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Abstract
Drought stress in plants is accompanied by several metabolic changes. One of them is the appearance of N-malonyltryptophan (MT) during leaf wilting of many species, but there is a significant number of plant species in which the appearance of MT did not occur. Plants of some species were able to synthesize also N-acetyltryptophan (AT). Excised tomato leaves incubated with D-amino acids (including D-Trp) transform them into malonyl- and acetyl-derivatives even without water deficit. However, MT which appeared during water deficit has been shown to contain L-Trp. Amino acid—1-amino-cyclopropane-1-carboxylic acid (ACC) is also malonylated during water deficit, but other L-amino acids were not malonylated. N-malonyl transferases specific for Trp and ACC have been found in several plants. The existence of N-malonyltransferase specific to L-Trp and appeared during water deficit in plants forming MT is supposed, but clear experimental proof has not been obtained yet. Plants can transform MT applied exogenously into Trp and further to indole-3-acetic acid (IAA). But no evidence has been appeared up to now that endogenous MT may be a source of IAA. It is unknown till now why it is necessary for plants of many species to malonylate only Trp during water deficit. How MT metabolized in animals and if it affects them is also unknown. The necessity to use molecular-genetic approaches for the elucidation of the physiological significance of MT formation during water deficit is underlined.
Highlights
2) Drought stress induces the synthesis of MT in plants of many species, but some numbers of them do not react to it by this way
3) Almost all higher plants can malonylate exogenous D-Trp and other D-amino acids in turgescent and wilted leaves, but MT which appeared during water deficit is synthesized from L-Trp
4) Leaf wilting does not cause N-acylation of other L-amino acids besides Trp. 5) Exogenous MT can release Trp which transformed further to indole-3-acetic acid (IAA), but no data found that this may occur from endogenous MT appeared during leaf wilting
Summary
Trp is one of 20 amino acids participating in protein synthesis and it is very important for plants. Trp is the precursor of IAA, melatonin (AMT), glucosinolates and other substances which participate in growth and development regulation and the resistance to drought stress. AMT increased the plant resistance to some abiotic and biotic stresses: drought, cold, chemical, salt, fungi and bacteria. It caused positive effects on the crop productivity [4, 5], In Brassicaceae, drought stress resistance was increased due to the induction of significant accumulation of Trp metabolite glucosinolates (cited from [6]), and it may be supposed to be a consequence of the increased Trp content. Trp content in plants is usually low when compared with the contents of other amino acids. It amounts to 1 - 10 ηmoles per 1 g fw in growing vegetative plant parts [1, 7], whereas other amino acids such as Asp, Glu, Pro and Ala are contained in quantities of 100 - 1000 ηmoles/mg fw [1, 8]
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