Plant tolerance mechanisms

Plants could maintain the necessary concentrations of essential metal ions in cells by homeostatic mechanisms. They are also involved in the reduction of damage induced by heavy metal toxicity. One of the major tolerance mechanisms is chelation of heavy metals by a family of peptide ligands, the phytochelatins (similar to other xenobiotics) [108], The molecular basis for the chelators and chaperone synthesis is well known and could be applied in the modification of tolerant plants. Tolerance to Cd and As is largely dependent on the phytochelatin pathway, but molecular biology of Cd hypertolerance in certain plant species, such as the... 

Overproduction of EPSPS has been observed in several plant cell cultures tolerant to glyphosate (12, 13, 29). In the case of glyphosate-tolerant Corydalis cultures, Smart et al. demonstrated by 2 D-gel electrophoresis, the overproduction of other proteins besides EPSPS. Since the levels of activity of several shikimate pathway enzymes were unaltered in the tolerant cell line compared to the parent cell line, it was concluded that these amplified proteins may not be involved in aromatic amino acid biosynthesis. It is possible that the other proteins may not have a role in the tolerance mechanism. Alterations in protein profiles between glyphosate-sensitive and tolerant petunia cell lines have also been observed. With the glyphosate tolerant carrot cell line, in addition to overproduction of EPSPS, the levels of aromatic amino acids were found to be enhanced (29). Based on the results with plant cell cultures, it was therefore not clear if overproduction of EPSPS was sufficient to obtain glyphosate tolerance in plants. 

Pineros MA, Magalhaes JV, Alves VMC and Kochian LV (2002) Thcphydology and biophysics of an aluminum tolerance mechanism based on root citrate exudation in maize. Plant Physiol 129 1194-1206. 

Bohnert, H.J., Q. Gong, P. Li, and S. Ma. 2006. Unraveling abiotic stress tolerance mechanisms—getting genomics going. Curr. Opin. Plant Biol. 9 180-188. 

Plants tolerant to imidazolinones have been produced by both transgenic and non-transgenic mechanisms. However, all of the imidazohnone-tolerant crops currently being sold have been developed by non-transgenic methods. The first imidazolinone-tolerant crop (maize) was introduced in 1992. Subsequently, four additional imidazolinone-tolerant crops (canola, rice, wheat and sunflower) have been commercialized [31]. All of the imidazohnone-tolerant crops are being sold under the CLEARFIELD trade name. 

It must be emphasized, however, that stress resistance or susceptibility is unlikely to reside in a single factor. Studies of the responses of plants to environmental stresses suggest rather that resistance results from the possession of a number of characteristics. Attempts to explain susceptibility or resistance of plants to environmental stresses in terms of single factors are therefore unlikely to result in plausible theories of environmental adaptation. Changes in nitrogenous compounds can only be regarded as components of the resistance or tolerance mechanisms. 

It could be concluded that, elements that can be hyperaccumulated include As (>0.1 %), Cd (>0.01 %), Co (>0.1 %), Cu (>0.1 %), Pb (>0.1 %), Mn (>1 %), Ni (>0.1 %), Se (>0.1 %) and Zn (>1 %). It could be also mentioned that, the element levels accumulated in these plants would be lethal to other organisms, yet cause no toxicity in hyperaccnmulators. The Se-tolerance mechanism of hyperaccumulators can be followed by nsing microfocused X-ray fluorescence (pXRF) mapping and micro-X-ray absorption near edge structure (pXANES) spectroscopy, which revealed a stark contrast in spatial distribution and chemical speciation of Se between hyperaccnmnlators and nonaccumulators. 

It is reported that, the toxic Se concentrations of nonaccumulator plants, resulting in a 10 % reduction of yield, without visible symptoms, range from Se contents of 2-330 mg kg- in rice and white clover, respectively. In accumulator plants, Se concentration may reach 4000 mg kg-, without negative effects (Kabata-Pendias 2011). Tolerance mechanisms involve processes of exclusion of active Se amino acids, thus... 

It could be defined a Se-tolerance mechanism as the ability of plants to convert Se into volatile compounds that are then released into the atmosphere. Thus, reducing their Se load is an important metabolic activity of a variety of different plant types. Rates of volatilization vary substantially between plant species and are related to a number of factors, including the concentration and chemical form of Se and of S in... 

Van Hoewyk D, Garifullina GF, Ackley AR, Abdel-Ghany SE, Marcus MA, Fakra S, Ishiyama K, Inoue E, Pilon M, Takahashi H et al (2005) Overexpression of AtCpNifS enhances selenium tolerance and accumulation in Arabidopsis. Plant Physiol 139 1518-1528 Van Hoewyk D, Takahashi H, Inoue E, Hess ATM, Pilon-Smits EAH (2008) Transcriptome analyses give insights into selenium-stress responses and selenium tolerance mechanisms in Arabidopsis. Physiol Plant 132 236-253... 

The techniques of molecular biology have particular potential for rapidly introducing small numbers of single genes. Unfortunately there is strong evidence that the complex compensation mechanisms that exist in plants, and the interactions between different whole-plant and biochemical responses to stress, will make the direct improvement of environmental stress tolerance in crop plants by genetic engineering rather more difficult... 

Polyols are present in desiccation tolerant lichens and liverworts, although not in mosses (Lewis, 1984). More generally starch hydrolysis and sugar accumulation occur in many plants experiencing severe water deficits (Hsiao, 1973). It is tempting to speculate that the accumulation of low molecular weight solutes in reponse to water stress represents a mechanism for the protection of membranes and proteins in the dry state. 

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