Involved in the biosynthesis of steviol glycosides in leaves (PubMed:15610349). Converts steviol to the mono-glycoside steviolmonoside (PubMed:15610349). Converts the mono-glycoside steviolmonoside to the bi-glycoside rubusoside (PubMed:15610349). Converts the bi-glycoside steviolbioside to the tri-glycoside stevioside (PubMed:15610349). Converts the tri-glycoside rebaudioside B to the tetra-glycoside rebaudioside A (PubMed:15610349). steviolmonoside + UDP-alpha-D-glucose = rubusoside + UDP steviolbioside + UDP-alpha-D-glucose = stevioside + UDP steviol + UDP-alpha-D-glucose = H(+) + steviolmonoside + UDP rebaudioside B + UDP-alpha-D-glucose = rebaudioside A + UDP Induced by blue light and far-red light. 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.