Engineering Plant Resistance to Thiazopyr Herbicide via Expression of a Novel Esterase Deactivation Enzyme

Plants were engineered to confer resistance to thiazopyr, a member of the pyridine herbicide family, via an esterase deactivation mechanism. Earlier studies showed that transformation of thiazopyr to its monoacid metabolite resulted in loss of herbicidal activity (P.C.C. Feng et al., 1995, Xenobiotica 35, 27). Based on thiazopyr hydrolytic activity, a 60-kDa esterase was purified from rabbit liver. The N-terminal amino acid sequence of purified pyridine-esterase demonstrated high homology to the published protein sequence of rabbit liver esterase isozyme 1 (RLE1). PCR primers designed based on the amino acid sequence of RLE1 recovered a novel cDNA (RLE3) whose derived amino acid sequence was 95% homologous to RLE1. Baculovirus-mediated expression of RLE3 cDNA in insect cells detected the 60-kDa esterase as well as activity against thiazopyr. Stable plant transformation of RLE3 cDNA was conducted in tomato and tobacco under a constitutive expression promotor. R 0 plants demonstrated wild-type phenotype, and analysis of leaf tissues confirmed the presence of the 60-kDa esterase. Transgenic seedlings demonstrated both in vitro and in vivo deactivation of thiazopyr to the monoacid. In growth chamber and greenhouse tests, R 1 seedlings from transgenic tomato and tobacco demonstrated enhanced resistance to thiazopyr. Resistance was directly correlated to the level of pyridine-esterase expression. A field study was conducted with transgenic tomato seedlings which further confirmed resistance to thiazopyr.