Metabolic Properties Of Assimilate Accumulation In The Source Sink Of Liquorice Seedlings And Its Response Mechanism To Drought Stress

Horticultural plants have a variety of species and a wide geographical distribution,and they have outstanding advantages in tapping nutritional health as well as ecologically functional plants.Liquorice（Glycyrrhiza uralensis Fisch.）is one of the first plant species selected for the catalog of medicinal food homologation in China,and also an important natural resource in arid and semi-arid areas.Exploring the performance of drought resistance adaptations and changes in metabolites of economic organs（roots）is important for the sustainable utilization of liquorice resources,as well as providing direct strategies for the conservation and utilization of dry plant resources.In this study,liquorice seedlings were used as materials,and natural drought was maintained in the diferent treatment levels,and soil moisture content was controlled by weighing and replenished the water,including normal water supply（CK）,light stress（LS）,moderate stress（MS）and severe stress（SS）,to study the photosynthetic,structural and product accumulation characteristics of liquorice source leaves under different drought stress;the characteristics of root growth and carbohydrate accumulation and transformation of sink organs were studied;and the response regulation characteristics of source-sink of liquorice to drought stress were preliminarily determined.On this basis,the proteomics and metabolomics methods were further used to analyze the changes of root proteins and metabolic components as economic organs and sink organs,so as to provide a basis for elucidating the source-sink regulation mechanism of liquorice in response to stress and establishing high-quality cultivation measures of liquorice.The main findings are as follows:（1）The shoot biomass of liquorice under drought stress showed a trend of increasing under LS and decreasing under SS,while the root-shoot ratio showed a continuous increasing trend with the increase of drought stress,indicating that the biomass of liquorice was preferentially deployed to the roots under drought stress.（2）Under LS,liquorice adapted to drought stress by adjusting the number and morphology of stomata and the density of glands,and the increase of stomatal density helped the plant to better control respiration,regulate the inlet and outlet of H₂O and CO₂in the body,and improve the photosynthetic efficiency of the plant when the water was suitable.After SS,liquorice may have a strong non-photochemical burst mechanism to dissipate the excess light energy,dissipate the absorbed light energy in the form of heat without photochemical reactions,thus reducing the generation of free radicals and playing a protective role for the photosynthetic mechanism;secondly,during SS,the water loss of leaves causes leaf thinning or wilting,the palisade-spongy ratio decreases,and the chloroplast membrane structure gradually blurs or even disappears.（3）The composition and content of sugars influenced the metabolic operation of the source reservoir and also influenced the formation of drought resistance as an important osmoregulatory substance.After LS and MS,fructose content was higher than sucrose in liquorice leaves,while sucrose content was higher than fructose in roots.Secondly,the contents of fructose,sucrose and glucose in leaves and roots were higher than CK after different drought stresses,which may be the result of the accumulation of the contents of liquorice to resist drought stress and improve the tolerance of the plant.（4）Based on the preliminary determination of the regulatory characteristics of the source-sink response of liquorice,proteomics was further used to analyze the information of protein fractions under different drought stress in the roots.A total of 7409 proteins were identifed in this study,of which 7305 total proteins could be quantifed.There were837 diferentially expressed proteins（DEPs）identifed after diferent drought stresses.Compared with CK,123 DEPs（80 up-regulated and 43 down-regulated）were found in LS;353 DEPs（254 up-regulated and 99 down-regulated）in MS;and 564 DEPs（312up-regulated and 252 down-regulated）in SS.Based on the above data,we performed a systematic bioinformatic analysis of the informative proteins.The analysis showed that in LS mainly the accumulation of osmolytic substances such as cottonseed sugar and proline improved the resistance of liquorice.In MS and SS,the oxidation of unsaturated fatty acids,as well as glucose and galactose accumulation were promoted in response to drought stress.In addition,SQS（Q8GSL6）was consistently up-regulated in expression after drought stress,leaving squalene synthesis unaffected,while squalene monooxygenase（A0A1S2YI26）was significantly down-regulated in expression after MS,affecting squalene 2,3-epoxide synthesis.Andβ-coumarin synthase（W5XM28,A0A0R0IYV7,Q84PE3）was also significantly down-regulated in expression after SS,which inhibited the synthesis ofβ-coumarin and consequently the synthesis of terpene precursor five-carbon compounds,ultimately leading to a decrease in glycyrrhetinic acid content.（5）Analysis of liquorice root extracts by ultra performance liquid chromatography tandem mass spectrometry（UPLC-MS/MS）resulted in the detection of 831 metabolites,including 177 flavonoids,140 lipids,113 phenolic acids,70 organic acids,66 amino acids and derivatives,53 nucleotides and derivatives,50 terpenoids,31 alkaloids,25lignans and coumarins,7 quinones The results showed that there were 97 metabolites,2ellagitannins,and 97 other metabolites.The correlation analysis of 11 flavonoids,9alkaloids,13 organic acids,and 7 amino acid substances that were significantly up-regulated in liquorice roots under SS revealed that 3-amino-2-naphthoic acid,L-proline,N-benzylmethylene isomethylamine,N-acetyl-5-hydroxytryptamine,L-cystine,4,5,6-trihydroxy-2-cyclohexene-1-ylideneacetonitrile,L-phenylalanine,2（3H）-Benzothiazolone were the most correlated group with other substances and may be the main metabolites for the evaluation of drought resistance in liquorice.The above results initially revealed the response mechanism of drought resistance adaptation and major metabolic pathways in liquorice,and screened some drought resistance candidate substances,which laid the theoretical foundation for the selection and utilization of drought resistance germplasm resources.