Physiological Mechanism Of Response To Soil Salt Stress By Excavation Of Castor Leaves

Castor（Ricinus communis L.）is an annual or perennial herb of Euphorbiaceae and Ricinus genus.Castor has strong adaptability in saline,drought and heavy metal stress,and is the world’s top ten oil crops and high-quality salt-tolerant energy plants.The growth of cotyledons is the most critical period in the life cycle of plants,which is easily affected by external environmental conditions and is also related to the survival of plants.For a long time,people have paid attention to the physiological functions of nutrition storage,transportation and photosynthesis of cotyledons,ignoring its role in the process of resistance to adversity stress during cotyledon excavation.In addition,root is the primary part of salt stress,which is also particularly important for plants to adapt to adversity.Under salt stress,the physiological mechanism of castor cotyledons responding to soil salt stress is not clear.Based on this,in this study,physiology,transcriptomics and metabolomics techniques were used to systematically analyze the physiological mechanisms of castor cotyledons and roots,and to investigate the correlation in the stress tolerance process,so as to provide scientific basis for further exploring the molecular mechanism of stress resistance in castor and the breeding of new varieties.The results show that:（1）Under salt stress,the biomass of castor cotyledons and roots decreased significantly（P<0.05）,and the fresh weight and dry weight of roots decreased more significantly.Compared with the control group,the root length and root bifurcation increased by 1.12 and 1.23 times,respectively,and the root diameter decreased from 0.43 to 0.4 mm.Photosynthetic physiology and chlorophyll fluorescence parameters were also affected by salt stress.Compared with the control,intercellular CO₂ had no significant difference,while transpiration rate,stomatal conductance and net photosynthetic rate decreased by 7.32,10 and 2.67 times,respectively.Chlorophyll a and chlorophyll b content decreased by 21.74%and 25.00%,respectively.Under salt stress,φPSII and qp decreased by 24.14%and 27.08%,respectively,and NPQ increased by63.64%.Therefore,150 m M salt stress inhibited the photosynthesis and root activity of castor cotyledons.（2）The accumulation of osmotic substances was a strategy for castor seedlings to adapt to salt stress.Under salt stress,MDA in leaves and roots of castor was 2.23 and 1.54 times higher than that of the control group（P<0.05）,and proline content increased by 7.34 and 1.82 times,respectively.Soluble sugar content in cotyledons decreased by 18.62%,and no significant changes were observed in roots.The Na⁺content in castor cotyledons and roots increased significantly（P<0.05）,and the K⁺content in cotyledons increased by 1.23 times,which decreased by 64.51%in roots.Therefore,the Na⁺/K⁺ratio in roots was higher than that in cotyledons.The activities of superoxide dismutase（SOD）in antioxidant enzymes increased in cotyledons and roots,and the activities of ascorbate peroxidase（APX）,catalase（CAT）and peroxidase（POD）in cotyledons decreased by 16.67%,36.36%and 27.06%,respectively.The activities of APX,CAT and POD in roots increased,and the POD activity in castor roots increased by 1.1 times under salt stress.（3）Analysis of differential genes in cotyledons and roots by high-throughput technology showed that there were 1580 differential genes in cotyledons under salt stress,of which 880genes were up-regulated and 700 genes were down-regulated.Differential genes are mainly enriched in plant hormone signal transduction,estrogen signal pathway,antigen treatment and delivery pathways.There were 1502 differential genes in roots,of which 732 genes were up-regulated and 770 genes were down-regulated.The differentially expressed genes are mainly enriched in the interconversion between pentose and glucuronic acid,phenylpropanoid biosynthesis,starch and sucrose metabolism and plant hormone signal transduction pathways.The three pathways of plant hormone signal transduction,starch and sucrose metabolism and arginine and proline metabolism changed significantly in cotyledon and root.The results showed that under the stimulation of salt stress,castor seedlings regulated the hormone signaling pathway through plant hormones bidirectionally,regulated the activities of enzymes related to glucose metabolism,and accumulated amino acids through arginine and proline metabolic pathways,thereby regulating carbon distribution and improving the adaptability of castor cotyledons and roots to salt stress.（4）The metabolites of cotyledons and roots were analyzed by liquid chromatography（LC）combined with electrospray ionization time of flight mass spectrometry（ESI-TOF-MS）.The results of principal component analysis（PCA）showed that the metabolites in cotyledons and roots were significantly different under salt stress.There are 38 differential metabolites in cotyledons,mainly including fatty acids,nucleic acids and organic acids,while only 19differential metabolites in roots,mainly including fatty acids and organic acids.In addition,Kyoto Encyclopedia of Genes and Genomes（KEGG）pathway enrichment analysis showed that flavonoid and flavonol biosynthesis,pantothenic acid and coenzyme A biosynthesis,citric acid cycle and carotenoid biosynthesis were common metabolic pathways in cotyledons and roots under salt stress.These results revealed the changes of metabolites in cotyledons and roots of castor seedlings under salt stress,and confirmed that castor seedlings could adjust carbon allocation strategies,re-plan a variety of metabolic pathways,and accumulate unsaturated fatty acids in cotyledons and roots to improve their salt tolerance.In addition,the amino acid and nucleic acid profiles in cotyledons had specific changes.Under salt stress,roots could specifically improve the activity of antioxidant enzyme system in scavenging reactive oxygen species.