| Strawberry is one of the model plants for berry fruit and genomic research in Rosaceae.Soluble sugar and organic acid serve as the essential basis of fruit flavor quality.It is important to explore the diversity of sugar/acid accumulation in strawberry fruits,and identify sugar transporters correlated with sugar accumulation,which not only has a guiding significance for improving fruit quality and strawberry breeding,but also holds a reference value for other Rosaceous plants.In this study,by analyzing the diversity of soluble sugars and organic acids in strawberry resources,as well as the genome-wide identification and bioinformatics analysis integrated with gene expression analysis of SWEET and MFS family sugar transporters,we revealed the accumulation features of sugar and acid,and the correlation between sugar content and sugar transporters in strawberry fruit.The main results are as follows:(1)Fruits of 506 strawberry resource representing 161 genotypes collected from Shanghai were analyzed by HPLC for soluble sugar(sucrose,glucose,fructose)and organic acid(malic acid,citric acid).The results showed that the total soluble sugar content of fruits from different strawberry resources varied greatly.The total sugar content ranged from 20.78 to 254.50 mg/g FW,with sucrose accounting for 1.00%-53.60%,while glucose(22.74%-43.32%)and fructose(23.66 %-56.52%)a relatively close range of the total sugar.The citric acid content in 93.7% samples accounted for more than 70% of the total organic acids,which supports that strawberry fruits preferentially accumulate citric acid.The sugar and acid levels of strawberries can be divided into three grades: high,medium and low.Germplasms with middle sugar and middle acid accounted for the largest proportion,being 50.3% and 60.3% of the population,respectively.(2)The accumulation features were characterized by comparing the changes of strawberry soluble sugar,organic acid and their components in different genotypes,under distinct cultivation conditions and seasonal changes.The results showed that the sugar and acid content in strawberry fruits were greatly influenced by genotype.The fruits from a commercial variety cv.Benihoppe had the highest sugar and acid content,which belonged to a high sugar and high acid type.And most of the commercial strawberries tested would be sorted into four types: high sugar and high acid,medium sugar and high acid,high sugar and medium acid,or medium sugar and medium acid.The composition of sugar and acid in specific genotypes changed greatly under the rain and less light conditions.Optimal sugar and acid quality of different strawberry resources was available at different seasons.As compared with soil cultivation,substrate cultivation brought about a significant decrease in the total sugar,its components and sugar-acid ratio of strawberry fruits.The total organic acids were also decreased in most germplasms when cultivated in substrate.(3)The availability of the strawberry database of Fv H4.0.a2 facilitated the identification of candidate genes of sugar transporters.Finally,we characterized 20 members of the SWEET family and 67 members of the MFS superfamily.The latter is further divided into 8 families,23 of the STP family,15 of the SFP family,7 of the PMT family,4 of the TMT and p Glc T families,and 3 of the INT and VGT families.(4)Bioinformatics analysis and spatiotemporal dynamic expression profiling showed that the SWEET family has the Mt N3?-slv(PF03083)domain,whose 20 members are distributed on all 7 chromosomes except for Fvb1.Most members contain 6 exons and 5 introns,encoding a protein of 235-310 amino acids with a 7 transmembrane helix structure.Except for Fv SWEET6 a,-8a,-8b,the rest 17 Fv SWEETs genes all were expressed in different tissues and organs of cv.Benihoppe.A total of seven genes including Fv SWEET1,-2a,-7,-9a,-9c,-10,-11 were identified as candidates contributing to sugar accumulation in strawberry fruits.(5)Phylogenetic analysis of the MFS superfamily updated the nomenclature of the strawberry homologous genes.When comparing current analysis based on Fv genome of 4.0.a2 with previous work based on v1.0,one member was reduced in STP and SFP subfamilies,while an additional member was added to p Glc T,TST,and VGT subfamilies.The chromosomal location,gene structure,and conserved motif analysis also have been optimized in v4.0.a2 database.It was found that 58 Fv STs showed different expression in cv.Benihoppe.Between Fragaria vesca and Fragaria × ananassa,nine and 11 Fv STs maintained similar expression patterns in the seeds and flesh,respectively.(6)The correlation between MFS expression level and sugar content was analyzed in three resources with significant differences in fruit sugar content.The results showed that 13 genes were significantly correlated,among which 9 genes,Fv SUT6,Fv STP7,Fv STP9,Fv STP15,Fv SFP8,Fv SFP9,Fv SFP10,Fv TST1 and Fv VGT1 displayed a positive correlation between total soluble sugar,sucrose,glucose,fructose content.The transcript abundance of the four genes Fv STP23 and Fv INT1-3 in fruits were negatively correlated with the contents of sucrose and total soluble sugars in mature fruits.Furthermore,the soluble sugar content in strawberry is closely related to sugar accumulation and sugar distribution.The transcription level of genes in the fruits at turning stage showed the highest correlation with the soluble sugar in the red fruit,indicating that this period is crucial for the sugar accumulation in the mature fruit.(7)The correlation of 13 Fv STs genes was re-examined in 19 varieties.The results showed that the dynamic expression patterns of these genes varied greatly among different varieties.There were 9 genes whose significant correlation was basically confirmed,among which 6 were positively correlated,including Fv STP7,Fv STP9,Fv STP15,Fv SFP8,Fv SFP9 and Fv VGT1.Three genes of INT subfamily Fv INT1-3 were negatively correlated.Based on the correlation research,a model of sugar transporter system cooperatively participating in the subcellular sugar distribution of strawberry fruit was proposed. |
PDF Full Text Request