Changes Of Sugar And Acid Content In Fruit Development Stages And Analysis Of Sugar Transporter Proteins In ’Gaoban’ Pummelo

Pummelo is the main citrus cultivation type,and its quality depends on the content of sugar and acid to some extent.However,there are no systematic studies on the variation of sugar and acid content and its key genes in pummelo fruit during its development period.Sugar transporter proteins(STP)plays an important role in the transport and distribution of carbon sources,affecting the metabolism and accumulation of sugar in plant tissues.Additionally,The characteristics and functions of STP gene family in pummelo are still unknown.To address these knowledge gaps,it is of great guiding significance to improve the fruit quality of pummelo by studying the change of sugar and acid content and the analysis of sugar transporter in pummelo fruit at development stages.In this study,’Gaoban’was used as the main experimental material to explore the formation rule of sugar and acid of pummelo fruit.We also investigated the expression patterns and physiological functions of sugar transporter proteins in Citrus grandis(CgSTP).The following main results were achieved:1.The rule of quality formation during the development period of pummelo fruit.It was found that the fruit of Gaoban reached maturity 270 d after flowering.With the development of fruits,the weight and volume of fruits increased rapidly from 90 d to 150 d after flowering,and then remained basically unchanged.The green color of the fruit peel gradually fades,while the yellow color gradually appears and deepens.The soluble solid content in juice sac increased gradually,and the titrable acid content increased gradually in the early stages and decreased slightly at last.The main organic acids in the juice sac were citric acid,malic acid and quinic acid,among which the content of citric acid was the highest.With the development of the fruit,the content of citric acid gradually accumulated in the early stages and reached the highest at 240 d,but decreased slightly in the maturity stage of 270 d.The main soluble sugars in the juice sac were sucrose,glucose and fructose,among which the content of sucrose was the highest.With the development of the fruit,the content of sucrose presents an "S"-shaped trend of first increasing,slightly decreasing at the middle developmental stage of fruit,and then gradually increasing.2.Identification and analysis of CgSTP family members.Bioinformatics analysis revealed that there were 14 members of CgSTP gene family in pummelo genome,whose gene structure and physicochemical properties of encoding products were highly conserved.They were unevenly distributed on seven chromosomes,most of which were on the chromosome 9.According to phylogenetic tree analysis,CgSTP family members were distributed in four clusters.The conserved motifs of 14 members showed high similarity,except for CgSTP8,all other members contain 12 conserved motifs in a consistent order.The results of promoter analysis showed that the promoter region of CgSTPs contained a large number of cis-acting elements,which suggested that the promoter of this gene family might respond to hormonal or external environmental signals.It was speculated that the CgSTPs may participate in various physiological functions,such as the growth and development of pummelo,fruit sugar accumulation,response to biotic and abiotic stress.3.Preliminary study on the expression patterns and functions of CgSTPs.It was found that CgSTP4,CgSTP6,CgSTP7,CgSTP9,CgSTP11,CgSTP13 and CgSTP14 were highly expressed in leaves,flowers and juice sac by q RT-PCR analysis.The results of subcellular localization showed that CgSTP4,CgSTP7,CgSTP9,CgSTP11,CgSTP13 and CgSTP14 were located in the plasma membrane,while CgSTP6 was located in the vacuole membrane.It was demonstrated that CgSTP4,CgSTP6,CgSTP11,CgSTP13 and CgSTP14 functions in glucose transport by sugar transport mutants in yeast.Further experiments on stable genetic transformation of Arabidopsis thaliana and transient transformation of pummelo pollen tubes had demonstrated that CgSTP6,CgSTP11 and CgSTP14 were involved in the accumulation of sugar in plant tissues,while CgSTP4 was involved in the carbon source absorption and growth process of pollen tubes.The above research preliminarily analyzed the quality formation rule of pummelo fruit during the development period,and accurate quantitative analysis were carried out on the changes of sugar and organic acid content.At the same time,the members of CgSTP gene family were identified,which proved the membrane positioning characteristics and sugar transport activity of some CgSTPs.In addition,physiological functions of CgSTP6,CgSTP11 and CgSTP14 in sugar transport and accumulation and CgSTP4 in pollen tube growth of pummelo were investigated.The research provides an important reference for the analysis and improvement of pummelo fruit quality.