Identification And Prelimary Functional Analysis Of Sweet Genes In Cassava(manihot Esculenta)a Thesis

Cassava（2n=36,Manihot esculenta Crantz）is a perennial shrub of the Euphorbiaceae cassava genus.It is the third largest food crop in tropical regions after rice and corn.The yield of cassava is extremely important for the food security of people living in tropical regions.Sugar transporters,as a transport carrier,play an important role in source to sink transportation of sugar in plants.SWEET（Sugars Will Eventually be Exported Transporters）is a new type of sugar transporter discovered in recent years,found in animals,plants and microorganisms.SWEET protein in plants contains two Mt N3＿slv domains and seven transmembrane helixs.SWEET protein can selectively transport monosaccharides（glucose,fructose,galactose,mannose,etc.）or disaccharides（sucrose）inside and outside the cell.Previous studies have shown that SWEET plays an important role in plant growth and development,hormone regulation,biotic and abiotic stress.In this study,23 MeSWEET genes were identified in the Phytozome database.The 23 MeSWEET genes were analyzed by bioinformatic analysis and their biological functions were preliminary investigated.The main results are as follows:Cassava contains 23 MeSWEET genes,they all contain 2 Mt N3＿slv domains,composed by 7 TMs and every 3 TMs constitute 1 Mt N3＿slv domain.All of which are in line with the basic characteristics of SWEET proteins in plants.Bioimformatic prediction of cis-elements in MeSWEET promoter shows that the regulation and expression of cassava MeSWEET is not only related to the growth cycle of cassava,but also related to tissue specificity,photosynthesis,response to external stimuli,biotic and abiotic stress and hormone balance.The MeSWEET family is divided into 4 branches in evolution,CladeⅠincludes 4members:MeSWEET1-1,1-2,3-1 and 3-2;CladeⅡincludes 4 members:MeSWEET4-1,4-2,6-1 and 6-2;CladeⅢincludes 12 members:MeSWEET10-1,10-2,10-3,10-4,10-5,10-6,10-7,10-8,10-9,11,15-1 and 15-2;Clade IV includes 3members:MeSWEET16-1,16-2 and 16-3.MeSWEET1-1,3-2,6-1,10-4,10-5,10-8,15-1,16-1 and 16-3 are all located on the cell membrane.MeSWEET1-1 has the ability to transport galactose and glucose,MeSWEET3-2 has the ability to transport galactose,and MeSWEET6-1 has the ability to transport fructose.The expression level of MeSWEET1-2,4-2,10-5,10-8,15-2,16-1 were induced by XamⅠ,The expression level of MeSWEET1-2,10-5,10-8,15-2,16-1 were induced by XamⅡ infection.XamⅠ contains 4 TALE effect factors,named TAL13,TAL14,TAL20,and TAL23 whereas XamⅡ contains 5 TALE effect factors,named TAL13,TAL14,TAL15,TAL20,and TAL23 according to the number of RVDs.The putative target of TAL20_XamⅠ and TAL20_XamⅡ is the promter region of MeSWEET10-5.Our results indicate that both XamⅠ and XamⅡ isolated in Hainan Province may secreate TAL20 effector,targeting the promoter sequence of MeSWEET10-5 to cause disease.Our study lays a solid foundation for exploring the molecular mechanism of the MeSWEET gene in response to the invasion of Xam,and provides a theoretical basis for molecular breeding of cassava resistance varieties.