| Day-neutral strawberries,also known as seasonal strawberries,have the characteristics of annual production and excellent resistance,which are valued by producers.However,compared with short-day strawberries,day-neutral strawberries are difficult to be widely promoted due to their low fruit sugar content.So far,the formation mechanism of low sugar in the fruit of day-neutral strawberry has not been analyzed,which restricts its variety improvement process.Based on the consideration of strawberry directional breeding strategy,this study used 5 day-neutral strawberry varieties(’Monterey’,’San Andreas’,’Portola’,’Diamante’,’Seascape’)and 5 short-day strawberry varieties(’Benihoppe’,’Akihime’,’Santa Red’,’Miaoxiang No.7’,’Ningyu’)as experimental materials,based on metabolic level omics,transcriptional level omics,genetic level omics and other methods to compare the reasons for the difference in sugar content between day-neutral strawberry and short-day strawberry varieties,and analyze the formation mechanism of low-sugar in day-neutral strawberry.The main results are as follows:1.The low sucrose content is the reason for the low sugar content of the day-neutral strawberry fruit.It was found that the content of soluble solids(TSS)and total soluble sugar(SSC)in day-neutral strawberry fruits were significantly lower than those in short-day strawberry varieties;The sucrose content(11.08-14.28 mg·g-1FW)was only about 25% of the short-day strawberry varieties(31.00-41.09 mg·g-1FW),but there was no significant difference in the components of glucose and fructose.Subsequently,ultra-high performance liquid chromatography(UPLC)analysis showed that the three major sugar components of short-day strawberry(’Benihoppe’)accumulated during the fruit ripening process,and the accumulation rate of sucrose was significantly higher than that of glucose and fructose;It increased rapidly after the chlorotic stage,and finally became the most abundant sugar component in the fruit at the ripening stage,accounting for about 43.00% of the total sugar content however,in comparison,the accumulation rate of sucrose content in day-neutral strawberries(’Monterrey’)was significantly lower than that in short-day strawberries(’ Benihoppe ’),and sucrose at fruit ripening stage only accounted for 20.37% of the total sugar content,and 22.63% lower than the ’ Benihoppe ’ strawberry The results showed that the difference in sugar content between day-neutral strawberries and short-day strawberries was caused by the accumulation of sucrose components.2.Analysis of sucrose accumulation pathway in strawberry fruit.In order to explore the associated genes regulating sucrose accumulation in strawberry fruit,this study combed the sucrose-related transport,synthesis and metabolism pathways;the sugar transport pathway includes three families,SWEET,SUT and MFS;the sucrose synthesis pathway includes two pathways,namely SUS synthesis pathway and SPS-SPP synthesis pathway;sucrose degradation pathway includes four pathways:SUS,CWINV,CIN and VIN.Based on the genomes of 9 Rosaceae species(Fragaria×ananassa,Fragaria vesca,Fragaria iinumae,Fragaria nilgerrensis,Fragaria viridis,Potentilla micrantha,Rubus occidentalis,Rosa chinensis,and Rosa multiflora),a total of 1209 members related to sucrose accumulation were screened and obtained,among which the cultivated A total of 344 members of the strawberry(‘Camarosa’)genome were identified and renamed according to the Arabidopsis nomenclature system.Based on the experimental dataset of fruit development transcriptome of 11 short-day strawberry varieties,the sugar transport pathways FanSWEET5 e.7,Fan TMT1.1,1.2,1.3,1.4 and Fan PMT1.1,1.3,the sucrose degradation pathway FanSUS2 a.1,2a.2,2a.3,2a.4 and FanCWINV1 c.1,1c.2 are potentially related to fruit sugar accumulation,but not involved in the sucrose synthesis pathway in fruit,suggesting that sucrose in mature fruit is obtained through sieve transport through SWEET genes,while glucose and fructose are obtained by decomposing sucrose by CWINV and SUS,and then transported into the vacuole by PMT and TMT for accumulation.3.FanSWEET5 e.7 involves the difference of sucrose content between day-neutral strawberry and short-day strawberry varieties.q RT-PCR experiments showed that the expression level of FanSWEET5 e.7 during fruit ripening of day-neutral strawberry(’Monterrey’)was significantly lower than that of short-day strawberry(’Benihoppe ’),while the expression levels of other candidate genes showed no regular and significant difference.Then,FanSWEET5 e.7 was shown to have the function of transporting sucrose by yeast heterologous expression experiments.Further,overexpression of FanSWEET5 e.7 gene in ’Monterrey’ strawberry fruit can significantly increase the sucrose,TSS and SSC contents of fruit at mature stage;meanwhile,inhibiting the expression of FanSWEET5 e.7 gene in ’Benihoppe’ strawberry fruit can significantly reduce the expression of FanSWEET5 e.7 gene at mature stage.Sucrose,TSS and SSC content.These results indicate that FanSWEET5 e.7 involves differences in sugar content between daily neutral strawberries and short-day strawberry varieties.The above results revealed a mechanism of the formation of low-sugar content fruit varieties of day-neutral strawberry due to the insufficient sucrose accumulation caused by the underexpression of FanSWEET5 e.7 gene during fruit ripening,which provided a theoretical basis for the cultivation of high-quality day-neutral strawberry varieties with high sugar content. |
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