Physiological And Proteomic Analysis Of Castor Cotyledons And Roots In Response To Alkaline Salt(NaHCO)3)Stress

Castor(Ricinus communis L.)is an annual or perennial herb of the genus castor in the euphorbia family.Its seeds contain more than 50%oil.and also considered to be energy plant.In addition,castor has strong resistance to stress conditions,and has good adaptability to drought and saline-alkali stresses.In recent years,it has been widely used in soil improvement and ecological environment construction in saline-alkali regions in Northeast China.In general,early seedling growth(cotyledon excavation process)is the period when the plant is most sensitive to stress conditions,which directly affects whether it can survive in such stress.For a long time,people have paid attention to the physiological functions of plant cotyledons,such as storage and transport of nutrients and photosynthesis,and ignored their role in resisting stress during early seedling stage.Moreover,root is the primary site of sensing salt stress.which also plays an important role in plant responses to stress.Therefore,this study focuses on castor cotyledon excavated process,and the physiological response of cotyledon and root under alkaline salt stress(NaHCO3)from the physiological and proteome level was measured,in order to further explore the molecular mechanism of castor and also provide scientific basis for breeding new varieties of this species.(1)The biomass of cotyledons and roots of castor oil plant was significantly reduced under alkaline salt stress(P<0.05).and the biomass of roots decreased more significantly.The fresh weight of cotyledons and roots under 90 mM NaHCO3 stress decreased by 28%and 47%,respectively,compared with the control group.Photosynthetic pigment content and chlorophyll fluorescence parameters of cotyledons were also affected by alkaline salt stress,Chlorophyll a,chlorophyll b and carotenoids which decreased by 31%,42%and 26%,respectively,compared with control.Root activity was increased by 38%when treated with low concentration of NaHCO3 stress,and decreased by 42%when treated with high concentration of NaHCO3.With the increasing of NaHCO3 stress,root activity was significantly inhibited.(2)With the increasing of NaHCO3 stress concentration,the contents of Na+,Ca2+ and Mg2+ in castor cotyledon and root were both increased,while the contents of K+ decreased.Castor root absorbed a large amount of Na+,and meanwhile inhibited K+absorption and transportion,and also disrupted the ion balance.The K+ content at 90 mM alkaline salt stress was inhibited by 52%.The accumulation of osmotic substances can be used as a physiological adaptation strategy for castor seedlings in response to NaHCO3 stress.The proline content in cotyledons increased significantly with the increase of stress(P<0.05).The proline content in cotyledons and roots under high concentration of NaHCO3 stress was 1.9 and 1.7 times of that in the control group,respectively.The soluble sugar content in cotyledons increased first and then decreased with the increase of stress concentration,while the soluble sugar content in the roots did not change significantly.(3)Castor under alkaline salt stress increased the activity of ascorbate peroxidase(APX),peroxidase(POD)and superoxide dismutase(SOD)to eliminate the damage caused by stress.The activity of APX and POD?SOD in roots increased gradually with the increasing of stress concentration.The activity of APX was significantly changed(P<0.05).At 90 mM NaHCO3 stress,APX in cotyledons was 3 times that of the control group,and APX in roots was 1.7 times that of control.The CAT activity in SOD and root in cotyledon decreased with the increasing of alkaline salt stress concentration.At the same time,MDA content in cotyledons first increased and then decreased,among which MDA content in cotyledons under 90 mM alkaline salt stress decreased by 39%compared with the control group,which may be caused by the damage of cell membrane caused by oxygen free radicals' failure to be cleared.(4)A total of 40 valid results were identified by MALDI-TOF/TOF.There are 15 differential proteins in castor cotyledons and 25 differential proteins in roots.In cotyledons,10 proteins abundance were up-regulated and 5 proteins abundance were down-regulated.In the root,14 proteins abundance were up-regulated and 11 proteins abundance were down-regulated.According to proteomics,alkaline salt stress mainly affects carbohydrate and energy metabolism,stress defense,amino acid metabolism,lipid metabolism in roots.The decreased expression abundance of 40S ribosomal proteins S20 in roots may be caused by the decrease of ribosomal proteins binding to rRNA during the protein synthesis process of roots,which may affect the protein synthesis.Meanwhile,the processing and modification of proteins may be inhibited.Alkaline salt stress mainly affects photosynthesis,carbohydrate and energy metabolism,stress defense and protein synthesis,folding and destination in cotyledons.Ribulose bisphosphate carboxylase/oxygenase activators are key enzymes involved in the Calvin cycle,and their expression abundance is decreased in cotyledons under alkaline salt stress.The abundance of stress defense-related proteins such as late embryonic rich protein LEA 14-A was increased in cotyledons and roots.Cotyledons may respond to alkaline salt stress through the expression of some photosynthetic,defense and energy-related proteins.Roots do not have the ability of photosynthesize,but can promote some metabolic pathways through the expression of other metabolism-related proteins.Cotyledons and roots have different mechanisms in response to alkaline salt stress,which are different and complementary for each other.