PLANT EXPRESSION OF A BACTERIAL CYTOCHROME-P450 THAT CATALYZES ACTIVATION OF A SULFONYLUREA PRO-HERBICIDE

The Streptomyces griseolos gene encoding herbicide-metabolizing cytochrome P450(SU1) (CYP105A1) was expressed in transgenic tobacco (Nicotiana tabacum). Because this P450 can be reduced by plant chloroplast ferredoxin in vitro, chloroplast-targeted and nontargeted expression were compared. Whereas P450(SU1) antigen was found in the transgenic plants regardless of the targeting, only those with chloroplast-directed enzyme performed P450(SU1)-mediated N-dealkylation of the sulfonylurea 2-methylethyl-2,3-dihydro-N-[(4,6-dimethoxypyrimidin-2-yl)aminocarbonyl]-1,2-benzoisothiazole-7-sulfonamide-1,1-dioxide (R7402). Chloroplast targeting appears to be essential for the bacterial P450 to function in the plant. Because the R7402 metabolite has greater phytotoxicity than R7402 itself, plants bearing active P450(SU1) are susceptible to injury from R7402 treatment that is harmless to plants without P450(SU1). Thus, P450(SU1) expression and R7402 treatment can be used as a negative selection system in plants. Furthermore, expression of P450(SU1) from a tissue-specific promoter can sequester production of the phytotoxic R7402 metabolite to a single plant tissue. In tobacco expressing P450(SU1) from a tapetum-specific promoter, treatment of immature flower buds with R7402 caused dramatically lowered pollen viability. Such treatment could be the basis for a chemical hybridizing agent.