Synthase catalytic subunit of the trehalose synthase complex that catalyzes the production of trehalose from glucose-6-phosphate and UDP-alpha-D-glucose in a two step process (PubMed:20439478). The disaccharide trehalose serves as a storage carbohydrate that is mobilized during conidial germination (PubMed:20439478). Regulates the level of trehalose as a protectant for cell integrity during thermal and oxidative stress (PubMed:20439478). D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Increases during conidiation. Induced by thermal stress (PubMed:20439478). Repressed by oxidative stress (PubMed:20439478). Simultaneous disruption of tpsA abolishes trehalose biosynthesis during hyphal and conidia formation, and delays germination (PubMed:20439478). Simultaneous disruption of tpsA results in abnormal cell wall formation, sensitivity to caspofungin, calcofluor white, thermal stress and oxidative stress, and hypervirulence in a mouse model of aspergillosis (PubMed:20439478). Belongs to the glycosyltransferase 20 family.