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Dehydrins (DHNs) are part of a large group of highly hydrophilic proteins known as LEA (Late Embryogenesis Abundant). They were originally identified as group II of the LEA proteins. The distinctive feature of all DHNs is a conserved, lysine-rich 15-amino acid domain, EKKGIMDKIKEKLPG, named the K-segment. It is usually present near the C-terminus. Other typical dehydrin features are: a track of Ser residues (the S-segment); a consensus motif, T/VDEYGNP (the Y-segment), located near the N-terminus; and less conserved regions, usually rich in polar amino acids (the Phi-segments). They do not display a well-defined secondary structure. The number and order of the Y-, S-and K-segments define different DHN sub-classes: Y(n)SK(n), Y(n)Kn, SK(n), K(n) and K(n)S. Dehydrins are distributed in a wide range of organisms including the higher plants, algae, yeast and cyanobacteria. They accumulate late in embryogenesis, and in nearly all the vegetative tissues during normal growth conditions and in response to stress leading to cellular dehydration (e.g. drought, low temperature and salinity). DHNs are localized in different cell compartments, such as the cytosol, nucleus, mitochondria, vacuole, and the vicinity of the plasma membrane; however, they are primarily localized to the cytoplasm and nucleus. The precise function of dehydrins has not been established yet, but in vitro experiments revealed that some DHNs (YSK(n)-type) bind to lipid vesicles that contain acidic phospholipids, and others (K(n)S) were shown to bind metals and have the ability to scavenge hydroxyl radicals [Asghar, R. et al. Protoplasma 177 (1994) 87-94], protect lipid membranes against peroxidation or display cryoprotective activity towards freezing-sensitive enzymes. The SK(n)-and K-type seem to be directly involved in cold acclimation processes. The main question arising from the in vitro findings is whether each DHN structural type could possess a specific function and tissue distribution. Much recent in vitro data clearly indicates that dehydrins belonging to different subclasses exhibit distinct functions.
A gene fusion system was used to study the expression pattern of the Dhn10 gene, encoding the DHN10 dehydrin protein in transgenic Solanum tuberosum plants carrying a combined GT-Dhn10 transgen in which the glucosyl transferase (GT) promoter region was fused to the coding sequence of the Dhn10 gene. Expression of the native Dhn10 gene and the GT-Dhn10 constructs was analysed in regenerated S. tuberosum transgenic plants, both at the transcript accumulation and protein levels. We showed that the expression of both the GT-Dhn10 transgen and the Dhn10 gene was regulated in the regenerated plants at the transcriptional level in an independent way, but only the protein product of the native Dhn10 expression was detected. The transcription product of the GT-Dhn10 transgen did not affect the expression of the Dhn10 gene either at the transcription level or at the protein level. The GT-Dhn10 plants did not show changes in freezing capacity compared to the control, non-transgenic ones.
This report deals with the effect of kinetin on the greening process, in relation to endogenous free polyamine levels and their metabolism in cucumber cotyledons. The kinetin response on free polyamine levels was found to be accompanied by an increase in free putrescine throughout the greening process. There was no significant difference in spermidine and spermine levels between control (water-treated) and kinetin-treated cotyledons; however, a slight increase in spermidine level, which was higher in control was observed at 4 h. In order to examine the action of kinetin on polyamine metabolism, particularly spermidine synthesis, the effect of kinetin on the level of S-adenosylmethionine decarboxylase mRNA and its enzyme activity were studied. First, an increase in the S-adenosylmethionine decarboxylase transcript level was observed at 30 min of illumination in water and kinetin-treated cotyledons, and next, the transcript decreased and was restored again at 2 h in kinetin-treated cotyledons and at 4 h in the control. This is the first report that demonstrates the light and kinetin regulation of S-adenosylmethionine decarboxylase transcript level. The highest S-adenosylmethionine decarboxylase activity was observed at 2 h of illumination, and it was higher in control when compared to kinetin-treated cotyledons. Spermidine and spermine levels observed in kinetin-treated cotyledons at 4 h of illumination may partly be a result of: lower S-adenosylmethionine decarboxylase activity inhibited by kinetin and/or higher by about 35% on kinetin polyamine oxidase activity. Experiments with methylglyoxal-bis (guanylhydrazone) and dicyclohexylamine showed that both spermidine synthesis inhibitors depressed chlorophyll accumulation in the greening cucumber cotyledons. Additionally, these results, indirectly confirm that polyamines may play some role in the greening process stimulated by kinetin.
The isolation of rye ß-Amy1 and ß-Amy2 gene promoters from nuclear DNA using the inverse polymerase chain reaction (IPCR) technique and characterization of their sequences are presented. The conservation of ß-amylasc coding sequences allowed for simultaneous IPCR amplification of two different promoters with primers designed on the basis of the single known cDNA sequence. Two ß-amylasc gene promoters display a low sequence similarity (47%). Beside consensus TATA and CCAAT boxes, other sequence motives common to both promoters were found. In addition, the homology of amino acid sequences of plant ß-amylases available in the database is discussed.
The expression pattern of a Solanum sogarandinum pGT::Dhn10 gene fusion encoding a dehydrin DHN10 protein and the potential role of that protein in cold tolerance in cucumber were analysed in three T1 transgenic lines. An accumulation of Dhn10 mRNA was detected in the leaves, cotyledons, hypocotyls and roots of the transgenic seedlings both under the control conditions and after a cold treatment at 6oC for 24 h. This was confirmed by RTPCR. However, no DHN10 protein was detected by the alkaline phosphataseconjugated antibody. The transgenic lines exhibited different levels of chilling tolerance. The TCC5/1 line showed a significant increase in its chilling tolerance compared to the non-transgenic line. No chilling injury was observed when the cold hardened (6oC, 24 h) TCC5/1 plants were subsequently exposed to a temperature of 2oC for 6 h. The other two transgenic lines, TCC2/1 and TCC3/2, exhibited a comparable level of chilling tolerance to that of the non-transgenic control. The transgenic lines showed similar or significantly decreased freezing tolerance compared to the non-transgenic control, as evaluated by an electrolyte leakage test. We concluded that the S. sogarandinum GT promoter is functional in the chilling sensitive species Cucumis sativus L., and that the pGT::Dhn10 gene fusion is expressed at the transcriptional level.
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