Mechanism Of Exogenous Polyamines To Citrus Fruit Postharvest Quality Preservation

Polyamines play important roles in plant adaptability,however,the role in regulating fruit senescence was still unknown.Fruit were gradually senescent during storage,in order to investigate the effects of exogenous spermine and spermidine on post-harvest senescence of citrus fruit,and reveal its molecular mechanism,Ponkan and Navel orange were used,and their storage quality after treatment were determined including weight loss rate,rot rate,respiration rate,TSS,TA,V_C content and fruit firmness.In addition,the physical changes of antioxidant indexes such as SOD,CAT,total phenols,flavonoids and carotenoids were also analyzed.Furthermore,differences of gene expression after treatment were also analyzed by using transcript sequence.Results as follows:1.Comparing the effects of spermidine and spermine treatment on storage quality.Ponkan were immersed in 1 m M exogenous spermine and spermidine separately,each group were evenly divided into two parts and stored at ambient temperature and 10℃respectively to select a better preservative.Under ambient temperature,fruit rot rate were reduced by 1 m M spermidine（SPD）while increased by 1 m M spermine（SPM）.Weight loss rate were reduced by SPD,but were not influenced by SPM.Citrus color index（CCI）showed no difference among CK,SPM and SPD.Respiration rate of SPD treated fruits was lower than CK,but that were higher in SPM treated fruits.The decline of TA during storage was retarded by SPM,however,accelerated by SPD.TSS showed no difference in SPD treated fruits comparing with CK,but higher than that in SPM treated fruit.Similar to the results under ambient temperature,on the condition of cold storage,SPD treatment could reduce fruit rot rate,and fruit respiration rate at 40～60 days after treatment.However,both SPM and SPD all had no effects on TSS and fruit weight loss.All in together,the effects of SPD on the storage quality of citrus fruit were better than that of SPM.2.Comprehensively analyzed the molecular mechanism of SPD on fruit quality preservation.In order to further study the mechanism underlining the storage quality maintaining by SPD,the sensory quality of Navel orange treated with 1 m M SPD and resistance physiology and gene expression profile were analyzed.Similar with the above results,SPD reduced the fruit decay rate and respiratory intensity,but had no effect on the weight loss rate.The increase of CCI value and the decrease of H°value from Navel orange during storage were delayed by SPD treatment,thereby retarding the color conversion process.TSS content was maintained at higher level,and the decrease of TA,soluble sugar and V_C contents were delayed after treatment.SPD inhibited C_X、PG、PME genes expression and related enzyme activity,delayed the decline of cellulose and hemicellulose content in the peel and flesh,then keep the firmness of fruits.Spermidine increased the activity of SOD and CAT in peel as well as CAT in flesh,decreased the accumulation of hydrogen peroxide in peel and pulp,induced DPPH and OH·scavenging ability in the peel and DPPH,OH·,O₂^-·Clear force and Fe³⁺reducing power,increased the accumulation of antioxidant compounds such as total phenol,flavonoid（3 mg/g and 8 mg/g higher respectively in peel）and carotenoids.Results of transcriptome sequencing showed that there were 335 and 811 genes respectively differentially expressed in peel and flesh after SPD treatment.Functional annotation and KEGG pathway enrichment analysis on differential expressed genes from flesh at 7 d（Flesh-CK-7 d vs Flesh-T-7 d）and 30 d（Flesh-CK-30 d vs Flesh-T-30 d）showed that phytohormone signal transduction pathway associated with fruit senescence were significantly enriched.Auxin early reaction related GH3 gene family,the ERF1/2gene involved in ethylene signalling,the JAZ（Jasmonate ZIM-domain）gene,and defense-related pathogenesis-related protein gene（PR-1）were significantly enriched.Genes related to ethylene and ABA biosynthesis were quantified by Q-PCR and the results showed that the expression levels of key genes SAMC,ACO1,ACS1 and NCED3 were significantly lower than those in the control.In summary,SPD can maintain the flavor quality through retarding the degradation of soluble sugar and TA,keep fruit firmness by reducing the gene expressions and enzyme activities of C_X、PG、PME and delay the declines of cellulose and hemicellulose contents,reduce fruit rot rate by inducing PR gene expression and disease resistance.SPD enhance SOD,CAT enzyme activities,induce the accumulation of antioxidant compounds such as total phenol,flavonoid and carotenoids,enhance the active oxygen scavenging ability,reduce the accumulation of H₂O₂,and then retard fruit senescence induced by active oxygen.