Physiological And Molecular Mechanism Of Grafted Watermelon In Response To Low Potassium Stress

Potassium is one of the necessary elements for plant,which plays an important role in plant growth and development.Watermelon is an important horticultural crop and often suffers from potassium deficiency in production of China.In this study,grafted watermelon was treated with short-term and whole growth period low potassium stress.The physiological and molecular mechanisms of grafting to improve the tolerance of watermelon to low potassium were elucidated from the aspects of growth,photosynthesis,endogenous hormones,fruit yield and quality,transcription group and fruit metabolism group of leaves and fruits.The main results of this study are as follows:1.Heterologous rootstock grafting maintains high photosynthetic characteristics,antioxidant activity and root potassium absorption efficiency of watermelon,which is key of rootstock grafting to alleviate the growth inhibition of low potassium stress on grafted watermelon seedlings.Watermelon plants[Citrullus lanatus(Thunb.)Matsum.and Nakai cv.Zaojia 8424],either self-grafted or grafted onto the rootstock‘Yongshi’(Citrullus lanatus sp.),‘Jingxinzhen No.1’(Lagenaria siceraria Standl.),and‘Qingyanzhen No.1’(Cucurbita maxima×C.moschata).Plants were subjected to 6.0mM K(normal K)and 0.1 mM K(low K)for 16 d.The leaf area,dry weight,root surface area,root average diameter,and root volume,which are higher in Z/Y,Z/J and Z/Q than in Z/Z,decrease under low potassium stress.Low potassium stress led to the decrease of photosynthetic rate and the accumulation of ROS in grafted watermelon,which caused membrane lipid peroxidation.Heterologous rootstock grafting increased the activities of antioxidant enzymes(SOD,CAT,and APX)of grafted watermelon seedlings under low potassium stress,effectively eliminated ROS,and decreased the degree of membrane lipid peroxidation,so that it could maintain normal growth and development under low potassium stress.Heterologous rootstocks use their strong roots to improve potassium uptake efficiency in low potassium stress.2.The differential expression of photosynthetic and redox related genes in leaves is the main molecular mechanism of low potassium tolerance in grafted watermelon.Comparing the two transcriptomes of grafted watermelon leaves treated with normal potassium supply(6 mM K~+)and low potassium stress(0.1 mM K~+)for 16 days.3223differentially expressed genes(699 down-regulated and 2524 up-regulated)were detected in Z/Z leaves.2887 differentially expressed genes(715 down-regulated and2172 up-regulated)were detected in Z/Q leaves.GO gene function analysis showed that the functions of differential genes were mainly involved in redox,carbohydrate metabolism,protein modification and phosphorylation,cell recognition and pollen recognition,photo reaction of photosynthesis and photosynthetic electron transport.KEGG metabolic pathway analysis showed that differentially expressed genes were significantly enriched in metabolic pathways such as photosynthesis,biosynthesis of secondary metabolites,biosynthesis of amino acids,starch and sucrose metabolism.The down-regulation of photosystem I,photosystem II,electron transport chain and carbon assimilate metabolism-related genes induced by low potassium stress is an important mechanism of photosynthetic decline induced by low potassium stress.Low potassium stress induced the up-regulation of antioxidant-related genes in the transcription group of heterologous rootstock grafted watermelon leaves,which may be the reason of the antioxidant activity of heterologous rootstock grafted watermelon was higher than that of self-grafted watermelon.3.Stock grafting improved single fruit weight and fruit quality of watermelon under low potassium stress.To study the effects of low potassium stress on the yield and quality of grafted watermelon fruit,four grafted combination watermelon(Z/Z、Z/Y、Z/J and Z/Q)were cultured with substrate and treated with normal potassium supply(6 mM K~+)and low potassium stress(0.1 mM K~+)for 60 days.The single fruit weight and fruit quality parameters of grafted watermelon pollinated after 30 d were decreased under low potassium stress.The potassium uptake and single fruit weight of Z/Y,Z/J,and Z/Q were significantly higher than those of Z/Z,and the relative single fruit weight was significantly higher than that of Z/Z in both potassium concentrations.Under low potassium stress,the contents of soluble solids,sucrose,Vc andβ-carotene of heterologous rootstock grafted watermelon fruit were higher than those of self-grafted watermelon.Heterologous rootstock alleviated the inhibitory effect of low potassium stress on the growth and development of grafted watermelon by absorbing K~+efficiently by roots and maintaining a high content of ZR(zeatin nucleoside)in shoot under low potassium stress.4.The fruit transcription group of self-married watermelon was more sensitive to low potassium stress than self-grafted watermelon seedling.Comparing the four transcriptomes of grafted watermelon fruit treated with normal potassium supply(6mM K~+)and low potassium stress(0.1 mM K~+).Under low potassium stress,There were 670(451 up-regulated,219 down-regulated),27(7 up-regulated,20 down-regulated),16(6 up-regulated,10 down-regulated)and 15(11 up-regulated,4 down-regulated)genes differentially expressed respectively in the fruits of grafted watermelon Z/Z,Z/Y,Z/J and Z/Q.Comparing the heterologous rootstock grafted watermelon fruit transcriptome was more sensitive to low potassium stress.GO gene function analysis showed that there were fewer enriched genes in Z/Y,Z/J,and Z/Q,and their functions were similar,which were mainly enriched in metabolic process and catalytic activity.There are 480 differentially expressed genes enriched in Z/Z,which are mainly enriched in biological processes such as glycerol phosphodiesterase activity,polyol metabolism,carbohydrate metabolism and so on.KEGG metabolic pathway analysis showed that the differentially expressed genes in Z/Z were mainly enriched in taurine metabolism,DNA replication,photosynthetic,plant hormone signal transduction,carbon fixation in photosynthetic organisms,nitrogen metabolism.5.Heterogenous stocks can increase the content of some functional substances in grafted watermelon fruit.38 metabolites were identified in fruits of four grafted watermelon treated with normal potassium supply(6 mM K~+)and low potassium stress(0.1 mM K~+),including 15 amino acids,8 organic salts,6 sugars,4 nucleic acids,and5 other compounds.The five most abundant metabolites are glucose,fructose,sucrose,citrulline,and glutamine.Heterologous rootstock grafting can increase the content of some functional substances in watermelon fruit.Under normal potassium supply,wild watermelon rootstock increased adenosine and methionine content,gourd rootstock increased adenosine,citrulline,glucose and methionine content,pumpkin rootstock increased asparagine content,Contents of citrulline,fumaric acid,glutamine,isoleucine,methionine,NAD+,phenylalanine,leucine,and valine.Under low potassium stress,wild watermelon rootstock increased the contents of asparagine,citrate,malate,NAD+,inositol,and alanine,while gourd rootstock increased the contents of asparagine,galactose,malate,and alanine.Pumpkin rootstock increased the content of inositol,malate,and asparagine.To sum up,the rootstock can improve watermelon tolerance to low potassium.The high root potassium absorption efficiency and the antioxidant enzyme activity,the high photosynthetic capacity and zeatin nucleosides content in leaves of grafted watermelon under low potassium stress,were the key for the tolerance of watermelon to low potassium.Rootstock grafting is also the main means to improve the nutritional quality of watermelon fruit.The genes responsable to low potassium discovered in this study laid the foundation for future research on the molecular mechanism of potassium nutrition in watermelon.