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:16946258). The disaccharide trehalose serves as a storage carbohydrate that is mobilized during conidial germination (PubMed:16946258). Trehalose also serves as a protectant for cell integrity during stress (PubMed:16946258). D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Decreases cellular trehalose level (PubMed:16946258). Sensitive to thermal stress (PubMed:16946258). Abnormal conidial germination on glucose or fructose carbon sources (PubMed:16946258). Belongs to the glycosyltransferase 20 family.