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Authors: ORSOLYA KINGA GONDOR, TIBOR JANDA, GABRIELLA SZALAI.
Salicylic acid (SA) was isolated first from bark of the willow tree (Salix) and used as a medicine. Its biosynthesis is part of the phenylpropanoid pathway where its first precursor is the phenylalanine. Two routes from phenylalanine to salicylic acid have been described that differ at the step involving hydroxylation of the aromatic ring. Phenylalanine is converted into cinnamic acid (CA) by phenylalanine ammonia lyase. Cinnamic acid can be hydroxylated to form ortho-hydroxycinnamic acid (oHCA) followed by oxidation of the side chain. Alternatively, the side chain of cinnamic acid is initially oxidized to give benzoic acid (BA), which is then hydroxylated in the ortho position. Flavonoids are important protectants during stresses and their biosynthesis derived from the cinnamic acid via trans-hydroxycinnamic acid. The aim of the work was to investigate physiological/biochemical processes induced by the different exogenous SA treatments. Mv Emese winter wheat variety was used for the experiments. Plants were grown in hydroponic solution. SA treatment was carried out either by soaking seeds in 0.5 mM SA for overnight before sowing or by addition of 0.5 mM SA to the hydroponics of seven-day-old plants for a day. Leaf and root samples were collected after 1 and 7 days of the hydroponic SA treatment. For detection of the oxidative stress the lipid peroxidation was measured via malondialdehyde (MDA) content spectrophotometricaly. 0.5 g plant material was used for determination of SA, BA, CA, oHCA and flavonoids. Methanol soluble free, methanol soluble bound and methanol insoluble bound fractions were measured. The analysis was carried out using an HPLC equipped with a UV-VIS and fluorescence detector. It can be seen from the results that the level of SA and its precursors changed after the treatments. The level of BA, CA did not change while the SA and oHCA content increased after 1 day of SA treatment in the leaves. The MDA concentration also increased compared to the control which alludes to the increased oxidative stress. oHCA can serve as an antioxidant so its elevated level can be a consequence of the stress. The BA content decreased, SA content increased while the oHCA content did not change in the methanol soluble bound fraction. The level of SA, BA, CA and oHCA did not change in the methanol insoluble bound fraction. Seed soaking did not have any effect on the content of the endogenous SA and its precursors in either of fractions. Free oHCA increased after seven days but there was no change in the bound fractions. The SA level increased in the case of hydroponic SA treated plants in all the fractions while the BA and CA content remained at the initial level. The CA content decreased while the oHCA content increased in the free fraction in the roots of the hydroponic SA treated plants after one day. The MDA concentration also increased as a marker of the oxidative stress. The SA content increased in all the fractions. The level of free SA increased after the seventh day of hydroponic SA treatment while the CA and BA did not change. Some of the flavonoids were also analysed. Myrecetin (M), kaempferol (K), quercetin (Q) and rutin (R) were measured. The M, Q and R level increased in the leaves after the SA treatments. R and K level decreased in the free fraction in the roots after one day and it still remained at this level for seven days compared to the control values. The content of the methanol soluble bound M and R decreased after one day but increased after seven days. The Q concentration increased only in the free fraction of the leaves after one day and a decrease could be observed in the roots in all the fractions. The level of free rutin slightly increased but it decreased in the roots after one day. An increase could be seen in the free rutin content in the roots after seven days. Acknowledgemens. I would like to thank the Department of Plant Physiology for help. This work was supported by the Hungarian National Scientific Research Fund (OTKA K101367).
