The Involvement Of Expansin In Wheat Cell Elongation Growth Regulated By Plant Hormone Under Water Stress

Drought is one of the important environment stresses during plant growth, and the disaster due to drought become more and more severe and frequent. Wheat (Triticum aesitivum) is the important food crop world widely. Water deficit is one of common stresses during wheat growth, and the lost of wheat yield by water deficit is very large. Therefore, it is very important and significant to study the mechanism of drought tolerance in wheat and to breed the new drought tolerant wheat cultivars.The growth inhibition is an essential and obvious phenomenon of plants under drought stress. The plant growth process includes cell extension and cell division, and cell extension is more sensitive to drought stress than cell division. Cell extension growth attribute to two factors, including turgor pressure and cell wall rigidity. Previous studies about cell extension focused mainly on turgor pressure, and suggested that turgor pressure play a leading role in cell elongation growth. Some studies indicated that plant growth inhibition under water stress may because of turgor pressure decrease. But plant cell wall yielding is generally recognized as a key factor for plant cell enlargement in recent report. More and more studies suggested that cell wall loosening and extension play an important role in response of plant to water stress. Expansin plays an important role in regulating cell elongation growth. Expansin is a cell wall protein, and it induces acid-dependent cell wall extension, by disrupting hydrogen bonds between cellulose microfibrils and matrix polymers. Some studies suggested that expansin was involved in the response of plant to drought stress.As signal factors, plant hormones are involved in the many processes of plant growth and development and play a significant role in plant drought tolerance. Many expansin genes expression were regulated by plant hormones. So if expansin is involved in the plant drought tolerance by plant hormones or not? The fact that wheat coleoptile growth is mainly dependent on cell elongation makes it an ideal model to study the mechanisms of cell elongation. In this paper, we used coleoptiles in 2 different wheat cultivars, a drought resistant cultivar HF9703 and a drought sensitive cultivar 921842, as material to study the effects of IAA and ABA on cell growth and expansin activity under different water stress condition. The purpose of paper was to analyze the role of expansin in drought tolerance regulated by plant hormones. The main results are as follows:1. Expansin was involved in wheat coleoptile cell growth under water stress(1) Expansin activity was associated with wheat cell elongation growth under water stressUnder drought stress condition, the growth of wheat coleoptiles was inhibited. The coleoptiles length in HF9703 was longer than that of 921842. The antibody of expansin, inhibitor of expansin function, inhibited the coleoptile growth, which suggested that expansin was involved in regulation of coleoptiles growth. Drought stress did not influence the susceptibility of expansin to cell wall, but induced the expansin activity increase. Moreover, the expansin activity in HF9703 was obviously higher than that in 921842. These results indicated that expansin was involved in the response of wheat coleoptile cell elongation to drought stress.(2) The up-regulation of expansin expression under water stress enhanced expansin activityWestern-blot analysis indicated that, water stress induced the increase of expansin expression in wheat coleoptiles. The expansin abundance in HF903 was more than that in 921842. This result was coincided with the response of expansin activity to water stress. Moreover, the recovery experiment indicated that expansin accumulation under water stress in wheat coleoptiles enhanced the rate of coleoptile cell growth. So these results suggested that the accumulation of expansin under water stress promoted the expansin activity, and expansin may play an important role in wheat drought tolerance.(3) Cell wall alkalinization induced by water stress was unfavorable to expansin activity and cell growthExpansin activity was dependent on acid condition. The detection of cell wall acidification and PM H+-ATPase activity indicated that Cell wall acidification was decreased by drought stress, which may be due to the decrease of PM H+-ATPase activity. Cell wall alkalinization was negative to expansin activity and cell elongation growth. Under water stress, exogenous acid condition promoted wheat coleoptile growth, which illustrated indirectly that cell wall alkalinization was associated with coleoptile growth inhibition. Water stress induced the expansin activity which enhanced the cell wall yielding and decreased the resistance of cell growth. However, the inhibition of coleoptile growth by water stress may be due to other factor, e.g. the decrease of cell turgor, which is the power driving cell elongation. The increase of expansin activity could relieve the inhibition of drought stress to cell elongation.2. Expansin was associated with wheat drought tolerance regulated by ABA(1) The using of exogenous ABA suggested that ABA was involved in the growth inhibition of wheat coleoptiles. Water stress induced the accumulation of ABA in coleoptiles and the content of ABA was more in HF9703 than that in 921842. FLU, the inhibitor of ABA synthesis, alleviated the growth inhibition induced by drought stress. It was suggested that ABA was involved in wheat coleoptile cell growth inhibition under water stress.(2) Exogenous ABA treatment induced the increase of expansin activity, which was consistent with the response of expansin activity to drought stress. FLU restrained the promotion of expansin activity by drought stress. Western-blot analysis suggested that ABA induced the increase of expansin expression, which may be the main reason of expansin activity increase.(3) ABA increased cell wall pH by decreasing PM H+-ATPase activity, which was negative to expansin activity. The PM H+-ATPase activity and cell wall acidification of HF9703 under water stress were higher than that of 921842. Cell wall alkalinization resulted in expansin activity decrease, which was negative to cell elongation growth.3. Cell wall acidification increase by IAA was favorable to expansin activity and cell growth(1) In detached coleoptiles, exogenous IAA promoted wheat coleoptile growth and increased expansin activity. The detection of cell wall acidification and PM H+-ATPase activity indicated that IAA induced the cell wall acidification and PM H+-ATPase activity increase. But IAA did not influence expansin expression obviously. It was suggested that IAA promoted expansin activity by regulating cell wall acidification and enhanced the cell growth of wheat coleoptiles. (2) However, IAA applied to the root of seedlings hardly affected the growth of coleoptiles, and expansin activity and abundance was not changed obviously. This result may attribute to absorption and transport of IAA in plant. The level of IAA in coleoptiles after IAA applied to the root was not changed obviously, which elucidates this possibility.(3) Under water stress, the level of IAA in wheat coleoptiles did not change. Exogenous IAA applied to the root did not influence the inhibition of coleoptile cell growth by water stress. Therefore, the function of IAA in wheat drought tolerance was not clear.4. The response of expansin genes to drought stress and IAA or ABA treatmentExpansin was coded by a large family. Six expansin genes was cheesed from different subfamilies in wheat, including 3 -expansin genes, TaEXPA1, TaEXPA2 and TaEXPA3, 3β- expansin genes, TaEXPB23, TaEXPB1 and TaEXPB2. The response of expansin gene expression to drought stress and IAA or ABA treatment was analyzed by RT-PCR. The results showed that TaEXPA1 and TaEXPA2 did not express in wheat coleoptiles and were not induced by drought stress or ABA/IAA treatment. The expression of else other 4 expansin genes under different condition was different from each other. TaEXPB23, TaEXPB1, TaEXPB2 and TaEXPA3 were up-regulated by drought stress, but did not affect by IAA. ABA induced the expression of TaEXPB23, TaEXPB2 and TaEXPA3 increase, but did not influence the expression of TaEXPB1. It was supposed that expansin in wheat was a large family and different member was control by different factors. Some expansin genes respond to drought stress and ABA treatment, which suggested that expansin was an important factor in wheat drought tolerance.