enzymes and concentration of non-enzymatic antioxidants such as proline in Al-treated plants. Membrane integrity was considerably improved in both leaf and root tissues in the presence of Al (Table 7).
Tolerance to plant pathogens
Aluminum can be toxic to pathogenic microorganisms, thus helping plants to avoid disease. Spore germination and vegetative growth of the black root rot pathogen, Thielaviopsis basicola Ferraris, were inhibited by 350 µM Al at pH 5. Similarly, mycelial growth and sporangial germination of potato late blight pathogen, Phytophthora infestans, were inhibited by 185 µM Al, and it was speculated that amendment of soils with Al might be used as a means of disease control (Miyasaka, Hue and Dunn 2007).
Conclusion and future perspective
Under both natural and cultivated ecosystems, plants often experience a combination of various stress factors including drought, high irradiance, UV radiation, chilling, flooding and salinity. An imbalanced nutrition accentuates effect of stress factors and hampers plant growth and productivity. In this chapter we tried to give evidences on how plants respond to a combination of micro-nutritional deficiencies and environmental stress factors. Antioxidant defense system is an important cross point between micronutrients and plants stress responses because of changes in the content and activity of its components due to both micro-nutrients deficiencies and environmental stress factors. However, new evidences on the effects of micronutrients on plants signaling events throw some light on the still poorly known aspects of micro-nutrients effects on plants interaction with their surrounding environment. Recent evidences on the effect of Mo on ABA signaling pathway and evidences on signaling effect of cell wall-bound B demonstrated that, micronutrients may also involve in plants signal transduction pathways either as components of important enzymes in the signaling or as structural components of a signaling molecule. More investigations are needed on this function of not only micronutrients but also elements that have been defined so far as beneficial elements.