Hydrogen Sulfide Treatment Improves Quality Attributes Of Postharvest Goji Berry (Lycium Barbarum L.) And Its Mechanisms

Goji berry（Lycium barbarum L.）is considered as a medicinal and food fruit,and the fresh fruit is susceptible to postharvest decay.Maintaining its postharvest nutritional quality and prolonging shelf life is of great importance for the promotion of fresh fruit for its consumption.Hydrogen sulfide has been shown to participate in a variety of physiological and biochemical processes after fruit harvesting,however the role of hydrogen sulfide as a gaseous signaling molecule in the postharvest preservation of goji fruit and its mechanism has not been reported.In this paper,we investigated the effects of hydrogen sulfide treatment on the physiological,biochemical,transcriptional and metabolic levels of postharvest goji berries,where used exogenous sodium sulfide solution as the donor of hydrogen sulfide,and further combined transcriptome-metabolome analysis to clarify the mechanism of hydrogen sulfide treatment on regulating ethylene biosynthesis and signal transduction in postharvest goji berries.This study investigated the mechanism of hydrogen sulfide regulation of postharvest senescence and signal transduction in goji fruit,and provided a theoretical basis for the use of hydrogen sulfide as a fresh fruit preservative in goji fruit.The main findings are as follows.（1）Effect of hydrogen sulfide treatment on the physiology and biochemistry of postharvest goji fruits.The exogenous sodium sulfide solution was used as the hydrogen sulfide donor to treat the fresh goji fruits and the optimal concentration of sodium sulfide solution was 1.4 m M.Hydrogen sulfide could reduce the rate of mold,decay of the fresh fruit,as well as delay the browning of fruit pedicels,decrease weight loss,hardness,soluble solids and color difference of fresh goji fruits,and maintain the appearance quality and full fruit of goji with bright red color;hydrogen sulfide could delay the degradation of total protein,total polysaccharide and betaine of fresh goji fruits after harvesting,meanwhile,it could also reduce the accumulation of reactive oxygen products such as H₂O₂,·O₂^-and membrane lipid peroxidation products MDA in fresh goji fruits,increase the activity of As A and GSH,improve the activity of CAT,APX,POD,GR and SOD,etc.The activity of antioxidant enzymes such as CAT,APX,POD,GR and SOD was increased,and the activity of LOX was down-regulated.These above results showed that hydrogen sulfide was able to maintain the nutrients and bioactive components of postharvest goji fruits at high levels.while increasing the activity of antioxidant enzymes and inhibiting the accumulation of reactive oxygen products,which enhanced the antioxidant and anti-aging ability of fresh goji berries,thus delaying the aging of fresh postharvest goji fruits.（2）Effect of hydrogen sulfide treatment on transcript levels of fresh goji fruits after harvest.RNA-seq was used to investigate the variation of differentially expressed genes with the storage time of fresh goji fruits after hydrogen sulfide treatment.There were 2919,1884 and 4339 differentially expressed genes in the three comparison groups of fresh goji fruits on day 2,day 4 and day 6 after hydrogen sulfide treatment respectively,and the total number of differentially expressed genes in the three comparison groups was 523.The metabolic pathways were more affected by differentially expressed gene enrichment after hydrogen sulfide treatment were photosynthesis,carotenoid biosynthesis,α-linolenic acid metabolic pathway,flavonoid biosynthesis,phenylpropane biosynthesis,starch and sucrose metabolism,and plant-pathogen interactions.Transcriptional expression profiling and q RT-PCR analysis showed that hydrogen sulfide treatment up-regulated the expression of antioxidant enzyme-related genes CAT,APX,POD,GR and SOD and down-regulated the expression of LOX genes,and the correlation results showed that hydrogen sulfide treatment was positively correlated with the expression of CAT,APX,POD,GR and SOD genes and negatively correlated with the expression of LOX genes,which was consistent with the determination of antioxidant enzyme activities in fresh goji fruits with hydrogen sulfide treatment,which further validated that hydrogen sulfide treatment regulated the antioxidant enzyme system in fresh goji fruits.In addition,hydrogen sulfide treatment up-regulated the expression of DCD and LCD genes,which were key enzymes of endogenous hydrogen sulfide synthesis,and increased the content of endogenous hydrogen sulfide in fresh goji fruits.（3）Effect of hydrogen sulfide treatment on the metabolic level of postharvest goji fruits.A liquid mass spectrometry approach was adopted to investigate the changes in metabolic profiles of fresh goji fruits after hydrogen sulfide treatment in different storage periods.After hydrogen sulfide treatment,the primary metabolite KEGG was analyzed,and 6263 metabolites annotated to KEGG were involved in 113metabolic pathways in positive and negative ion modes.Secondary metabolite KEGG analysis showed that the metabolic pathways enriched in differential metabolites varied with the time of hydrogen sulfide treatment.At the overall level,the main metabolic pathways enriched to fresh goji fruits metabolites after hydrogen sulfide treatment were biosynthesis of plant secondary metabolites,amino acid biosynthesis,biosynthesis of phenylpropanoids,ABC transport,aminoacyl t RNA biosynthesis,phenylalanine metabolism,2-oxocarbonyl acid metabolism,carbon metabolism,biosynthesis of alkaloids of the mangiferous acid pathway,ornithine,lysine and nicotinic acid alkaloids biosynthesis,etc.（4）Hydrogen sulfide treatment regulates the mechanism of ethylene biosynthesis and its signal transduction in postharvest goji fruits.The expression of key genes in ethylene biosynthesis and ethylene signaling were changed to different degrees after hydrogen sulfide treatment.The expression of ACS,ACO and met K,key enzymes of ethylene biosynthesis,and ethylene signaling factors SIMKK,MPK6,EBF1/2,EIN3,ERF1 and ERF2 were also significantly reduced.The expression of CTR1 was up-regulated,and the content of L-methionine,S-adenosyl-L-methionine and 1-aminocyclopropane-1-carboxylate,the key metabolites in ethylene biosynthesis pathway,was reduced,which finally led to the inhibition of ethylene biosynthesis in fresh goji fruits after harvesting by hydrogen sulfide treatment,thus delaying the senescence decay and extending the shelf life of goji fruits.The above findings systematically reveal the response mechanism of postharvest goji fruits to hydrogen sulfide from the phenological,physicochemical,transcriptional,and metabolic levels,elucidate how the metabolism of intracellular compounds is mobilized in postharvest goji fruits by hydrogen sulfide treatment to delay postharvest senescence and provide a theoretical basis for the application of sulfide to the storage and preservation of postharvest goji fruits.