Study On The Physiological Response And Molecular Mechanism Of Grafted Cucumber Seedlings Under Low Temperature Stress

Cucumber(Cucumis sativa L.)is one of the main crops planted in facilities in North of China.It often happens cold injury in cold winter and spring,which eventually leads to yield and quality reduction of cucumber.But using black-seed pumpkin as stock for grafting is an effective way to improve cold resistance of cucumber.The lower temperature(18/12?)and critical low temperature(10/5?)are common phenomena of facility agriculture which were simulated by artificial climate chamber and the materials used black-seed pumpkin as stock and cucumber ‘Youbo1-5' as scion in this study.The purpose of this study was to investigate the response mechanism of own-rooted and grafted cucumber seedlings in the levels of endogenous hormones,anti-oxidation system,photosynthesis,and further explain the regulating mechanism of grafted cucumber with favorable chilling resistance from the gene transcription level.The main results are as follows:1.The lower temperature and critical low temperature stress decreased the dry weight accumulation of cucumber seedlings.But the dry weigh of roots,shoots and whole plant maintained higher values in grafted cucumber seedlings,the results suggested that grafting could significantly promote the growth of cucumber seedlings under low temperature stress.After low temperature stress,the contents and balance of endogenous hormones in cucumber seedlings changed.The contents of ABA,IAA and ZR increased in grafted cucumber seedlings,but the contents of GA3 decreased significantly.Compared with own-rooted cucumber seedlings,grafted plants enhanced ABA/GA3,but decreased ABA/ZR and ABA/IAA.2.With the degree and time of low temperature stress increased,own-rooted and grafted cucumber seedlings accumulated a lot of O2·-and H2O2 which could increase the membrane lipid peroxidation.Meanwhile,the contents of proline,soluble sugar and soluble protein increased significantly,and the activities of SOD?POD and CAT also improved.Grafting could regulate gene expression by promoting the activity of antioxidant enzymes and increase the contents of osmotic regulation substances in low temperature,this was the reason why the grafted plants showed a lower level of oxidative damage.3.The lower temperature and critical low temperature had various influences on photosynthesis of cucumber seedlings.The photosynthetic depression under lower temperature was due to stomatal factors.In order to accommodate the lower assimilatory power of cucumber under this circumstance,the plants down-regulated the gene expression of calvin cycle key enzymes which include Rubisco,SBPase,FBA and TK.The decreased of net photosynthetic rate was restricted by non-stomatal factors under critical low temperature stress,which might be the results of assimilation capacity declined.However,the grafted plants showed the greater photosynthetic capacity,because they could increase the chlorophyll content,the opening of stomates,the activities and gene expression of calvin cycle key enzymes.4.Under lower temperature stress,the donor and acceptor sides of PSII were not affected;although RC/CSo and PIABS tended to decline,they followed with a complete recovery at 25/18? after 2d.The results suggesting the function of PS? was not damaged under lower temperature.At the same time,in order to response the decline of carbon assimilation,ABS/RC,TRo/RC and ETo/RC all decreased significantly,but increased TRo/RC of cucumber seedlings.The critical low temperature caused irreversible damage of cucumber photosynthetic apparatus and decreased light energy utilization efficiency.The primary victims was the donor side of PSII,the electron transport of which was restrained.Meanwhile,as the critical low temperature changed PSII structure which reduced the storage of acceptor side,the electron transport of acceptor was also restrained.The grafted plants had a good performance on the light energy utilization under low temperature stress,since its opening degree and the electron transport ability of PS? reaction center increased.5.In this study,high-throughput sequencing technique was used to sequencing the mRNAs of grafted and own-rooted seedlings under critical low temperature and normal temperature.The expression profiles of the own-rooted and grafted seedlings under normal temperature were CO and CG,respectively which under low temperature and critical low temperature were LO and LG.The results showed that there were 1893 DEGs in CG vs CO,including 949 up-regulated genes and 944 down-regulated genes;CG vs LO had 5117 DEGs,including 2611 up-regulated genes and 2506 down-regulated genes;LG vs CG had 1724 DEGs,including 962 up-regulated genes and 762 down regulated genes;LG vs CO had 3119 DEGs,including 1504 up-regulated genes and 1615 down regulated genes;LG vs LO had 2737 DEGs,including 1511 up-regulated genes and 1266 down regulated genes;LO vs CO had 4786 DEGs,including 2246 up-regulated genes and 2540 down regulated genes.6.The enrichment of GO and KEGG in four libraries showed that there were 46 DEGs related to photosynthetic and 78 DEGs related to Carbon Assimilation Enzyme.In hormone signaling pathways,such as IAA,JA,GA,ETH and SA signaling pathways were more active,there were 47,45,26,49 and 32 DEGs respectively.7.69 gene families of transcription factors were identified in the four libraries,the gene number of each gene family ranged from 1 to 54.For example,the number of the AP2-EREBP transcription factor gene family was 54;the number of MYB transcription factor gene family was 46;the number of HB transcription factor gene family was 45;the number of bHLH transcription factor gene family was 42;the number of bZIP transcription factor gene family was 24;the number of AUX/ IAA transcription factor gene family was 20.