The Involvement Of Expansins In Response To Water Stress During Leaf Development In Wheat

Drought is one of the adversities that crops frequently encounter during growth and development. In recent years, the drought caused by climate change are becoming more frequent. Wheat is a worldwide crop, whose growth and yield are directly related to the human food supply. However, water stress can seriously inhibit the normal growth and development of wheat and affect the quality and yield. The organic matter needed for wheat growth and grain filling depends mainly on the leaf supply, as this is the primary organ for manufacturing carbohydrates. Understanding the determinants of wheat leaf growth and their response to drought has great significance in production.Leaf growth mainly relates to cell division and cell elongation, and the elongation growth of the cells is more sensitive to drought stress than cell division. Cell elongation depends on loosening of cell walls, uptake of water into expanding cells and synthesis of cell wall polysaccharides. Because cell turgor pressure changes little along the growing region, the primary factors that regulate plant growth are most often attributed to cell wall loosening and cell wall extensibility.Expansins are a family of cell wall proteins proposed to play a key role in the regulation of tissue elongation and cell wall differentiation. Expansin is hypothesized to induce an increase in cell wall extensibility in vitro and cell expansion in vivo, by breaking hydrogen bonds between hemicelluloses and cellulose microfibrils. During growth, plant cells secrete expansin, which unlocks the network of wall polysaccharides, permitting turgor-driven cell enlargement. Various analyses have revealed that expansins are expressed in all expanding parts of plants or organs that undergo cell wall modifications. Meanwhile, there is increased evidence showing that expansins are vitally involved in the responsive mechanisms of plant species to abiotic stresses.In this study, we used two wheat cultivars, a drought-resistant cultivar HF9703 and a drought-sensitive cultivar 921842, as material to study the role of expansin on cell growth of wheat leaf under different water stress condition. The purpose of paper was to analyse the function of expansin in drought tolerance of wheat. The main results are as follows:1. The expansin activity was changed with the development of wheat leaves, which was higher in days 6 to 8 from leaf 3 initial, consistent with the relative elongation rate of leaf 3. Meanwhile, five expansin genes from wheat reached maximum level at about the 6th day from leaf initial, this time point is consistent with the maximum length of lamina epidermis. These results suggested that the major involvement of expansin in leaf growth.2. Water restriction negatively affected wheat leaf growth in a dose-dependent pattern, acid-induced cell wall extensibility was also reduced after water stress treatment, but not in a dose-dependent pattern. However, moderate water stress up-regulated expansin activity, in particular in the leaf segment of 40-80 mm, but intermediate and severe water stress down-regulated expansin activity. At the same time, cell wall susceptibility to expansin raised with increased degrees of water stress. Although acid-induced cell wall extension was down-regulated by water stress, it could maintain at a certain level when water stress was strengthened, which was possibly due to the increased cell wall susceptibility to expansin with increased water stress. The growth of wheat leaves was almost recovered to the control level after treated by moderate water stress and re-watering, but this accelerated growth was not observed in intermediate and severe water stress. This phenomenon was possibly due to the increased expansin activity and expression level of expansin genen under increased water stress. These results suggested that expansin and cell wall susceptibility were involved in the response of wheat leaf growth to water stress.3. The length of leaf 3 in the drought-resistant line HF9703 and the drought-sensitive line 921842 were inhibited by 13.1% and 26% under moderate water stress conditions, respectively. A higher relative elongation rate was observed in HF9703 at the earlier stage during water stress treatment, and even at day 8, HF9703 could maintain a certain elongation rate under moderate and intermediate water deficit conditions, but a rapid decline occurred in 921842, suggesting different water stress tolerance between HF9703 than 921842. Furthermore, the expansin activity and transcriptional level in HF9703 were always higher than that in 921842 whether it is normal water condition or water stress condition. The drought resistance of HF9703 was higher than 921842, associated with higher expansin activity and transcriptional level in leaves of HF9703.