The alpha subunit is responsible for the aldol cleavage of indoleglycerol phosphate to indole and glyceraldehyde 3-phosphate. In bacteria, tryptophan synthase alpha (TSA) activity is almost completely dependent on formation of an active alpha2beta2 complex with tryptophan synthase beta (TSB), and indole is usually not released during tryptophan synthesis. In maize, the TSA homolog BX1 catalyzes the formation of free indole from indole-3-glycerol phosphate, independently of TSB. (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate = D-glyceraldehyde 3-phosphate + indole (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate + L-serine = D-glyceraldehyde 3-phosphate + H2O + L-tryptophan kcat is 0.02 sec(-1) with indole-3-glycerol phosphate as substrate. Secondary metabolite biosynthesis; 2,4-dihydroxy-1,4-benzoxazin-3-one biosynthesis; 2,4-dihydroxy-1,4-benzoxazin-3-one from indoleglycerol phosphate: step 1/5. Amino-acid biosynthesis; L-tryptophan biosynthesis; L-tryptophan from chorismate: step 5/5. Tetramer of two alpha and two beta chains for the tryptophan synthase activity. Homodimer of alpha chains for the indole-3-glycerol phosphate lyase activity. Mutant rescue experiments indicate the existence of a second tryptophan synthase alpha gene whose product might be involved in tryptophan biosynthesis while BX1 might be restricted to free indol production. Belongs to the TrpA family.