The alpha subunit is responsible for the aldol cleavage of indoleglycerol phosphate to indole and glyceraldehyde 3-phosphate. Required for tryptophan biosynthesis. 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. Mostly expressed in roots, hypocotyls and leaves, and, to a lower extent, in seedlings, cotyledons, stems, inflorescences, flowers, siliques and seeds. Compressed root wave phenotype on tilted agar surfaces. Reduced accumulation of tryptophan, but increased levels of auxin. Belongs to the TrpA family.