Abstract

One of the most dynamic areas of plant molecular biology is the investigation of the actions of three classes of herbicides: s‐triazines (atrazine, simazine), glyphosate, and sulfonylureas (chlorsulfuron, sulfometuron methyl) (Figure 1). The results of this work are expected to provide the first significant applications of plant biotechnology: directly, in the genetic engineering of crop plants resistant to specific herbicides and, indirectly, in providing a molecular basis for the rational design of new herbicides for specific biological targets.s‐Triazines affect photosynthesis by inhibiting the binding of quinones to the chloroplast membrane QB protein. An s‐triazine resistant QB protein isolated from weeds in fields consistently treated with the herbicide has a serine in place of a glycine in this highly conserved protein. Glyphosate inhibits 5‐enolpyruvyl‐shikimate‐3‐phosphate synthase (EPSP synthase), an enzyme in the aromatic amino acid biosynthetic pathway. Mutagenized bacteria produce a resistant EPSP synthase with a substitution of serine for proline. Sulfonylureas inhibit the acetolactate synthase (ALS) of bacteria, yeast, and higher plants; this enzyme catalyzes the first step in the synthesis of branched chain amino acids. Resistant ALS has been found in bacteria, yeast and tobacco with a proline substituted by serine in yeast ALS. These findings provide a strong basis for developing projected plant biotechnology applications.