The alpha subunit is responsible for the aldol cleavage of indoleglycerol phosphate to indole and glyceraldehyde 3-phosphate (By similarity). Contributes to the tryptophan-independent indole biosynthesis, and possibly to auxin production. (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate + L-serine = D-glyceraldehyde 3-phosphate + H2O + L-tryptophan (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate = D-glyceraldehyde 3-phosphate + indole Amino-acid biosynthesis; L-tryptophan biosynthesis; L-tryptophan from chorismate: step 5/5. Tetramer of two alpha and two beta chains. Ubiquitously expressed at low levels in seedlings, roots, hypocotyls, cotyledons, stems, leaves, inflorescences, flowers, siliques and seeds. Mostly present in vascular tissues. In flowers, expressed in carpels, stamens and pollen. Belongs to the TrpA family.