Improvement Of Tomato Fruit Sweetness By Overexpression Of SugarTransporter Protein Fgr

Tomatoes（Solanum lycopersicum）are originated in Central and South America and are very popular around the world for their unique flavor and are now widely grown as a global vegetable food.During the domestication of wild-type tomatoes,the planting density and yield of tomatoes have been continuously increased,and the increasing fruit weight led to the continuous decrease in tomato flavor due to the negative correlation between soluble sugar content and fruit weight.With the improvement of people’s living standards,people’s demand for tomatoes has become more diversified,and the improvement of tomato flavor is gradually being emphasized.The soluble sugars in the fruit of modern cultivated tomatoes consist mainly of fructose and glucose,which are the main criteria for measuring the quality of tomato fruits.The sweetness index of fructose is 2.26 times higher than glucose,and increasing the proportion of fructose will result in sweeter tomato fruit with the same total sugar content.Fgr（Fructose To Glucose Ratio）transporter proteins in tomato belong to the SWEETs protein family have the ability to bidirectionally transport sugars,and because of their higher affinity for glucose,Fgr transporter proteins have a higher transport efficiency for glucose.This enhances glucose efflux from the fruit during tomato ripening and makes the accumulation of fructose in the tomato fruit.The fructose and glucose contents are approximately the similar in wild-type tomato fruits,while Fgr transporter proteins have higher affinity for glucose and higher transport efficiency.During tomato ripening,Fgr transporter proteins promote glucose efflux from the fruit,while sweeter fructose is accumulated in the fruit.This study was aimed to improve the fruit flavor of cultivated tomato Ailsa Craig.Firstly,a pCAMBIA1300b-CaMV35S-Fgr overexpression vector was constructed and transgenic stable genetic plants were obtained by Agrobacterium-mediated transformation.To determine the sugar soluble content of tomato fruits,we established a high performance liquid chromatographic（HPLC）method to determine the sugar content of tomato fruits over-expressed with Fgr by designing gradient experiments.In addition,data on various agronomic traits of Fgr transgenic strains were investigated.The main results of the study are as follows.we obtained 11 T₀ generation Fgr transgenic positive plants at the initial stage by Agrobacterium-mediated genetic transformation experiments,followed by self-propagation and molecular identification before finally obtaining two Fgr overexpression stable genetic lines.Determination of T₂ generation Fgr transgenic tomatoes by HPLC revealed that the fructose glucose ratio was 10%-30%higher than that of the wild-type and the fructose percentage in the fruit was significantly increased.By setting gradient experiments on temperature,flow rate and mobile phase,a method for the determination of soluble sugars in tomato fruits using HPLC coupled with a differential detector was established,which avoids complex derivatization of sugars,saves experimental cost and provides a reference for the establishment of soluble sugar content determination methods for other fruits and vegetables.Although the transgenic tomatoes did not differ significantly from wild-type tomatoes in terms of tomato fruit weight and plant growth and developmental status,the early ripening phenomenon appeared in Fgr transgenic tomatoes was found for the first time,and the fruit color change period was significantly shortened by 4-5 days compared with wild-type tomatoes.This suggests that altered fructose content or ratio in tomato fruits can cause early ripening and that fructose or fructose to glucose ratio may act as a signaling molecule to influence tomato ripening.In conclusion,we obtained an early-ripening tomato with a high fructose-to-glucose ratio trait and found for the first time that overexpression of the Fgr gene caused an early ripening trait in tomato,which provides a reference for studying the relationship between sugar transporter proteins and the growth,development,and ripening of tomato fruits,and also has important implications for the exploration of new functions of sugar transporter proteins and provides new ideas for the study of sugar signaling in tomato.