Analyses Of Physiological And Biochemical Mechanism And Gene Expression Profile Of Fruit Quality Development In Watermelon

Fruit quality development of watermenlon is a complex process, mainly including changes of fruitshape and size, sugar content and composition, flesh color, texture and flavor. In this study, thepurpose is to investigate the variation of substances and enzymes that related to fruit quality, geneexpression profiling of fruit quality formation during watermelon fruit development. East-Asia Ecotypewatermelon accession97103(Citrullus lanatus subsp. vulgaris East-Asia Ecotype) with high sugarcontent and the wide type watermelon accession PI296341-FR（Citrullus lanatus subsp. lanatus）withvery low sugar content were used to study the physiological and biochemical mechanism and geneexpression profiling in several critical stages of fruit quality formation during watermelon fruitdevelopment via physiological and biochemical analysis and high-throughput sequencing technology.The main results are as following:1. The comparison of physical differences between97103with high sugar content andPI296341-FR fruit with low sugar content1.1. The significant difference of dry matter content between97103and PI296341-FR fruitindicated the differences of photosynthates unloading and partitioning. Unloaded photosynthates in97103matured fruit was about1.3-1.5times higher than that in PI296341-FR fruit. More unloadedphotosynthates were mainly stored in97103flesh tissue in the form of soluble sugars. In comparison,less photosynthates were unloaded into PI296341-FR fruit and mainly stored in seeds.1.2. The contents of sucrose, fructose and glucose in97103central flesh were high, while thecontent of sucrose, fructose and glucose were very low in97103mesocarp, PI296341-FR central fleshand mesocarp. The accumulation of sucrose was the key factor determining the final sweetness of97103fruit flesh.1.3. The lower content of pectin, crude fiber and malate in97103fruit flesh than that inPI296341-FR lead to the attractive agricultural traits of cultivated watermelon, including crispy textureand distinct flavor.2. The physiological and biochemical differences for watermelon fruit between97103andPI296341-FR2.1. In comparison, the enzyme activity of acid alfa-galactosidase in sweet tissue was significanthigher than in non-sweet tissues. It indicated that more photosynthates were unloaded into sweet tissuethan non-sweet tissues, and accumulated in the form of soluble sugars in sweet tissue. There was almostno difference in the enzyme activity of alkaline alfa-galactosidase between sweet tissue (97103centralflesh) and non-sweet tissue (97103mesocarp, PI296341-FR flesh and mesocarp PI29634).2.2. The enzyme activity of Insoluble acid invertase (IAI) were higher in sweet tissue than innon-sweet tissues.It indicated that more photosynthates were unloaded into sweet tissue than non sweettissues and IAI played an important role in photosynthates unloading. No difference was observed inthe enzyme activity of soluble acid invertase between sweet tissue and non-sweet tissues in the latter stage of fruit development. The enzyme activities of sucrose synthase (SuSy) and sucrose phosphatesynthase (SPS) were significant higher in sweet tissue than those in non-sweet tissues. These resultsindicated that sucrose accumulation was closely related to the enzyme activities of SuSy and SPS.2.3. The enzyme activities of polygalacturonase (PG) and cellulose (Cx) in97193fruit flesh weresignificant higher than in other tissues including97103mesocarp, PI296341-FR flesh and mesocarp.These results indicated that the two enzymes should be involved in fruit texture solfting. The significantdifference of malate content between97193fruit flesh and PI296341-FR flesh may be due to theenzyme activity difference of malate dehydrogenase (MDH).3. The comparison of gene expression profiling between cultivated97103and wild typePI296341-FR during fruit quality developmentFour critical stages for97103and six critical stages for PI296341-FR had been chosen forstrand-specific RNA-seq analysis in97103flesh,97103mesocarp and PI296341-FR flesh during fruitdevelopment.2,452,826and322genes were identified that were differentially expressed in97103flesh,97103mesocarp and PI296341-FR flesh, respectively, during fruit development. GO termenrichment analysis indicated that, during fruit development, biological processes such as cell wallbiogenesis, biogenesis and defense responses were significantly altered in97103flesh,97103mesocarpand PI296341-FR flesh, whereas hexose and monosaccharide metabolic processes were onlysignificantly altered in97103flesh, supporting sugar contents observed among three tissues.The annotated watermelon genome contains a total of62sugar metabolic enzyme genes. Ourtranscriptome analysis identified13sugar metabolic genes that are differentially expressed during97103flesh development and among three tissues. The differences of differentially expressed sugarmetabolic genes can be better supported in transcriptome level and more genes were different expressedin sweet tissue than in non-sweet tissues of watermelon fruit. Some differentially expressed sugarmetabolic genes correlated with activities of sugar metabolic enzymes and some not, possible reasonwas the mechanism for sugar metabolic processes were complex.Based on the analyses of differentially expressed sugar metabolic genes, we propose a model forsugar metabolism in cells of watermelon fruit flesh: During watermelon flesh development,α-galactosidase (AGA), insoluble acid invertase (IAI), sucrose phosphate synthase（SuSy）, sucrosephosphate synthase (SPS), UDP-glucose4-epimerase (UGE), soluble acid invertase (SAI) andUDP-galactose pyrophosphorylase (UGGP) had one or more expressed genes in enzyme families, andthese expressed genes involved in regulating sugar unloading and metabolism as key genes.