A DELETION IN AN INDOLE SYNTHASE GENE IS RESPONSIBLE FOR THE DIMBOA-DEFICIENT PHENOTYPE OF BXBX MAIZE

The biosynthesis of DIMBOA, a pesticidal secondary metabolite of maize , branches off the tryptophan pathway. We have previously demonstrated that indole is the last intermediate common to both the tryptophan an d hydroxamic acid pathways. The earliest discovered mutant in the DIMB OA pathway, bxbx (benzoxazineless), is deficient in the production of DIMBOA and related compounds. This paper presents evidence that a gene identified by Kramer and Koziel [Kramer, V. C. & Koziel, M. G. (1995) Plant Mel. Biol. 27, 1183-1188] as maize tryptophan synthase alpha (T SA) is the site of the genetic lesion in the DIMBOA-deficient mutant m aize line bxbx. We demonstrate that the TSA gene has sustained a 924-b p deletion in bxbx compared with its counterpart in wild-type maize. W e report that the TSA gene maps to the same location as the bxbx mutat ion, on the short arm of chromosome 4. We present evidence that the ve ry early and very high level of expression of TSA corresponds to the t iming and level of DIMBOA biosynthesis but is strikingly different fro m the expression of the maize tryptophan synthase beta (TSB) genes. We show that feeding indole to bxbx seedlings restores their ability to synthesize DIMBOA. We conclude that the maize enzyme initially named t ryptophan synthase alpha in fact is a DIMBOA biosynthetic enzyme, and we propose that it be renamed indole synthase. This work confirms and enlarges upon the findings of Frey et al. [Frey, M. Chomet, P., Glawis chniq, E., Stettner, C., Grun, S., Winklmair, A, Eisenreich, W., Bache r, A., Meeley, RE., Briggs, S. P., Simcox, K. & Gierl, A. (1997) Scien ce 277, 696-699],which appeared while the present paper was in review.