Tolerance mechanisms Selection parameters for pigeonpea cajan

Kaur Nayyar : Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms 1 1 commonly found in leaves, floral parts, and pollens. They usually accumulate in the plant vacuole as glycosides, but they also occur as exudates on the surface of leaves and other aerial plant parts. Flavonoids are suggested to have many functions in flowers, fruits, and seed pigmentation, protection against UV light, defence against phytopathogens pathogenic microorganisms, insects, animals, role in plant fertility and germination of pollen and, molecules in plant- microbe interaction. Apart from the above roles, flavonoids have as antioxidative activity Brown et al.1998. Besides having the function of ROS scavenging , flavonoids are able to function as chelators for metals, depending on the molecular structure Brown et al.1998 and hence can take part in plant defence. In Arabidopsis thaliana, the relation between flavonoids and heavy metal tolerance were investigated. Both Arabidopsis wild type and mutant lines with a defect in flavonoid biosynthesis were grown on media containing different heavy metals. Results revealed that root length and seedling weight were reduced in mutants more than in the wild type when grown on cadmium, while on zinc only root length was affected Keilig and Muller 2009.

5. Tolerance mechanisms

Heavy metals in the plant environment operate as stress factors that cause physiological strain and in doing so they reduce the plant vigour and totally inhibit the plant growth in extremes However, plants have evolved several physiological mechanisms which enable them to tolerate metal toxicity Baker 1987. The development of metal tolerance in plants is a major way to reduce the harmful effects of excessive exposure to heavy metal ions Tyler et al.1989. There are various potential cellular and other mechanisms available for metal detoxification and tolerance in higher plants Hall 2002, which have been reported to function in legumes. 5.a. Role of Arbuscular mycorrhizal fungi Mycorrhizal association is a symbiotic non-pathogenic relationship between plant roots and fungal hyphae with a fungal connection between the soil and the root Harley and Smith 1983. It has been reported that the host plant receives support from AM fungi, with the help of its symbiotic association, in the aspect of uptake of phosphorus and other nutrients, enhancement of growth hormones, increase of protein content, increase of lipid, sugars, amino acid levels, increase of tolerance to heavy metals, increase of salinity tolerance, and resistance to root-borne pathogens Upadhyaya et al. 2010. Recently, the symbiotic association with mycorrhizal fungi has been proposed as one of the major mechanisms of plant HM-tolerance Hall 2002, Joachim et al. 2009. However, alleviating heavy metal toxicity by AMF colonization can vary to a large extent, depending on which heavy metal is involved, its concentration in the soil, the fungal symbiosis partner and the conditions of plant growth Turnau 1998. There are many strategies adopted by AM which can alleviate heavy metal threats in mixed culture systems and, thus, from the food chains Joschim et al. 2009. These include the immobilisation of metal compounds, precipitation of p o l y p h o s p h a t e granules in the soil, adsorption to chitin in the fungal cell walls and chelation of heavy metals inside the fungus Joachim et al. 2009. Generally, AM binds to heavy metals beyond the plant rhizosphere by releasing an insoluble glycoprotein commonly known as glomalin Gonzalez- Chavez et al. 2004. The roles of AM are summarized in the Fig.5. Numerous studies have indicated that AMF can decrease the metal uptake of the host plants, thus protecting them against HMs toxicity Leyval et al. 1997. Many heavy metal contaminated sites are reported to have mycorrhizae Weissenhorn and Leyval 1993.This indicates that these fungi have evolved a HM-tolerance and that they may play a role in the phytoremediation of the site Khan et al. 2000. Mycorrhizae were found to ameliorate the toxicity of trace metals in polluted soils growing in soybean and lentil plants Jamal et al. 2002. Increased heavy metal tolerance of plants by dual inoculation of an arbuscular mycorrhizal fungi and nitrogen-fixer Rhizobium bacterium was reported in cowpea Al-Garni 2006. The effects of dual inoculation with arbuscular mycorrhizal AM fungus and Rhizobium N-fixing bacteria, NFB on the host plant cowpea Vigna sinensis in pot cultures were investigated at six concentrations of Zn 0.0-1000 mgkg dry soil and Cd 0.0-100 mgkg dry soil. The study provides evidence for benefits of NFB to AM fungi in the protection of host plants against the detrimental effects of heavy metals and provides the mechanisms for metal tolerance against them.A greenhouse pot experiment was done to investigate the effects of the colonization of arbuscular mycorrhizal fungus AMF Glomus mosseae on the growth and metal uptake of three leguminous plants Sesbania rostrata, Sesbania cannabina , Medicago sativa grown in multi-metal contaminated soil Lin et al. 2007. The results revealed that AMF colonization increased the growth of the legumes thereby indicating that AMF colonization increased the plant’s resistance to heavy metals. The effect was also enhanced on the formation of root nodules and N and P uptake increased, which may be due to the heavy metal tolerance mechanisms conferred by the AMF. Fig: 5 Steps for alleviating heavy metal stress adopted by AM fungi 1 2 Journal of Food Legumes 263 4, 2013 5.b. Role of metallothioneins and phytochelatins Metallothioneins MT’s belong to a family of cysteine- rich low molecular weight metal-binding proteins generally induced during the metal stress Corbett and Goldsbrough 2002. Metallothioneins generally form complexes with heavy metal ions and are present in almost all forms of life and have a role in protecting cells from the deleterious effects of high concentration metal ions. The function of MT is to detoxify non-essential metals such as mercury, cadmium and essential metals such as zinc and copper. Phytochelatins PCs, a type of MT’s, are synthesized in plants in response to heavy metal stress and due to various metals. PCs consist of only the three amino acids: Glu, Cys and Gly, the Glu, and Cys residues linked through a g- carboxylamide bondCobbet 2000.Recent research indicates that PCs are present in a wide variety of plant species and in some microorganisms. They are structurally related to glutathione GSH, g-Glu-Cys-Gly and were presumed to be the products of a biosynthetic pathway. In addition, a number of structural variants, for example, g-Glu-Cysn-b- Ala, g- Glu-Cysn-Ser, and g-Glu-Cysn-Glu, have been identified in some plant species Rauser 1999, Zenk 1996. Activation of the detoxicative-phytochelatin system was observed in the cytosol of root cells of three legume species, Vicia faba , Pisum sativum, and Phaseolus vulgaris when they were exposed to lead ions Piechalak et al. 2002. This system was composed of phytochelatins PCs in roots of V. faba, homophytochelatins hPCs in P. vulgaris roots, and both PCs and hPCs in P. sativum roots. 5.c. Organic acids and amino acids Some amino acids, particularly histidine and proline, also play very important roles in the chelation of metal ions both within plant cells and in the xylem sap Rai 2002. Kerkeb and Krämer 2003have reported that in Alyssum lesbiacum and Brassica juncea , an enhanced release of Ni into the xylem is associated with concurrent release of histidine from an increased root free His pool. Other amino acids such as citrate, malate and histidine are potential ligands for heavy metals and could play a role in tolerance and detoxification Rauser 1999. Citrate, malate and oxalate have been involved in transport of metal ions through the xylem and vacuolar sequestering Rauser 1999. It is reported that citric acid to be a major Cd 2+ ligand at low Cd 2+ concentrations Wagner 1993 and has been shown to form complexes with Ni 2+ in Ni- hyperaccumulation plants Sagner et al. 1998. It is also suggested that malate is a cytosolic zinc chelator in zinc- tolerant plants Mathys 1977. Kramer et al.1996 reported that the significant and proportional change in amino acid or organic acid concentration elicited by a change in metal exposure was shown by histidine response in plants that accumulate nickel. The presence of different concentrations of organic acids among various ecotypes of metal-tolerant plants in their natural habitat has deemed these substances as likely cellular chelators Rauser 1999. 5.d. Polyamines Polyamines PAs are nitrogenous compounds present in all living cells. They are not only involved in various cellular processes like growth promotion and cell division but also in the inhibition of ethylene production and senescence Tiburcio et al. 1997. They influence a variety of growth and development processes in plants which have been suggested to be a class of plant growth regulators and to act as second messengers Evans and Malmberg 1989, Kakkar and Sawhney 2002. The polyamines are cations due to protonation at cytoplasmic pH, i.e. putrescine 2+ , spermidine 3+ , and spermine 4+ , which accounts for their binding ability to nucleic acids Flink and Pettijohn 1975.It has been reported that the levels of polyamines and the activities of their biosynthetic enzymes in plants increase under environmental stresses Evans and Malmberg 1989. Polyamine contents are highly altered in response to the exposure to heavy metals. For example, the response of different polyamines to Cd treatment strongly varied in Phaseolus vulgaris in an organ-specific manner. Putrescine increased in root, hypocotyl, and epicotyl whereas spermidine increased in hypocotyl, decreased in leaves, and did not change in roots.In soybean phospholipids, using membrane vesicles Weinstein et al. 1986. Tadolini et al. 1984 showed that polyamines inhibit lipid peroxidation when bound to the negative charges on the vesicle surface. In addition, polyamines namely, spermine, spermidine, putrescine, and cadaverine have been demonstrated to scavenge free radicals in vitro Drolet et al.1986. Furthermore, polyamines block one of the major vacuolar channels, the fast vacuolar cation channel, and their accumulation could decrease ion conductance at the vacuolar membrane to facilitate metal ion compartmentation Bru¨ggemann et al.1998. Their roles in metal tolerance remain to be explored in detail. Conclusion Heavy metal contaminations seriously threaten the productivity of plants and particularly the legumes which are important atmospheric nitrogen fixers and an excellent source of protein to both animals and human beings. These metals prove to be deleterious for the legume growth and physiology, and ultimately enter the food chain to affect human population. In spite of presence of diverse tolerance mechanisms to toxic metals, legumes suffer due to their cultivation in contaminated soils. The knowledge about metal tolerance mechanisms gained from model plants such as Arabidopsis needs to be explored in legumes too to induce tolerance to metals. Various mechanisms involving phytochelatins, thiols, transporters based in plasma membrane and tonoplast need to be manipulated through genetic means to enhance the metal tolerance in legumes. 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