Involved in the biosynthesis of steviol glycosides in leaves (PubMed:26358188, PubMed:27923373). Converts the mono-glycoside steviolmonoside to the bi-glycoside steviolbioside (PubMed:26358188, PubMed:27923373). Converts the bi-glycoside rubusoside to the tri-glycoside stevioside (PubMed:27923373). Converts the tri-glycoside stevioside to the tetra-glycoside rebaudioside E (PubMed:27923373). Converts the tetra-glycoside rebaudioside A to the penta-glycoside rebaudioside E (PubMed:27923373). steviolmonoside + UDP-alpha-D-glucose = H(+) + steviolbioside + UDP rubusoside + UDP-alpha-D-glucose = H(+) + stevioside + UDP stevioside + UDP-alpha-D-glucose = H(+) + rebaudioside E + UDP rebaudioside A + UDP-alpha-D-glucose = H(+) + rebaudioside D + UDP Leaves of the 'sweet herb' Stevia rebaudiana contain a mix of steviol glycosides, compounds that are unique in the plant world because of their intense sweetness and high concentration in leaf tissue (Probable). Stevia leaves have been used as natural sweeteners in South America for centuries (Probable). Belongs to the UDP-glycosyltransferase family.