Drought Resistance Character And Molecular Basis For Rapeseed(Brassica Napus L.) With Different Root System Architecture

Rapeseed is an important oil crop in China,and severe autumn and spring droughts occurred frequently during its growing season.The root system architecture plays a crucial role in plant resistance to drought,and root morphology is closely related to drought resistance.Therefore,elucidating the regulation mechanism of rapeseed root morphogenesis and its response to drought stress is significant in improving rapeseed yield under drought conditions.In this study,30 rapeseed materials were selected and explored to drought stress simulated by PEG 6000 hydroponic.The root morphology of these materials was investigated under control and drought at the seedling stage.According to the ratio of the primary root length to total root length,and the number of lateral roots,they were divided into three clusters:primary root type,primary and lateral root equilibrium type,and lateral root type.Thereafter,two materials with contrasting root architecture and drought resistance were screened out for the next experiment.The two-dimensional dynamic scanning and three-dimensional reconstruction were conducted to analysis the root growth changes.Meanwhile,endogenous root hormone and transcriptome were measured to clarify the regulatory mechanism of rapeseed root morphogenesis under drought stress.Besides,21 materials with similar growth stages were subjected to drought stress at the flowering stage by pot experiment to study the effect of water stress on the yield and yield components of rapeseed with different root system architectures.The main findings are as follows:1.Drought at the seedling stage significantly reduced the root length,the number of root tips and root surface area of most materials,and the reduction range of drought-resistant varieties was small or increased.In the seedling stage of rapeseed,PEG 6000 was used to simulate drought stress and the results showed that the treatment of15%（w/v）PEG 6000（-0.3 MPa,25°C）could significantly reduce the primary root length,total root length,the number of root tips,root surface area and root volume.We found that the primary root length,total root length,the number of root tips,root surface area,and root volume of the primary root type variety Fuyou4 were higher than those of the primary and lateral root equilibrium type variety CY12NY-7.Meanwhile,the root three-dimensional reconstruction results showed that the primary root length,total root length,the number of root tips,and root surface area of Fuyou4 increased under drought stress.It indicated that the drought-resistant variety can further improve its water absorption capacity or stimulate its stress-resistant growth potential under larger water deficit.2.Focusing on the drought-resistant varieties of the primary root type at the seedling stage may be an effective way to breed drought-resistant varieties.Comparing the biomass accumulation of the three types under drought stress,it can be found that the root to shoot ratio of primary root type materials was increased under drought stress at the seedling stage,and the decrease of root and shoot weight are 16.26%and 15.70%lower than that of equilibrium type material,and 3.87%and 14.21%lower than that of lateral root type material.Under flowering drought,the decrease of the dry weight of root,biomass yield and total biomass were 5.19%,6.54%and 5.96%lower than those of equilibrium type materials,and 32.73%,1.49%and 7.56%lower than those of lateral root materials.In addition,the yield per plant,biomass yield and harvest index of primary root type variety Fuyou4 under drought at flowering stage were 19.2 g,83.3 g and 0.23,respectively,which were significantly higher than those of the primary and lateral root equilibrium type variety CY12NY-7.The results indicated that the primary root type varieties with drought-resistant screened out under drought conditions at seedling stage might have higher biomass and yield under drought stress at different growth stages.3.Drought-resistant varieties should have comprehensive resistance to drought such as good root system architecture and high photosynthetic rate.The net photosynthetic rate,stomatal conductance,intercellular CO₂ concentration,transpiration rate and mesophyll conductance of different materials are decreased,but the instantaneous water use efficiency increased under drought conditions,and drought at flowering stage had a greater effect on photosynthesis of rapeseed.There were great differences among materials with three root system architecture,but the decrease in net photosynthetic rate of primary root type is the lowest under drought.For example,the net photosynthetic rate of gl302-1,primary root type,has a lower decrease under drought,and decreases by 29.3%and 49.5%under drought stress at the seedling and flowering stage,respectively,which were 8.02%and 16.35%lower than the average reduction of the used materials.Meanwhile,its yield increased by 9.1%under drought at the flowering stage,indicating that its comprehensive drought resistance and growth recovery ability are stronger.4.Transcriptome analysis revealed that hormones play an important role in root morphogenesis.Analysis of two varieties with different root traits and drought resistance showed that the differentially expressed genes were mainly enriched in response and resistance to stress,thylakoid and thylakoid membrane,photosynthesis and ADP binding.The KEGG pathway was mainly enriched in phenylalanine metabolism,photosynthesis,photosynthetic antenna protein,phenylpropane biosynthesis,and other metabolic pathways and cellular processes.Nine genes related to roots were identified by transcriptome analysis.The expression of auxin（IAA）related genes BnaC04g00740D,gibberellin（GA）related gene BnaCnng22380D and BnaCnng73240D,jasmonic acid（JA）related gene BnaC04g01170D and BnaA04g29730D,and brassinosteroids（BR）related gene BnaA03g52520D in Fuyou4 with long primary root length and strong drought resistance were in high levels.In conclusion,we found that the primary root type drought-resistant material selected at seedling stage could be considered as an excellent drought-resistant material if it still had high photosynthetic rate and growth recovery ability under flowering drought.The results showed that it is more likely to screen out drought-resistant varieties with higher biomass and yield under drought at different growth stages based on the ratio of primary root length to total root length and the number of lateral roots,combined with relevant indicators such as biomass under drought conditions at the seeding stage.