The Effect And Mechanism Of Dwarfing Rootstock ’Yunnan Quince’ On Fruit Sugar Content Of ’Zaosu’ Pear

Dwarfing rootstock is an important method to achieve dwarfing and dense planting of pear.Different rootstock types affect the growth and fruit quality of scion varieties.Sugar synthesis and accumulation are important factors affecting fruit quality.Currently,the research on the effect of rootstocks on sugar metabolism in pear varieties is mainly focused on the synthesis of sugar in fruit,and there are few studies on the mechanism of sugar transport,and the research on the effect of dwarfing rootstock on sugar transport in pear fruit is still unclear.In this study,‘Zaosu’ pear(Pyrus bretschneideri Rehd.)grafted on the dwarfing rootstock ‘Yunnan quince’(Cydonia oblonga Mill.)and the arbor rootstock ‘Duli’(Pyrus betulifolia)were studied.The fruit quality of different stock-scion combinations was measured and the transporter PbSWEET6 related to assimilate transport was screened.The biological function of PbSWEET6 in sugar transportation was verified.The main results are as follows:1.‘Yunnan quince’ as the rootstock significantly increased the sugar content in the fruit of ‘Zaosu’ pear.The sugar content in the mature fruit of ‘Zaosu’ / ‘Quince’ and ‘Zaosu’ /‘Duli’ was determined.The results showed that the contents of sucrose,glucose and fructose in the fruit of ‘Zaosu’ / ‘Quince’ were higher than that of ‘Zaosu’ / ‘Duli’ fruit.The synthesis and transportation of sugar are important factors affecting fruit quality.The capacity of leaf photosynthetic and assimilate distribution capacity of different stock-scion combinations were analysed,the results showed that the photosynthetic capacity was similar between‘Zaosu’ / ‘Quince’ and ‘Zaosu’ / ‘Duli’ leaves.However,in the ’Zaosu’ pear with ’Yunnan quince’ as the rootstock,more assimilates were transported to the fruit.These results indicated that the distribution of assimilates was an important factor for the difference in sugar content between ‘Zaosu’ / ‘Quince’ and ‘Zaosu’ / ‘Duli’.2.As sugar transporters involved in the transport of assimilates,the SWEETs(Sugars Will Eventually be Exported Transporters)are of great significance in regulating sugar transport.A total of 18 PbSWEETs sugar transporter genes have been identified in pear.Based on the expression analysis in ‘Zaosu’ / ‘Quince’ and ‘Zaosu’ / ‘Duli’ fruit,PbSWEET6 were screened out which was highly expressed in ‘Zaosu’ / ‘Quince’.The sequence analysis showed that full length of the pear PbSWEET6 gene was 918 bp,encoded a protein with a length of 305 aa and had 7 transmembrane domains.Phylogenetic analysis found that PbSWEET6 belonged to the clade III of the SWEET family.The PbSWEET6 protein was localized in the cytoplasmic membrane,indicated that it could function on the cell membrane.3.The PbSWEET6 gene affected sugar content in fruit and might be involved in regulating the transport of sugar from source to sink.Firstly,the function of PbSWEET6 was verified in the fruit of ‘Zaosu’ / ‘Quince’ and ‘Zaosu’ / ‘Duli’ by transient transformation system.Overexpression of PbSWEET6 in ‘Zaosu’ / ‘Duli’ fruit resulted in increases in the sucrose and glucose contents,while silencing of PbSWEET6 in ‘Zaosu’ / ‘Quince’ fruit decreased the sucrose content.The function of PbSWEET6 was further verified by transformation of tomato and pear fruit calli.Compared with the wild-type tomato,the contents of sucrose,fructose and glucose in tomato fruit of PbSWEET6 heterologously expressed lines were significantly increased,the sugar contents in leaves were significantly decreased.PbSWEET6-overexpressed transgenic pear fruit calli was cultured on medium containing different sugar sources(3% sucrose,3% fructose and 3% glucose)to verify the ability of PbSWEET6 to transport different sugars.The sucrose and glucose contents of PbSWEET6-overexpressed calli grown on 3% sucrose medium were increased compared with wild-type,and showed the same trend cultured on 3% glucose medium.The results showed that the increased expression of PbSWEET6 in ‘Zaosu’ / ‘Quince’ might be directly or indirectly involved in the accumulation of glucose and sucrose in ‘Zaosu’ fruit.