Identification And Related Mechanisms Of Waterlogging Tolerance At The Seedling Emergence Of Oilseed Rape

Yangtze River Basin is the most important oilseed rape production region in China,and the main farming system is"rice-rape"rotation for the oilseed rape production in this region.However,continuous rainfall often occurs during the sowing period of oilseed rape in this region,combined with the heavy soil texture,hard drainage in paddy field,which frequently results in waterlogging or flooding disasters and seriously affects the seedling emergence and reduces the seed production of oilseed rape.In this study,23 cultivars of Brassica napus those are planted in Yangtze River Basin were used as materials to identify the waterlogging tolerance of different cultivars at the radicle elongation stage by 3 waterlogging imposition methods.Based on this,the cultivars with different tolerances were selected and the appropriate identification method was established.Furthermore,the responses of seedling emergence,plasma membrane permeability and transcriptome of cultivars with different tolerances to various waterlogging stress intensities were compared.The regulation of waterlogging tolerance at the radicle elongation of a sensitive cultivar was also tested using aminooxyacetate（AOA）,an ethylene synthesis inhibitor.The main purpose of this study is expected to provide theoretical and technical guidelines for breeding new cultivars with high waterlogging tolerance and developing cultivation methods to combat waterlogging in oilseed rape production.The main results are as follows:1.By comparing the results of different methods,it was determined that waterlogging treatment for 36 h at the radicle elongation stage could be a reliable identification method.There are abundant genetic variations in waterlogging tolerance at radicle elongation stage among oilseed rape cultivars.The tolerant cultivar consumed less oxygen and showed lower membrane peroxidation and higher plasma membrane stability under waterlogging stress.2.The seeds of H32,a tolerant cultivar,and F958,a sensitive cultivar,were subjected to waterlogging for 12 h,24 h,36 h and 48 h,respectively at the radicle elongation,and then the transcriptomic analyses were performed on the different samples using the non-waterlogged seeds at the same stage as controls.The results revealed that oilseed rape was sensitive to the changes in ambient oxygen level and its ability to scavenge H₂O₂,a reactive oxygen species,was decreased by hypoxia.Different cultivars had a similar adaptation mechanism to waterlogging in DNA damage repair.Based on the functional annotation of genes,it was found that the differential expressions of phosphoethanolamine methyltransferase（PEAMT）and Lysophosphatidyltransferase（LPCAT）genes might be involved in the stability of membrane structure in cells of different cultivars under waterlogging stress.Through the screening and identification of candidate genes for ethylene metabolism and ethylene signal transduction pathway,it was found that low level ethylene synthesis may play an important role in the regulation of waterlogging tolerance in rapeseed at the radicle elongation.The timely and accurate expression response of key genes involved in ethylene biosynthesis may be an important mechanism for the tolerant cultivar to adapt to waterlogging stress.Ethylene responsive transcription factor（ERF）was significantly induced in the whole waterlogging process of the two cultivars,indicating that ERF may improve waterlogging tolerance of oilseed rape by inducing the expression of some hypoxia responsive genes.3.The pretreatment of seeds with suitable concentration of AOA significantly relieved the waterlogging stress on the seedling emergence of sensitive cultivar at the radicle elongation stage with 5 m M being mostly effective,compared with distilled water pretreatment,the emergence of waterlogged seeds was increased by 10.4percentages.Nevertheless,high concentration（20 m M）of AOA pretreatment significantly worsened the waterlogging stress on the seedling emergence.Pretreatments with different concentrations of AOA did not show adverse effects on the seedling growth after waterlogging and agronomic traits at maturity,indicating that AOA is safe to be used in the waterlogging tolerance regulation of oilseed rape at the radicle elongation stage.4.Observation on the cell ultrastructure showed that,the cells in radicle and germ of oilseed rape were damaged by waterlogging at the radicle elongation with enhancing damage as the stress intensity increases.The tolerance of the germ cells to waterlogging was stronger than that of the radicle cells in structure stability.Pretreatment with AOA significantly improved the integrity of the cell structure in radicle and germ of different cultivars under waterlogging by reducing the damage to plasma membrane and mitochondrial structures.Meanwhile,AOA induced the relative autophagic activity in the cells of radicle and germ to digest the damaged organelles and metabolites under waterlogging stress,thus enhanced the tolerance of oilseed rape.