The Characteristics And Functions Of Expansins In Wheat Coleoptiles

The growth is the essential phenomenon of plants, which includes cell extension and cell division. Previous studies about cell extension focused mainly on turgor pressure, while the plant cell wall behaves as a tough polymeric network to restrain cell turgor, wall yielding is generally recognized as a key control point for plant cell enlargement. Further more, cell wall proteins play a significant role in regulating cell wall extensibility. Expansins are cell wall proteins that induce pH-dependent wall extension and stress relaxation, by disrupting hydrogen bonds between cellulose microfibrils and matrix polymers, which play an important role in regulating plant development course and the response of plants to adverse environment. Since they are first isolated from cucumber hypocotyls, expansins have been identified in many plant species and organs.Wheat (Triticum aesitivum L) is the important crop worldwide. The growth and yield of wheat directly affect the food supply of people. Expansins have been identified in wheat. In this study, we demonstrate the identification, characteristics and functions of expansins in wheat coleoptiles, in particular, their response to water stress. The main results are as follows:1) Firstly, in order to identify and characterize expansins from wheat coleoptiles, we constructed an improved extensometer with a high-sensitive LVDT as the core of the apparatus, which could measure cell wall extension availably.2) The identification of expansins from wheat coleoptiles was confirmed on the basis of expansin activity assays, immunoblot analysis and antibody inhibitor.3) We adopted two methods of Hepes and SDS to extract expansin proteins and compared the efficiency of different methods on protein activity and abundance level. Our data suggested that SDS was more efficient than Hepes in extracting expansin proteins from wheat coleoptiles, whereas, expansin proteins extracted with Hepes possessed more activity, but there was little expansins activity extracted with SDS, that maybe because of being denatured by SDS. The expansins were specifically associated with the cell wall and were hardly detected in the soluble cytoplasmic fraction.4) Endogenous and reconstituted wall extension activities of wheat coleoptile expansins showed similar sensitivity to pH, with an optimum pH range of 4.0 to 4.5, a little difference from that of other expansins including cucumber hypocotyl. Once heat-inactivated walls have been pretreated with exogenous expansins, the reconstituted wall required no further expansin for extension. Native cell wall extension was observed in acidic solution and abolished in neutral solution, and this characteristic could be repeated by alternately switching incubation buffer again and again. Expansins extracted from wheat coleoptiles could induce cell wall extension of cucumber hypocotyls, and the converse was also true, albeit with differences noted in extension activity. Reconstituted wall extension of wheat coleoptiles suggested that expansin activity was induced by DTT, K+ and Mg2+, but inhibited by Zn2+, A13+ and Ca2+(10mM), similar to those found in cucumber hypocotyls.5) Cell division rarely occurs in wheat coleoptile. Its elongation is primarily attributed to cell extension and consequently makes coleoptile an ideal unit to investigate the mechanism of cell elongation. The currently available data showed a correlation between expansin expression and temporal growth of wheat coleoptile, and the growth of wheat coleoptile was mediated by the amount and the activity of expansins. These results suggest that expansins play a significant role in mediating elongation of wheat coleoptiles.6) The coleoptiles are the protective tissue of cotyledon, and the growth inhibition of plant cell is one of the most sensitive responses to diverse environment, while expansin genes are differentially regulated by environmental signals, evaluating the role of expansin in wheat coleoptile is useful to understand the molecular mechanism of growth response to diverse environment in wheat. This study revealed that coleoptiles grown under low water potential were more extensible than high water potential, and the expression of coleoptile expansins was increase in response to water stress. Considering that the growth of wheat coleoptiles is slow under water stress, we speculate that besides expansins, other factors including cell turgor and the cell wall susceptibility to water stress are also involved in the process of coleoptile growth. It is necessary to study further the relationships between drought resistance of wheat and expansins activity, cell turgor, and the cell wall susceptibility to water stress using different drought resistant cultivars or species as experimental materials.7) Expansins activity and abundance in wheat coleoptiles were response to light, suggesting that expansins may involve in the process of light to plant growth light.