| Drought is an important limiting factor for crop growth and yield. In recent years, the occurrence of drought in the Yangtze River of China is increasing. Rapeseed, as the main winter crop of this region, is also harmed by the autumn or winter drought, harming the emergence of rapeseed and affecting the yield severely. Furthermore, plant hormone ABA plays a large role in the drought tolerance and induing the gene expression related to drought tolerance. Recent years, people has done lots of research work about the stress hormone ABA in respone to abiotic stresses especially the drought signal transduction and application of ABA to improve crop resisitance, and also made many achievements. However, the many limitations of ABA itself have limited its application in the agricultural disaster mitigation aspects seriously. At this time, the emerging of AM1, an ABA analog, offseted the defects of ABA and had a broad market prospect. As a new type of ABA analogues, the actual drought-resistant effect of AM1 remains to be verified, and the mechanism of ABA in improving the drought resistance of crop still needs to explore further. In view of the above considerations, this research used the pot experiment and selected the different drought-tolerant genotypes of rapeseed, one is Qinyou 8(sentive), another is Q2(resistant). And we studied the response mechanism to drought stress of the two genotypes and the mechanism of the exogenous sprayed ABA or AM1 in improving their drought resistance. We hope that these works could provide the theoretical basis for improving the drought resistance of crop and the technical support for the actual application of AM1. So we studied the response of rapeseed to drought and the effect and mechanism of AM1 to relieve the drought damage to rape using the morphological, physiological, biochemical and transcriptome sequencing methods. The main conclusions are as follows:1. Drought stress reduced the rape of relative growth rate, leaf area, dry matter accumulation and leaf relative water content, root moisture content, but increased the ratio of root to shoot; In addition, Exgenous ABA or AM1 application slowed their decreasing trend, increased the ratio of root to shoot further. The results of some agronomic parameters showed that: drought reduced the relative growth rate, leaf area, dry matter accumulation and leaf relative water content, root moisture content significangtly, the reduction percentages are Qinyou 8&Q2: 77.42&54.55%, 54.64&24.75%, 70.75&48.88%, 11.36&6.35% and 8.18&4.92%, especially Qinyou 8. Furthermore, the exogenous ABA or AM1 treatment slowed the reduced amplitude of corresponding parameters, and improved the root/shoot ratio further. However, their promoting effects on the root moisture content are not obvious.2. Drought stress damaged the photosynthetic apparatus of rape to some extent. The determination of photosynthetic parameters, chlorophyll fluorescence, chlorophyll content and the chloroplast ultrastructure by transmission electron microscopy observation showed that:Drought stress reduced the net photosynthesis rate, transpiration rate and stomatal conductance, the reduction percentages are 44.42 & 23.15%,49.71 & 28.37% and 66.64 & 47.16%, but increased the water use efficiency significangtly, especially Qinyou 8(Qinyou 8&Q2: 50.15&28.60%). Furthermore, drought has also decreased photosynthetic efficeincy(Fv/Fm), effective quantum yield of PSII(ΦPSII), photochemical quenching coefficient(q L), and electron transport rate(ETR), improved non-photochemical quenching(NPQ). The reduction percentages are Qinyou 8&Q2: 24.65&12.83%,18.62&8.52%,33.28&7.23%, 10.55&3.27% and 11.03&16.78%. The chloroplast TEM images showed that drought has damaged to the chloroplasts of rapes to some extent. The damage of Qinyou 8 is more serious. Specifically speaking, the chloroplast structure is fuzzy matrix, the number of granum thylakoids is little, starch grains were decomposed, platoglobuli became multi and small, and so on. Surely, the damage to Qinyou 8 was more serious. In addition, the ABA or AM1 treatment has decreased the net photosynthesis rate, transpiration rate and stomatal conductance further, while increased the water use efficiency further, the decreases of Fv/Fm and ΦPSII became lower. And the extent of the damage of chloroplast ultrastructure under the drought +ABA or drought +AM1 treatment was lower significantly than those under only drought.3. Drought stress could indue the occurrence of a series of physiological and biochemical reactions in rapes, and the treatment of exogenous ABA or AM1 changed the reaction speed or process. Firstly, drought stress caused the membrane lipid peroxidation: increased the contents of MDA and active oxygen(O2.- 、H2O2), especially in Qinyou 8. At this time, plants could start its antioxidant system(antioxidant enzyme: SOD, POD and CAT), and antioxidants: ASA and GSH) to reduce the damage by the reactive oxygen species(ROS) caused by drought. Specially, the activities of SOD、POD and CAT have been improved and the contents of ASA and GSH have been increased. The exogenous ABA or AM1 treatment promoted the role of antioxidant system, reduced the content of MDA and ROS further, then reduced the damage to rapes from ROS caused by drought. In addition, drought stress has changed the content and distribution of endogenous hormones in rapeseed, promoted the synergy between hormones: drought stress increased the content of ABA, JA and SA, but reduced the content of IAA, which indicated that drought stress led the hormone levels towards slowing the growth of plant and promoting stomatal closure. The exogenous sprayed ABA promoted the induction process(i.e., to improve the contents of ABA, SA and JA, decrease the content of IAA further); However, While the treatment of AM1 has promoted the formation of JA and SA and reduced the content of IAA, had not promote the generation of endogenous ABA. It may be interpreted as an alternative mechanism of competition. Furthermore, drought stress significantly up-regulated expression of the Bn NCED3(the key gene coding endogenous ABA biosynthesis enzyme), Bn RAB18(ABA responsive key genes in drought signal pathway) and Bn PYL5(encoding ABA downstream of the receptor protein gene). And exogenous ABA treatment promoted up-regulated expression of the three genes further under drought stress, however, exogenous AM1 treatment only up-regulated expression of Bn RAB18 and Bn PYL5, but down-regulated expression of Bn NCED3, which may explain the changing trend of the endogenous ABA content. Finally, drought stress also induced the formation of osmotic adjustment substances(proline and soluble sugar), and the effect of exogenous ABA or AM1 to enhance the content of the two rapes is obvious.4. The preliminary analysis of transcriptome data showed that drought stress could induce the related gene expression, and the numbers of gene expression is little under the three days-drought treatment. Furthermore, the numbers increased several hundred times with the drought time prolonged or the degree depened. There were differences between Qinyou 8 and Q2. In addition, for Qinyou 8, the comparison between drought+ABA and drought+AM1 showed that: the two treatments could induce the same gene expression under the same drought degree(same: 5389; difference: 130). So we can say that the AM1 could induce the same gene expression in response to drought stress as ABA. |
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