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  • Author or Editor: D. Szegő x
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S-methylmethionine (SMM) is an important intermediary compound in the sulphur metabolism and has been shown to play a possible role in moderating the damaging effects of low temperature stress. The present work investigated the extent to which SMM is capable of influencing the activity of antioxidant enzymes when the subtropical species maize is exposed to chilling temperatures during the early developmental phase. SMM was found to contribute to the protection of maize seedlings against low (<14°C) temperature stress by enhancing the activity of certain antioxidant enzymes to varying extents, and thus helping to neutralise the reactive oxygen species (ROS) formed at this temperature. Results obtained in a gradient plant growth chamber revealed that, with the exception of catalase, SMM increased the activity of all the antioxidants studied (glutathione reductase, glutathione-S-transferase, guaiacol peroxidase, ascorbate peroxidase), particularly in the lower ranges of the temperature gradient (6–14°C).

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S-methylmethionine (SMM), a naturally occurring, biologically active compound, is a free amino acid derivative, which is increasingly recognised as playing an important part in the plant metabolism. SMM, which is synthesised from methionine, is involved in crucial processes in the S metabolism, such as the regulation of methionine and S-adenosyl methionine levels, the methylation processes taking place in cells, and the transport and storage of sulphur in certain phases of development. It is of great importance in the development of resistance to abiotic and biotic stress factors, as it is a direct precursor in the biosynthesis of the osmoprotectants and other S-containing compounds involved in defence mechanisms, while also influencing the biosynthesis of major plant hormones such as polyamines and ethylene. The present paper discusses our increasing understanding of the role played by SMM in the plant metabolism and its possible role in the improvement of traits that enable plants to overcome stress.

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The plant hormones auxin, cytokinin and gibberellic acid, which stimulate plant growth and development, induce significant changes in the isoacceptor spectra of various tRNAs. The present experiments revealed that the treatment of wheat seedlings with auxin, cytokinin or gibberellic acid resulted in the appearance of new isoacceptors in the spectra of three tRNA groups specific for amino acids (methionine, tyrosine and valine). These new isoacceptors may be beneficial for the synthesis and regulation of the proteins induced by the plant hormones.

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The work was aimed at investigating short-term metabolic changes caused by S-methylmethionine (SMM) and at clarifying the gene expression background of these changes in order to gain a better understanding of the protective effect of SMM against stress. When examining the expression of genes coding for the enzymes responsible for the biosynthesis of polyamines, which play an important role in responses to low temperature stress, and that of the C-repeat binding transcription factor (CBF1) gene, it was found that both SMM and cold treatment increased the expression of genes responsible for the polyamine synthesis pathway starting from arginine. It caused only a slight increase when applied alone, but when SMM pre-treatment was followed by cold stress, it resulted in a considerable extent of up-regulation. SMM caused a similar increase in the expression of CBF1. The changes in the expression of genes responsible for the polyamine synthesis were clearly reflected in changes in the putrescine and agmatine contents, while the greater increase in the spermidine content was indicative of the role of SMM as a direct precursor in spermidine biosynthesis. The results demonstrated that, in addition to its direct effect on the sulphur metabolism and on polyamine biosynthesis, the protective effect of exogenous SMM was chiefly manifested in its influence on the expression of genes responsible for the biosynthesis of the polyamines important for stress responses and on the CBF1 transcription factor gene that acts as a regulator in cold stress.

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