Effect Of Chitosan On Postharvest Juice Sac Granulation In Majiayou Pummelo And The Possible Mechanisms

Majiayou pummelo（citrus grandis（L.）Osbeck cv.Majiayou,MP）is an excellent local pummelo variety with unique genetics and superior quality.However,it is prone to juice sac granulation during storage,along with dim fruits color,pulp juice sac dehydration and gelation,and even yellowish and rough,lignification and severe hardened,which seriously damages its postharvest quality and hinders sustainable development of MP industry.In order to explore the occurrence mechanism and regulation of postharvest juice sac granulation for MP,the matural MP fruits were applied as experimental materials in this study.The transcriptomics differences for pulp juice sac of different granulation degrees during room-temperature（20±2℃）storage for 150 days was analyzed by transcriptome technology.Meanwhile,the effects of 1.5%chitosan coating on postharvest juice sac granulation for MP fruits during above storage time were also investigated.The main results are as follows:1.Transcriptome analysis was performed on the pulp juice sac stored at 0 d（none granulation,NG）,90 d（initial granulation,IG）and 135 d（moderate granulation,MG）,totally acquiring 6376 differentially expressed genes（DEGs）,including 4623 DEGs between NG and IG,4405 DEGs between NG and MG.The KEGG pathways that annotated more genes included metabolic pathways,biosynthesis of secondary metabolites,plant-pathogen interaction,plant hormone signal transduction and phenylpropanoid biosynthesis.KEGG and GO significantly enriched in secondary metabolic process,MAPK signaling pathway-plant,amino sugar and nucleotide sugar metabolism,response to chitin,cell wall organization and oxidation activities.In addition,the transcription factors that annotated more genes were close to plant stress response,like WRKY,AP2/ERF-ERF,MYB,C2H2 and NAC.Combined with the regulation of related gene expressions in these pathways,it was found that the postharvest juice sac granulation process for MP fruits was closely related to the response to external stress and cell wall modifications.2.Compared with the control（CK）,1.5%chitosan（CT）significantly inhibited the granulation index increasing,maintained better fruits color,higher total soluble solids content（TSS）,total sugar content,ascorbic acid（As A）and reduced glutathione（GSH）content after day 45,and higher titratable acidity（TA）content at the end of storage.At the same time,CT effectively inhibited the accumulation of superoxide anion（O₂^-）,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）,increased the activities of superoxide dismutase（SOD）,ascorbate peroxidase（APX）and glutathione reductase（GR）,and up-regulated the genes expressions（Mn SOD,APX1 and GR2）.In correlation analysis,the granulation index was significantly negatively correlated with fruit color index,sugar and acid content,As A and GSH content,antioxidant enzyme activities（SOD,APX,GR）and gene CAT1 expression level（p<0.05）,but had significantly positive correlations with the contents of O₂^-,H₂O₂ and MDA（p<0.01）.Above shown that 1.5%chitosan coating could delay the postharvest juice sac granulation of MP fruits via mitigating the reactive oxygen damage.3.As for the cell wall modifications process,CT significantly inhibited the increasing of cell wall materials,lignin,cellulose and hemicellulose,and suppressed the activities of peroxidase（POD）,phenylalanine ammonia lyase（PAL）and cinnamic alcohol dehydrogenase（CAD）after day 90.In the meantime,CT promoted protopectin hydrolysis duing storage to maintain a high water-soluble pectin content.Besides,CT held stronger activities of pectin methylesterase（PME）,polygalacturonidase（PG）,cellulase（Cx）and xyloglucan endoglycosyltransferase（XTH）activities,and up-regulatedcell wall metabolism related genes expressions（PME,PG,Cx and XTH33）,but down-regulatedcell lignin synthesis related genes expressions（POD,PAL and CAD）In the correlation analysis,the granulation index was significantly positively correlated with contents of cell wall materials,cellulose and lignin,and the activities and gene relative expressions of POD,PAL and CAD（p<0.01）,but significantly negatively correlated with the pectin materials contents,cell wall hydrolase activities（PME,PG and Cx）and gene relative expressions（PG and Cx;p<0.01）.It was revealed that 1.5%chitosan effectively promoted cell wall hydrolysis and inhibited the cell wall hyperplasia and thickening process,thereby alleviating the postharvest juice sac granulation of MP fruits.