Effect Of Temperature On Metabolism Of Polysaccharide And Ganoderic Acids In Postharvest Ganoderma Lucidum

Ganoderma lucidum is one of the well-known Chinese herbal medicines in our country,which has pharmacological activities such as anti-tumor,liver protection,immune regulation and so on,and has high medicinal value.The secondary metabolites of G.lucidum,ganoderic acid（GA）and intracellular polysaccharide（IPS）,have attracted much attention of researchers due to their important pharmacological activities.At present,the reported methods to increase the content of GA and IPS mainly include the optimization of medium components and cultivation conditions,as well as the addition of exogenous promotion or enhancement of gene expression.G.lucidum lost its nutrient matrix after harvest,but the life activities of cells were still continuing,and studies on postharvest physiology and metabolism of active substances of G.lucidum were rarely reported.In this study,the postharvest mycelia and fruiting bodies of G.lucidum were taken as research objects respectively,and the effects of different temperature treatments on the biomass of postharvest G.lucidum,the contents of secondary metabolites GA and IPS,the antioxidant properties of cells,and the expression of key enzyme genes in secondary metabolites synthesis pathway were explored.Mature G.lucidum mycelium and fruiting bodies were respectively placed in different temperature environments.Compared with normal conditions,the content of GA in postharvest mycelia of G.lucidum increased significantly at 20℃and 30℃,reaching 3.04 mg/100 mg DW and 3.11 mg/100 mg DW,which were 2.58 and 2.63 times higher than that untreated（stored directly after harvest without temperature storage）,respectively.The contents of four kinds of individual GAs（GA-Mk,GA-T,GA-S,GA-Me）also increased to varying degrees,reaching 233.86μg/100 mg DW,292.77μg/100 mg DW,559.19μg/100 mg DW and 194.52μg/100 mg DW,respectively.Although the content and yield of GA were at a low level when stored at 40℃,the ratios of GA-T,GA-S and GA-Me were significantly higher than other temperatures.The content of GA in postharvest fruiting bodies of G.lucidum was different from that of mycelia,and the overall effect was the best when stored at 40℃,with the highest content of 1.11 mg/100 mg DW.Compared with the fruit bodies untreated group,it increased by25.45%.And four kinds of GA monomers were also increased to some extent.The IPS content of G lucidum reached 24.42 mg/100 mg DW when mycelia were stored at 4℃,which was 25%higher than that untreated.When stored at 4,10,20 and 30℃for 1 d,the contents of IPS were 1.34,1.50,1.63 and 1.54 times than untreated respectively,while the rest was significantly lower than untreated.The physiological state of postharvest G.lucidum stored at different temperatures also changed greatly.For postharvest G.lucidum mycelia,temperature not only affected the growth and metabolism of G.lucidum mycelia,but also affected the characteristics of antioxidant enzymes.At 40℃,the mycelia suffered more serious damage,with MDA（malondialdehyde）and H₂O₂（hydrogen peroxide）contents higher than 20℃and30℃,while POD（peroxidase）,CAT（catalase）and SOD（superoxide dismutase）activities were at the lowest state.The activities of POD,CAT and SOD at all temperatures showed a trend of rising first and then declining.The maximum of CAT and SOD activities appeared at 20℃and 30℃,339.60μmol/min/mg prot and 9.76 U/mg prot respectively.The fruiting bodies of G.lucidum were similar to the mycelium of G.lucidum.The maximum of CAT and SOD activities also appeared at 20℃and 30℃,512.95μmol/min/mg prot and 12.97 U/mg prot respectively.The higher MDA content was,the more serious the lipid level of G lucidum cell membrane was.In G.lucidum mycelia and fruiting bodies,MDA content was higher when stored at 40℃,which indicated that cell damage was more serious.After storage at 20,30 and 40℃,the oxidation level of the harvested mycelium and fruiting bodies gradually increased,while the antioxidant system was activated and increased at the initial stage and gradually decreased at the later stage.In order to study the effect of temperature on the secondary metabolism of G.lucidum,the expression of key enzyme genes in the synthetic pathway of GA and IPS was analyzed by fluorescence quantitative PCR.After saving at different temperatures,the key enzyme genes of IPS and GA synthesis pathway of postharvest G.lucidum were also improved to some extent.When stored at 30℃,the expression levels of key enzyme genes hmgr,sqs and ls of G.lucidum mycelia after harvest were 2.81 times,2.32 times and 2.17 times higher than that untreated respectively.However,the expression levels of pgm,ugp and gls of the key enzyme genes of IPS synthesis pathway were at a low level at 20℃,30℃and 40℃,and were lower than that untreated.For G.lucidum fruiting bodies,the expression level of hmgr,sqs and ls genes was the highest when stored at40℃,which are 1.34 times,1.35 times and 1.55 times higher than that untreated respectively.However,the expressions of pgm,ugp and gls genes didn′t increase at this temperature.After storage at different temperatures,the changes of gene expression levels of key enzymes for the synthesis of GA and IPS were as same as their content trends,regardless of mycelium or fruiting bodies.Under temperature stress on the postharvest mycelia of G.lucidum,the highest IPS content was 23.04 mg/100 mg DW after 12 h of stress at 0℃,which was 15.56%higher than that without stress.The content of GA reached the maximum value of 2.17 mg/100mg DW at 0℃for 6 h,which was 1.50 times higher than that without stress.The contents of four kinds GA monomers were also better under 0℃stress,and the contents of GA-T,GA-S and GA-Me all reached the maximum value at 6 h,which were 1.25,1.21 and 1.37 times higher than that without stress,respectively.Under 40℃stress,the contents of four kinds GA monomers generally decreased with the extension of time.The expressions of key enzyme genes in the synthesis pathway of GA and IPS were similar to the changes of their contents respectively.Under 0℃stress,the expressions of hmgr and ls genes were 1.60 times higher than that without stress at 6 h.Under 0℃stress the expression levels of pgm,ugp and gls genes were higher than at 40℃.In this paper,the effects of temperature on the physiology and metabolism of postharvest G.lucidum cells were explored,which can provide reference for the postharvest biology research of higher fungi and the preservation and application of postharvest quality of G.lucidum.