| Platycodon grandiflorum is a perennial herb of Platycodon genus in Platycodon family.This genus has only one species and one variety in the world.Platycodon grandiflorum is widely distributed in humid and semi-humid areas from temperate to subtropical zone in eastern China,which is a large number of commonly used Chinese medicinal materials and a typical medicinal and food homologous plant in China.The root of Platycodon grandiflorum is a famous Chinese medicinal material,which has the functions of promoting lung,removing phlegm,dispersing cold,relieving cough,reducing swelling and discharging pus.An essential component to characterize the quality of Platycodon grandiflorum are the platycodin,and platycodin D is the standard component stipulated in the Pharmacopoeia of the people’s Republic of China.The technological innovation of medicinal materials production is based on the formation and control of medicinal materials quality.Water is the basic substance of plant photosynthesis,and which is also an important medium and participant in various physiological metabolic processes of plants.Water regulation is also an important agronomic measure to improve crop yield and quality in agricultural production.Therefore,it is important to study the influence of water factors on the yield and quality of medicinal materials,especially the effect of drought stress on the quality of medicinal materials.The effects of water regulation on yield and quality of Platycodon grandiflorum were less studied.The physiological and molecular ecological response of water to drought stress is still blank.To this end,we used pot experiments to simulate the natural environment,and Platycodon grandiflorum of the same source and same age were used for water regulationthe.Drought stress of Platycodon grandiflorum during different growth periods(vegetative,flowering and fruiting)was observed.The responses of photosynthetic physiology,stress resistance physiology,platycodin content and biosynthesis gene expression of Platycodon grandiflorum to drought stress were studied.The molecular mechanism of drought response(gene regulation network)was revealed by transcriptome analysis,and the molecular ecological process of platycodin synthesis in Platycodon grandiflorum was explained.The aim is to provide scientific basis for water ecological regulation in the production of Platycodon grandiflorum.The main conclusions are as follows:(1)The response mechanism of primary metabolites(12 amino acids and 8 nucleosides)to water signal was different under drought stress during different growth periods of Platycodon grandiflorum.During the DS,the primary metabolites of Platycodon grandiflorum(6 species)and fruit stage(11 species)were negatively affected by drought stress.Only 4 nucleoside components of Platycodon grandiflorum were negatively affected by flower period drought stress.Only the ALA content increased significantly in AT(autumn)stage of vegetative period of Platycodon grandiflorum,while the accumulation of 7 and 4 primary metabolites increased significantly in flower and fruit periods,respectively.The contents of ALA,hypoxanthine and allopurinol,decreased significantly in the DS stage of vegetative and flower periods Platycodon grandiflorum,while that of ALA,hypoxanthine and allopurinol increased 28.04 %,31.09 %and 22.39 % in fruit periods Platycodon grandiflorum.In general,when drought occurs,the response mode of primary metabolites to water was different due to the different fertility conditions of Platycodon grandiflorum.Judging from the direct effects(DS)and long-term effects(AT),drought stress had a negative effect on the accumulation of primary metabolites of vegetative and fruit periods Platycodon grandiflorum,but had mostly positive effect on the flower period of Platycodon grandiflorum.At the same time,for drought stress,the biosynthesis regulation of amino acids or nucleoside components of Platycodon grandiflorum during the same growth period showed dual molecular ecological response strategies to promote and inhibit mutual transformation.(2)The regulation of fruit Platycodon grandiflorum by drought stress may be more beneficial to the improvement of the quality of Platycodon grandiflorum.The PD3、DPD accumulation in the DS stage of Platycodon grandiflorum in the three growth periods was consistent,all of them were significantly higher than that in CK(p <0.05).It can be seen that Platycodon grandiflorum may actively cope with adverse environmental conditions through the accumulation of secondary metabolites,which may be a means for Platycodon grandiflorum to resist short-term drought stress.During the AT stage,the PD3 and SPD contents increased only in flower period Platycodon grandiflorum,and the platycodin(PD、PD3、DPD、SPD)did not accumulate significantly in flower period Platycodon grandiflorum,while the contents of platycodin(PD 、 PD3 、 DPD)and total saponins(SPD)of fruit period Platycodon grandiflorum increased by 8.95 %,11.67 %,12.54 % and 21.67 %,respectively.Hypoxanthine is an important primary metabolite that affects the biosynthesis of PD and PD3 in platycodon grandiflorum during vegetative period.A variety of amino acids and nucleosides significantly promoted the accumulation of PD in platycodon grandiflorum.Guanine and ALA significantly promoted the accumulation of platycodin at fruit stage,while VAL was mostly negative on the accumulation of platycodon at fruit stage,indicating that guanine,ALA and Val were important factors affecting the accumulation of platycodon grandiflorin at fruit stage and important primary metabolites regulating the quality of platycodon grandiflorin at fruit stage.From the long-term effect of drought stress,compared with the vegetative stage and flowering stage,drought stress induced the synthesis of a large number of platycodon in Platycodon grandiflorum at fruit stage,which was the key period to control the quality of Platycodon grandiflorum.(3)Studies on the response mechanism of photosynthetic physiology,resistance physiology and secondary metabolism of Platycodon grandiflorum in different growth stages to drought stress showed: The three growth periods of Platycodon grandiflorum accumulated a large amount of reactive oxygen species in the DS stage,which affected photosynthetic physiology.They all resist the oxidative damage caused by drought stress,by increasing the TSOD、POD and CAT activity of protective enzymes.In DS stage,the SS、PRO content in flower period of Platycodon grandiflorum was 2.37 times and 1.53 times of the CK,and the SP content decreased significantly,the content of PRO、SP did not increase significantly in vegetative period of Platycodon grandiflorum,but the SP、PRO increased significantly in fruit period of Platycodon grandiflorum.It is speculated that different osmotic regulators may respond to drought stress by synergistic or antagonistic action in Platycodon grandiflorum.The response of the same osmotic adjustment to drought stress in Platycodon grandiflorum was not consistent.There were obvious positive correlation between PD、PD3 and T-SOD,DPD and CAT、POD,SPD and SS,in vegetative period of Platycodon grandiflorum(p < 0.05).In flower period of Platycodon grandiflorum,the DPD and PD3 were positively correlated with the root dry weight、PRO and φPSII,respectively(p < 0.05).The PD was positively correlated with SS、φPSⅡ,PD、PD3 were positively correlated with CAT and WUE in fruit period of Platycodon grandiflorum(p < 0.05).To sum up,there was some correlation between the secondary metabolite(platycodin)accumulation and photosynthesis,chlorophyll fluorescence,osmotic regulator,protective enzyme in Platycodon grandiflorum.(4)Analysis of the key enzymes in the biosynthesis of platycodin during different growth periods showed that: The difference in the number of up-regulated genes(8,6,3)in DS stages of the three growth periods of Platycodon grandiflorum,which was the differential response to water in different growth periods,and also the main factor affecting the difference in the synthesis of platycodin at this stage by drought stress.Ridge regression analysis showed that,the significant up-regulation of HMGR,IPPI,MVD,PMK,SS and β-A28 O in Platycodon grandiflorum at the vegetative period,HMGR and UGT1 in Platycodon grandiflorum at the flowering period,and PMK in the fruit period were the important influencing factors for the accumulation of platycodin at each stage in DS stage.At the same time,β-A28 O and β-AS were significantly up-regulated at the AT stage,which were the key enzyme genes for the significant accumulation of platycodin in fruit period in autumn.(5)At different stages(F＿DS、F＿RW、F＿AT)of fruit period Platycodon grandiflorum,transcriptome analysis of showed: In the stages of F＿DS、F＿RW、F＿AT,contained DEGs2087、1453、1447,respectively,which indicated that most of the DEGs were induced in the F＿DS stage.The five DGEs in the biosynthesis pathway of triterpenoids showed significantly down-regulated in the stages of F＿DS 、 F＿RW 、 F＿AT(TRINITY＿DN18371＿c0＿g1 、TRINITY＿DN20252＿c1＿g4 、 TRINITY＿DN22757＿c0＿g2 、 TRINITY＿DN21641＿c0＿g5 、TRINITY＿DN24994＿c0＿g2).These genes may be important regulatory genes involved in drought stress response in the synthesis pathway of platycodin in fruit period Platycodon grandiflorum.Drought stress had obvious inhibitory effect on genes of Group1 and Group 3 in NAC transcription factors of Platycodon grandiflorum,but had obvious induction effect on genes of Group 4,and on Group1 and Group 3 genes in transcription factors.And drought stress had obvious inhibitory effect on genes of Group1 and Group 3 in C3 H transcription factors of Platycodon grandiflorum.NAC and C3 H transcription factors may be closely related to the resistance of Platycodon grandiflorum to drought stress. |
