| Oxygen is essential for the growth and metabolism of living cells in higher plants.Tomato(Solanum lycopersicum L.)is one of the most common cultivated vegetables in the world.However,the root system often suffers from hypoxia problems such as soil waterlogging,high groundwater level,and tight soil texture in the conventional cultivars of tomato.In the soilless cultivation,there is also face insufficient oxygen in the hydroponic environment,resulting in hypoxia stress,which seriously affects the normal growth,development and physiological metabolism of tomato.Therefore,it is very significance and practical value to study the effect of oxygen on the growth and development of tomato and the mechanism in response to hypoxia.In this study,17 varieties from different sources of tomato were used as the test materials.In the seedling stage,four kinds of rhizosphere oxygen concentration treatments:low oxygen(nitrogen to nutrient solution,maintaining oxygen concentration between 1.5-2 mg/L),normal(air to nutrient solution,maintaining oxygen concentration around 8.5mg/L),medium oxygen(oxygen to nutrient solution,maintaining oxygen concentration around 20mg/L)and high oxygen(oxygen to nutrient solution,maintaining oxygen concentration around 34mg/L)were carried out to test the materials under the hydroponic nutrient solution culture condition.Next,the response effects of materials with different oxygen sensitivity to rhizosphere oxygen concentration were measured,and then the molecular mechanism of tomato insensitivity to low oxygen were investigated.The results were as follows:(1)17 tomato varieties were cultivated 12 days,with the different rhizosphere oxygen concentration,for measuring the morphological indexes such as leaf number,plant height,stem diameter,root length,root dry weight,shoot dry weight,total dry weight,and chlorophyll content of tomato were investigated.Then oxygen sensitivity of tomato varieties was evaluated by subordinate function value,strong seedling index and principal component analysis.The results showed that‘Fenzhenzhu'was high oxygen sensitive variety,Huangbaoshi'was medium oxygen sensitive variety,‘Henan No.4'was a normal oxygen sensitive variety,' Huangzhenzhu' was hypoxic sensitive variety,and '178'was insensitive to oxygen variety,respectively,(2)The representative varieties with different oxygen sensitivities,such as'Fenzhenzhu'(high oxygen sensitivity variety),'Henan No.4'(normal oxygen sensitivity variety)and 178'(oxygen insensitivity variety)were used as the test materials.Different rhizosphere oxygen concentrations were carried out to investigate the indexes,such as dry weight of root and shoot,plant height,stem diameter,longest root length,root surface area,root volume,root diameter,root activity,root anatomical structure,photosynthetic pigment content,hormone content,O2 flux,and so on to study the response of growth development and physiological characteristics to rhizosphere oxygen concentration.The results showed that the tomato varieties with different oxygen sensitivities were response differently to the rhizosphere oxygen concentration,and the roots with average diameter equal or lesser than 1.0mm played an important role in the tolerance of low oxygen stress.The content of GA and ABA were no difference between'Fenzhenzhu' and ' 178',whereas the IAA content was significantly decreased in'Fenzhenzhu',while IAA content was slightly increased in ' 178',but not reached to significant level under low oxygen treatment.Inflow rates of O2 increased by 120.25%after hypoxic treatment compared with that of normal oxygen in 'Fenzhenzhu' roots,while only increased by 60.16%after the hypoxic treatment compared with that of control in ' 178 ' roots,obviously lower than that of 'Fenzhenzhu',which further proves that‘178'is more tolerant to hypoxic stress.(3)Oxygen insensitive variety '178'and high oxygen sensitive variety‘Fenzhenzhu'were selected to analyze tomato seedlings in response to different rhizosphere oxygen concentration by RNA-seq.There were 3985 and 5040 genes differentially expressed under hypoxia treatment in 'Fenzhenzhu' and‘178',while 1145 and 2740 genes differentially expressed under medium oxygen treatment in ' Fenzhenzhu'and'178',respectively.In the 1178 differentially expressed genes shared by'178' and'Fenzhenzhu ' under hypoxia treatment,genes involved in biosynthesis and signal transduction of plant hormones,including IAA,JA,GA,Eth,ABA and SA,which may play important roles in tomato seedlings in response to different rhizosphere oxygen concentration were significantly enriched.Furthermore,differentially expressed genes involved in carbon metabolism were also got enriched in‘178'under hypoxia treatment,indicating that carbon metabolism related pathways,such as glycolysis,could be activated to provide more energy for tomato plants to resist the adverse environment of hypoxia in'178'.In addition,genes involved in phenylpropane biosynthesis were also got enriched in '178'under medium oxygen treatment,which implied that biosynthesis of phenylpropane played certain roles in '178'under medium oxygen treatment.(4)Oxygen insensitive variety '178'was used to investigate the differentially expressed miRNAs response to hypoxia of tomato by high-throughput sequencing technology.A total of 33 known miRNAs were lowly expressed,whereas only three miRNAs showed higher expression in hypoxia-treated '178' root compared with untreated‘178'root.At the same time,two novel miRNAs were also validated by bioinformatics.Furthermore,two known and lowly expressed miRNAs,miR171 and miR390,were deactivated by Short Tandem Target Mimic(STTM)technology in Arabidopsis.And the results indicated that the number and length of lateral roots were more in STTM171 and STTM390 transgenic lines compared with that of wild type plant,which partly phenocopy the increase root number and shortening the root length in hypoxia-treated tomato root.All these results indicated that the differentially expressed miRNAs between hypoxia-treated tomato and control root,which contribute to the auxin homeostasis,morphologic change,and stress response,might result in reduction in the biomass and length of the root in hypoxia conditions. |
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